[v4,2/2] vhost: Perform memory section dirty scans once per iteration

Commit Message

Si-Wei Liu March 14, 2024, 8:27 p.m. UTC

On setups with one or more virtio-net devices with vhost on,
dirty tracking iteration increases cost the bigger the number
amount of queues are set up e.g. on idle guests migration the
following is observed with virtio-net with vhost=on:

48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14

With high memory rates the symptom is lack of convergence as soon
as it has a vhost device with a sufficiently high number of queues,
the sufficient number of vhost devices.

On every migration iteration (every 100msecs) it will redundantly
query the *shared log* the number of queues configured with vhost
that exist in the guest. For the virtqueue data, this is necessary,
but not for the memory sections which are the same. So essentially
we end up scanning the dirty log too often.

To fix that, select a vhost device responsible for scanning the
log with regards to memory sections dirty tracking. It is selected
when we enable the logger (during migration) and cleared when we
disable the logger. If the vhost logger device goes away for some
reason, the logger will be re-selected from the rest of vhost
devices.

After making mem-section logger a singleton instance, constant cost
of 7%-9% (like the 1 queue report) will be seen, no matter how many
queues or how many vhost devices are configured:

48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14

Co-developed-by: Joao Martins <[e-mail]>
Signed-off-by: Joao Martins <[e-mail]>
Signed-off-by: Si-Wei Liu <[e-mail]>

---
v3 -> v4:
  - add comment to clarify effect on cache locality and
    performance

v2 -> v3:
  - add after-fix benchmark to commit log
  - rename vhost_log_dev_enabled to vhost_dev_should_log
  - remove unneeded comparisons for backend_type
  - use QLIST array instead of single flat list to store vhost
    logger devices
  - simplify logger election logic
---
 hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
 include/hw/virtio/vhost.h |  1 +
 2 files changed, 62 insertions(+), 6 deletions(-)

Comments

Jason Wang March 15, 2024, 4:03 a.m. UTC | #1

On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
>
> On setups with one or more virtio-net devices with vhost on,
> dirty tracking iteration increases cost the bigger the number
> amount of queues are set up e.g. on idle guests migration the
> following is observed with virtio-net with vhost=on:
>
> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
>
> With high memory rates the symptom is lack of convergence as soon
> as it has a vhost device with a sufficiently high number of queues,
> the sufficient number of vhost devices.
>
> On every migration iteration (every 100msecs) it will redundantly
> query the *shared log* the number of queues configured with vhost
> that exist in the guest. For the virtqueue data, this is necessary,
> but not for the memory sections which are the same. So essentially
> we end up scanning the dirty log too often.
>
> To fix that, select a vhost device responsible for scanning the
> log with regards to memory sections dirty tracking. It is selected
> when we enable the logger (during migration) and cleared when we
> disable the logger. If the vhost logger device goes away for some
> reason, the logger will be re-selected from the rest of vhost
> devices.
>
> After making mem-section logger a singleton instance, constant cost
> of 7%-9% (like the 1 queue report) will be seen, no matter how many
> queues or how many vhost devices are configured:
>
> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
>
> Co-developed-by: Joao Martins <[e-mail]>
> Signed-off-by: Joao Martins <[e-mail]>
> Signed-off-by: Si-Wei Liu <[e-mail]>
>
> ---
> v3 -> v4:
>   - add comment to clarify effect on cache locality and
>     performance
>
> v2 -> v3:
>   - add after-fix benchmark to commit log
>   - rename vhost_log_dev_enabled to vhost_dev_should_log
>   - remove unneeded comparisons for backend_type
>   - use QLIST array instead of single flat list to store vhost
>     logger devices
>   - simplify logger election logic
> ---
>  hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
>  include/hw/virtio/vhost.h |  1 +
>  2 files changed, 62 insertions(+), 6 deletions(-)
>
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 612f4db..58522f1 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -45,6 +45,7 @@
>
>  static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
>  static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
>
>  /* Memslots used by backends that support private memslots (without an fd). */
>  static unsigned int used_memslots;
> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
>      }
>  }
>
> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
> +{
> +    assert(dev->vhost_ops);
> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
> +
> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);

A dumb question, why not simple check

dev->log == vhost_log_shm[dev->vhost_ops->backend_type]

?

Thanks

Si-Wei Liu March 15, 2024, 6:44 p.m. UTC | #2

On 3/14/2024 9:03 PM, Jason Wang wrote:
> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
>> On setups with one or more virtio-net devices with vhost on,
>> dirty tracking iteration increases cost the bigger the number
>> amount of queues are set up e.g. on idle guests migration the
>> following is observed with virtio-net with vhost=on:
>>
>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
>>
>> With high memory rates the symptom is lack of convergence as soon
>> as it has a vhost device with a sufficiently high number of queues,
>> the sufficient number of vhost devices.
>>
>> On every migration iteration (every 100msecs) it will redundantly
>> query the *shared log* the number of queues configured with vhost
>> that exist in the guest. For the virtqueue data, this is necessary,
>> but not for the memory sections which are the same. So essentially
>> we end up scanning the dirty log too often.
>>
>> To fix that, select a vhost device responsible for scanning the
>> log with regards to memory sections dirty tracking. It is selected
>> when we enable the logger (during migration) and cleared when we
>> disable the logger. If the vhost logger device goes away for some
>> reason, the logger will be re-selected from the rest of vhost
>> devices.
>>
>> After making mem-section logger a singleton instance, constant cost
>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
>> queues or how many vhost devices are configured:
>>
>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
>>
>> Co-developed-by: Joao Martins <[e-mail]>
>> Signed-off-by: Joao Martins <[e-mail]>
>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>
>> ---
>> v3 -> v4:
>>    - add comment to clarify effect on cache locality and
>>      performance
>>
>> v2 -> v3:
>>    - add after-fix benchmark to commit log
>>    - rename vhost_log_dev_enabled to vhost_dev_should_log
>>    - remove unneeded comparisons for backend_type
>>    - use QLIST array instead of single flat list to store vhost
>>      logger devices
>>    - simplify logger election logic
>> ---
>>   hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
>>   include/hw/virtio/vhost.h |  1 +
>>   2 files changed, 62 insertions(+), 6 deletions(-)
>>
>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>> index 612f4db..58522f1 100644
>> --- a/hw/virtio/vhost.c
>> +++ b/hw/virtio/vhost.c
>> @@ -45,6 +45,7 @@
>>
>>   static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
>>   static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
>>
>>   /* Memslots used by backends that support private memslots (without an fd). */
>>   static unsigned int used_memslots;
>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
>>       }
>>   }
>>
>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
>> +{
>> +    assert(dev->vhost_ops);
>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
>> +
>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
> A dumb question, why not simple check
>
> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
Because we are not sure if the logger comes from vhost_log_shm[] or 
vhost_log[]. Don't want to complicate the check here by calling into 
vhost_dev_log_is_shared() everytime when the .log_sync() is called.

-Siwei
> ?
>
> Thanks
>

Jason Wang March 18, 2024, 3:22 a.m. UTC | #3

On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 3/14/2024 9:03 PM, Jason Wang wrote:
> > On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
> >> On setups with one or more virtio-net devices with vhost on,
> >> dirty tracking iteration increases cost the bigger the number
> >> amount of queues are set up e.g. on idle guests migration the
> >> following is observed with virtio-net with vhost=on:
> >>
> >> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
> >> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
> >> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
> >> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
> >>
> >> With high memory rates the symptom is lack of convergence as soon
> >> as it has a vhost device with a sufficiently high number of queues,
> >> the sufficient number of vhost devices.
> >>
> >> On every migration iteration (every 100msecs) it will redundantly
> >> query the *shared log* the number of queues configured with vhost
> >> that exist in the guest. For the virtqueue data, this is necessary,
> >> but not for the memory sections which are the same. So essentially
> >> we end up scanning the dirty log too often.
> >>
> >> To fix that, select a vhost device responsible for scanning the
> >> log with regards to memory sections dirty tracking. It is selected
> >> when we enable the logger (during migration) and cleared when we
> >> disable the logger. If the vhost logger device goes away for some
> >> reason, the logger will be re-selected from the rest of vhost
> >> devices.
> >>
> >> After making mem-section logger a singleton instance, constant cost
> >> of 7%-9% (like the 1 queue report) will be seen, no matter how many
> >> queues or how many vhost devices are configured:
> >>
> >> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
> >> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
> >>
> >> Co-developed-by: Joao Martins <[e-mail]>
> >> Signed-off-by: Joao Martins <[e-mail]>
> >> Signed-off-by: Si-Wei Liu <[e-mail]>
> >>
> >> ---
> >> v3 -> v4:
> >>    - add comment to clarify effect on cache locality and
> >>      performance
> >>
> >> v2 -> v3:
> >>    - add after-fix benchmark to commit log
> >>    - rename vhost_log_dev_enabled to vhost_dev_should_log
> >>    - remove unneeded comparisons for backend_type
> >>    - use QLIST array instead of single flat list to store vhost
> >>      logger devices
> >>    - simplify logger election logic
> >> ---
> >>   hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
> >>   include/hw/virtio/vhost.h |  1 +
> >>   2 files changed, 62 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >> index 612f4db..58522f1 100644
> >> --- a/hw/virtio/vhost.c
> >> +++ b/hw/virtio/vhost.c
> >> @@ -45,6 +45,7 @@
> >>
> >>   static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
> >>   static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
> >> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
> >>
> >>   /* Memslots used by backends that support private memslots (without an fd). */
> >>   static unsigned int used_memslots;
> >> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
> >>       }
> >>   }
> >>
> >> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
> >> +{
> >> +    assert(dev->vhost_ops);
> >> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
> >> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
> >> +
> >> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
> > A dumb question, why not simple check
> >
> > dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
> Because we are not sure if the logger comes from vhost_log_shm[] or
> vhost_log[]. Don't want to complicate the check here by calling into
> vhost_dev_log_is_shared() everytime when the .log_sync() is called.

It has very low overhead, isn't it?

static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
{
    return dev->vhost_ops->vhost_requires_shm_log &&
           dev->vhost_ops->vhost_requires_shm_log(dev);
}

And it helps to simplify the logic.

Thanks

>
> -Siwei
> > ?
> >
> > Thanks
> >
>

Si-Wei Liu March 18, 2024, 10:16 p.m. UTC | #4

On 3/17/2024 8:22 PM, Jason Wang wrote:
> On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
>>
>>
>> On 3/14/2024 9:03 PM, Jason Wang wrote:
>>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
>>>> On setups with one or more virtio-net devices with vhost on,
>>>> dirty tracking iteration increases cost the bigger the number
>>>> amount of queues are set up e.g. on idle guests migration the
>>>> following is observed with virtio-net with vhost=on:
>>>>
>>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
>>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
>>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
>>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
>>>>
>>>> With high memory rates the symptom is lack of convergence as soon
>>>> as it has a vhost device with a sufficiently high number of queues,
>>>> the sufficient number of vhost devices.
>>>>
>>>> On every migration iteration (every 100msecs) it will redundantly
>>>> query the *shared log* the number of queues configured with vhost
>>>> that exist in the guest. For the virtqueue data, this is necessary,
>>>> but not for the memory sections which are the same. So essentially
>>>> we end up scanning the dirty log too often.
>>>>
>>>> To fix that, select a vhost device responsible for scanning the
>>>> log with regards to memory sections dirty tracking. It is selected
>>>> when we enable the logger (during migration) and cleared when we
>>>> disable the logger. If the vhost logger device goes away for some
>>>> reason, the logger will be re-selected from the rest of vhost
>>>> devices.
>>>>
>>>> After making mem-section logger a singleton instance, constant cost
>>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
>>>> queues or how many vhost devices are configured:
>>>>
>>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
>>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
>>>>
>>>> Co-developed-by: Joao Martins <[e-mail]>
>>>> Signed-off-by: Joao Martins <[e-mail]>
>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>
>>>> ---
>>>> v3 -> v4:
>>>>     - add comment to clarify effect on cache locality and
>>>>       performance
>>>>
>>>> v2 -> v3:
>>>>     - add after-fix benchmark to commit log
>>>>     - rename vhost_log_dev_enabled to vhost_dev_should_log
>>>>     - remove unneeded comparisons for backend_type
>>>>     - use QLIST array instead of single flat list to store vhost
>>>>       logger devices
>>>>     - simplify logger election logic
>>>> ---
>>>>    hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
>>>>    include/hw/virtio/vhost.h |  1 +
>>>>    2 files changed, 62 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>>>> index 612f4db..58522f1 100644
>>>> --- a/hw/virtio/vhost.c
>>>> +++ b/hw/virtio/vhost.c
>>>> @@ -45,6 +45,7 @@
>>>>
>>>>    static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
>>>>    static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
>>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
>>>>
>>>>    /* Memslots used by backends that support private memslots (without an fd). */
>>>>    static unsigned int used_memslots;
>>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
>>>>        }
>>>>    }
>>>>
>>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
>>>> +{
>>>> +    assert(dev->vhost_ops);
>>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
>>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
>>>> +
>>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
>>> A dumb question, why not simple check
>>>
>>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
>> Because we are not sure if the logger comes from vhost_log_shm[] or
>> vhost_log[]. Don't want to complicate the check here by calling into
>> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
> It has very low overhead, isn't it?
Whether this has low overhead will have to depend on the specific 
backend's implementation for .vhost_requires_shm_log(), which the common 
vhost layer should not assume upon or rely on the current implementation.

>
> static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
> {
>      return dev->vhost_ops->vhost_requires_shm_log &&
>             dev->vhost_ops->vhost_requires_shm_log(dev);
> }
>
> And it helps to simplify the logic.
Generally yes, but when it comes to hot path operations the performance 
consideration could override this principle. I think there's no harm to 
check against logger device cached in vhost layer itself, and the 
current patch does not create a lot of complexity or performance side 
effect (actually I think the conditional should be very straightforward 
to turn into just a couple of assembly compare and branch instructions 
rather than indirection through another jmp call).

-Siwei

>
> Thanks
>
>> -Siwei
>>> ?
>>>
>>> Thanks
>>>

Jason Wang March 20, 2024, 3:27 a.m. UTC | #5

On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 3/17/2024 8:22 PM, Jason Wang wrote:
> > On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
> >>
> >>
> >> On 3/14/2024 9:03 PM, Jason Wang wrote:
> >>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
> >>>> On setups with one or more virtio-net devices with vhost on,
> >>>> dirty tracking iteration increases cost the bigger the number
> >>>> amount of queues are set up e.g. on idle guests migration the
> >>>> following is observed with virtio-net with vhost=on:
> >>>>
> >>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
> >>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
> >>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
> >>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
> >>>>
> >>>> With high memory rates the symptom is lack of convergence as soon
> >>>> as it has a vhost device with a sufficiently high number of queues,
> >>>> the sufficient number of vhost devices.
> >>>>
> >>>> On every migration iteration (every 100msecs) it will redundantly
> >>>> query the *shared log* the number of queues configured with vhost
> >>>> that exist in the guest. For the virtqueue data, this is necessary,
> >>>> but not for the memory sections which are the same. So essentially
> >>>> we end up scanning the dirty log too often.
> >>>>
> >>>> To fix that, select a vhost device responsible for scanning the
> >>>> log with regards to memory sections dirty tracking. It is selected
> >>>> when we enable the logger (during migration) and cleared when we
> >>>> disable the logger. If the vhost logger device goes away for some
> >>>> reason, the logger will be re-selected from the rest of vhost
> >>>> devices.
> >>>>
> >>>> After making mem-section logger a singleton instance, constant cost
> >>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
> >>>> queues or how many vhost devices are configured:
> >>>>
> >>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
> >>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
> >>>>
> >>>> Co-developed-by: Joao Martins <[e-mail]>
> >>>> Signed-off-by: Joao Martins <[e-mail]>
> >>>> Signed-off-by: Si-Wei Liu <[e-mail]>
> >>>>
> >>>> ---
> >>>> v3 -> v4:
> >>>>     - add comment to clarify effect on cache locality and
> >>>>       performance
> >>>>
> >>>> v2 -> v3:
> >>>>     - add after-fix benchmark to commit log
> >>>>     - rename vhost_log_dev_enabled to vhost_dev_should_log
> >>>>     - remove unneeded comparisons for backend_type
> >>>>     - use QLIST array instead of single flat list to store vhost
> >>>>       logger devices
> >>>>     - simplify logger election logic
> >>>> ---
> >>>>    hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
> >>>>    include/hw/virtio/vhost.h |  1 +
> >>>>    2 files changed, 62 insertions(+), 6 deletions(-)
> >>>>
> >>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >>>> index 612f4db..58522f1 100644
> >>>> --- a/hw/virtio/vhost.c
> >>>> +++ b/hw/virtio/vhost.c
> >>>> @@ -45,6 +45,7 @@
> >>>>
> >>>>    static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
> >>>>    static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
> >>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
> >>>>
> >>>>    /* Memslots used by backends that support private memslots (without an fd). */
> >>>>    static unsigned int used_memslots;
> >>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
> >>>>        }
> >>>>    }
> >>>>
> >>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
> >>>> +{
> >>>> +    assert(dev->vhost_ops);
> >>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
> >>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
> >>>> +
> >>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
> >>> A dumb question, why not simple check
> >>>
> >>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
> >> Because we are not sure if the logger comes from vhost_log_shm[] or
> >> vhost_log[]. Don't want to complicate the check here by calling into
> >> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
> > It has very low overhead, isn't it?
> Whether this has low overhead will have to depend on the specific
> backend's implementation for .vhost_requires_shm_log(), which the common
> vhost layer should not assume upon or rely on the current implementation.
>
> >
> > static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
> > {
> >      return dev->vhost_ops->vhost_requires_shm_log &&
> >             dev->vhost_ops->vhost_requires_shm_log(dev);
> > }

For example, if I understand the code correctly, the log type won't be
changed during runtime, so we can endup with a boolean to record that
instead of a query ops?

> >
> > And it helps to simplify the logic.
> Generally yes, but when it comes to hot path operations the performance
> consideration could override this principle. I think there's no harm to
> check against logger device cached in vhost layer itself, and the
> current patch does not create a lot of complexity or performance side
> effect (actually I think the conditional should be very straightforward
> to turn into just a couple of assembly compare and branch instructions
> rather than indirection through another jmp call).

Thanks

>
> -Siwei
>
> >
> > Thanks
> >
> >> -Siwei
> >>> ?
> >>>
> >>> Thanks
> >>>
>

Si-Wei Liu March 20, 2024, 9:02 p.m. UTC | #6

On 3/19/2024 8:27 PM, Jason Wang wrote:
> On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <[e-mail]> wrote:
>>
>>
>> On 3/17/2024 8:22 PM, Jason Wang wrote:
>>> On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
>>>>
>>>> On 3/14/2024 9:03 PM, Jason Wang wrote:
>>>>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>> On setups with one or more virtio-net devices with vhost on,
>>>>>> dirty tracking iteration increases cost the bigger the number
>>>>>> amount of queues are set up e.g. on idle guests migration the
>>>>>> following is observed with virtio-net with vhost=on:
>>>>>>
>>>>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
>>>>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
>>>>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
>>>>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
>>>>>>
>>>>>> With high memory rates the symptom is lack of convergence as soon
>>>>>> as it has a vhost device with a sufficiently high number of queues,
>>>>>> the sufficient number of vhost devices.
>>>>>>
>>>>>> On every migration iteration (every 100msecs) it will redundantly
>>>>>> query the *shared log* the number of queues configured with vhost
>>>>>> that exist in the guest. For the virtqueue data, this is necessary,
>>>>>> but not for the memory sections which are the same. So essentially
>>>>>> we end up scanning the dirty log too often.
>>>>>>
>>>>>> To fix that, select a vhost device responsible for scanning the
>>>>>> log with regards to memory sections dirty tracking. It is selected
>>>>>> when we enable the logger (during migration) and cleared when we
>>>>>> disable the logger. If the vhost logger device goes away for some
>>>>>> reason, the logger will be re-selected from the rest of vhost
>>>>>> devices.
>>>>>>
>>>>>> After making mem-section logger a singleton instance, constant cost
>>>>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
>>>>>> queues or how many vhost devices are configured:
>>>>>>
>>>>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
>>>>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
>>>>>>
>>>>>> Co-developed-by: Joao Martins <[e-mail]>
>>>>>> Signed-off-by: Joao Martins <[e-mail]>
>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>
>>>>>> ---
>>>>>> v3 -> v4:
>>>>>>      - add comment to clarify effect on cache locality and
>>>>>>        performance
>>>>>>
>>>>>> v2 -> v3:
>>>>>>      - add after-fix benchmark to commit log
>>>>>>      - rename vhost_log_dev_enabled to vhost_dev_should_log
>>>>>>      - remove unneeded comparisons for backend_type
>>>>>>      - use QLIST array instead of single flat list to store vhost
>>>>>>        logger devices
>>>>>>      - simplify logger election logic
>>>>>> ---
>>>>>>     hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
>>>>>>     include/hw/virtio/vhost.h |  1 +
>>>>>>     2 files changed, 62 insertions(+), 6 deletions(-)
>>>>>>
>>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>>>>>> index 612f4db..58522f1 100644
>>>>>> --- a/hw/virtio/vhost.c
>>>>>> +++ b/hw/virtio/vhost.c
>>>>>> @@ -45,6 +45,7 @@
>>>>>>
>>>>>>     static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
>>>>>>     static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
>>>>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
>>>>>>
>>>>>>     /* Memslots used by backends that support private memslots (without an fd). */
>>>>>>     static unsigned int used_memslots;
>>>>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
>>>>>>         }
>>>>>>     }
>>>>>>
>>>>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
>>>>>> +{
>>>>>> +    assert(dev->vhost_ops);
>>>>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
>>>>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
>>>>>> +
>>>>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
>>>>> A dumb question, why not simple check
>>>>>
>>>>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
>>>> Because we are not sure if the logger comes from vhost_log_shm[] or
>>>> vhost_log[]. Don't want to complicate the check here by calling into
>>>> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
>>> It has very low overhead, isn't it?
>> Whether this has low overhead will have to depend on the specific
>> backend's implementation for .vhost_requires_shm_log(), which the common
>> vhost layer should not assume upon or rely on the current implementation.
>>
>>> static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
>>> {
>>>       return dev->vhost_ops->vhost_requires_shm_log &&
>>>              dev->vhost_ops->vhost_requires_shm_log(dev);
>>> }
> For example, if I understand the code correctly, the log type won't be
> changed during runtime, so we can endup with a boolean to record that
> instead of a query ops?
Right now the log type won't change during runtime, but I am not sure if 
this may prohibit future revisit to allow change at the runtime, then 
there'll be complex code involvled to maintain the state.

Other than this, I think it's insufficient to just check the shm log 
v.s. normal log. The logger device requires to identify a leading logger 
device that gets elected in vhost_dev_elect_mem_logger(), as all the 
dev->log points to the same logger that is refenerce counted, that we 
have to add extra field and complex logic to maintain the election 
status. I thought that Eugenio's previous suggestion tried to simplify 
the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that 
gets expanded to use the lh_first field for the QLIST would simply 
satisfy the basic need. Why extra logic to make the check ever more 
complex, is there any benefit by adding more fields to the vhost_dev?


Thanks,
-Siwei

>
>>> And it helps to simplify the logic.
>> Generally yes, but when it comes to hot path operations the performance
>> consideration could override this principle. I think there's no harm to
>> check against logger device cached in vhost layer itself, and the
>> current patch does not create a lot of complexity or performance side
>> effect (actually I think the conditional should be very straightforward
>> to turn into just a couple of assembly compare and branch instructions
>> rather than indirection through another jmp call).
> Thanks
>
>> -Siwei
>>
>>> Thanks
>>>
>>>> -Siwei
>>>>> ?
>>>>>
>>>>> Thanks
>>>>>

Jason Wang March 21, 2024, 3:56 a.m. UTC | #7

On Thu, Mar 21, 2024 at 5:03 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 3/19/2024 8:27 PM, Jason Wang wrote:
> > On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <[e-mail]> wrote:
> >>
> >>
> >> On 3/17/2024 8:22 PM, Jason Wang wrote:
> >>> On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>
> >>>> On 3/14/2024 9:03 PM, Jason Wang wrote:
> >>>>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>> On setups with one or more virtio-net devices with vhost on,
> >>>>>> dirty tracking iteration increases cost the bigger the number
> >>>>>> amount of queues are set up e.g. on idle guests migration the
> >>>>>> following is observed with virtio-net with vhost=on:
> >>>>>>
> >>>>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
> >>>>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
> >>>>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
> >>>>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
> >>>>>>
> >>>>>> With high memory rates the symptom is lack of convergence as soon
> >>>>>> as it has a vhost device with a sufficiently high number of queues,
> >>>>>> the sufficient number of vhost devices.
> >>>>>>
> >>>>>> On every migration iteration (every 100msecs) it will redundantly
> >>>>>> query the *shared log* the number of queues configured with vhost
> >>>>>> that exist in the guest. For the virtqueue data, this is necessary,
> >>>>>> but not for the memory sections which are the same. So essentially
> >>>>>> we end up scanning the dirty log too often.
> >>>>>>
> >>>>>> To fix that, select a vhost device responsible for scanning the
> >>>>>> log with regards to memory sections dirty tracking. It is selected
> >>>>>> when we enable the logger (during migration) and cleared when we
> >>>>>> disable the logger. If the vhost logger device goes away for some
> >>>>>> reason, the logger will be re-selected from the rest of vhost
> >>>>>> devices.
> >>>>>>
> >>>>>> After making mem-section logger a singleton instance, constant cost
> >>>>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
> >>>>>> queues or how many vhost devices are configured:
> >>>>>>
> >>>>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
> >>>>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
> >>>>>>
> >>>>>> Co-developed-by: Joao Martins <[e-mail]>
> >>>>>> Signed-off-by: Joao Martins <[e-mail]>
> >>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
> >>>>>>
> >>>>>> ---
> >>>>>> v3 -> v4:
> >>>>>>      - add comment to clarify effect on cache locality and
> >>>>>>        performance
> >>>>>>
> >>>>>> v2 -> v3:
> >>>>>>      - add after-fix benchmark to commit log
> >>>>>>      - rename vhost_log_dev_enabled to vhost_dev_should_log
> >>>>>>      - remove unneeded comparisons for backend_type
> >>>>>>      - use QLIST array instead of single flat list to store vhost
> >>>>>>        logger devices
> >>>>>>      - simplify logger election logic
> >>>>>> ---
> >>>>>>     hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
> >>>>>>     include/hw/virtio/vhost.h |  1 +
> >>>>>>     2 files changed, 62 insertions(+), 6 deletions(-)
> >>>>>>
> >>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >>>>>> index 612f4db..58522f1 100644
> >>>>>> --- a/hw/virtio/vhost.c
> >>>>>> +++ b/hw/virtio/vhost.c
> >>>>>> @@ -45,6 +45,7 @@
> >>>>>>
> >>>>>>     static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
> >>>>>>     static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
> >>>>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
> >>>>>>
> >>>>>>     /* Memslots used by backends that support private memslots (without an fd). */
> >>>>>>     static unsigned int used_memslots;
> >>>>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
> >>>>>>         }
> >>>>>>     }
> >>>>>>
> >>>>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
> >>>>>> +{
> >>>>>> +    assert(dev->vhost_ops);
> >>>>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
> >>>>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
> >>>>>> +
> >>>>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
> >>>>> A dumb question, why not simple check
> >>>>>
> >>>>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
> >>>> Because we are not sure if the logger comes from vhost_log_shm[] or
> >>>> vhost_log[]. Don't want to complicate the check here by calling into
> >>>> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
> >>> It has very low overhead, isn't it?
> >> Whether this has low overhead will have to depend on the specific
> >> backend's implementation for .vhost_requires_shm_log(), which the common
> >> vhost layer should not assume upon or rely on the current implementation.
> >>
> >>> static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
> >>> {
> >>>       return dev->vhost_ops->vhost_requires_shm_log &&
> >>>              dev->vhost_ops->vhost_requires_shm_log(dev);
> >>> }
> > For example, if I understand the code correctly, the log type won't be
> > changed during runtime, so we can endup with a boolean to record that
> > instead of a query ops?
> Right now the log type won't change during runtime, but I am not sure if
> this may prohibit future revisit to allow change at the runtime,

We can be bothered when we have such a request then.

> then
> there'll be complex code involvled to maintain the state.
>
> Other than this, I think it's insufficient to just check the shm log
> v.s. normal log. The logger device requires to identify a leading logger
> device that gets elected in vhost_dev_elect_mem_logger(), as all the
> dev->log points to the same logger that is refenerce counted, that we
> have to add extra field and complex logic to maintain the election
> status.

One thing I don't understand here (and in the changelog) is why do we
need an election here?

> I thought that Eugenio's previous suggestion tried to simplify
> the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that
> gets expanded to use the lh_first field for the QLIST would simply
> satisfy the basic need. Why extra logic to make the check ever more
> complex, is there any benefit by adding more fields to the vhost_dev?

I don't get here, the idea is to just pick one shared log which should
be much more simpler than what is proposed here.

Thanks

>
>
> Thanks,
> -Siwei
>
> >
> >>> And it helps to simplify the logic.
> >> Generally yes, but when it comes to hot path operations the performance
> >> consideration could override this principle. I think there's no harm to
> >> check against logger device cached in vhost layer itself, and the
> >> current patch does not create a lot of complexity or performance side
> >> effect (actually I think the conditional should be very straightforward
> >> to turn into just a couple of assembly compare and branch instructions
> >> rather than indirection through another jmp call).
> > Thanks
> >
> >> -Siwei
> >>
> >>> Thanks
> >>>
> >>>> -Siwei
> >>>>> ?
> >>>>>
> >>>>> Thanks
> >>>>>
>

Si-Wei Liu March 21, 2024, 9:42 p.m. UTC | #8

On 3/20/2024 8:56 PM, Jason Wang wrote:
> On Thu, Mar 21, 2024 at 5:03 AM Si-Wei Liu <[e-mail]> wrote:
>>
>>
>> On 3/19/2024 8:27 PM, Jason Wang wrote:
>>> On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <[e-mail]> wrote:
>>>>
>>>> On 3/17/2024 8:22 PM, Jason Wang wrote:
>>>>> On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>> On 3/14/2024 9:03 PM, Jason Wang wrote:
>>>>>>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>>>> On setups with one or more virtio-net devices with vhost on,
>>>>>>>> dirty tracking iteration increases cost the bigger the number
>>>>>>>> amount of queues are set up e.g. on idle guests migration the
>>>>>>>> following is observed with virtio-net with vhost=on:
>>>>>>>>
>>>>>>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
>>>>>>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
>>>>>>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
>>>>>>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
>>>>>>>>
>>>>>>>> With high memory rates the symptom is lack of convergence as soon
>>>>>>>> as it has a vhost device with a sufficiently high number of queues,
>>>>>>>> the sufficient number of vhost devices.
>>>>>>>>
>>>>>>>> On every migration iteration (every 100msecs) it will redundantly
>>>>>>>> query the *shared log* the number of queues configured with vhost
>>>>>>>> that exist in the guest. For the virtqueue data, this is necessary,
>>>>>>>> but not for the memory sections which are the same. So essentially
>>>>>>>> we end up scanning the dirty log too often.
>>>>>>>>
>>>>>>>> To fix that, select a vhost device responsible for scanning the
>>>>>>>> log with regards to memory sections dirty tracking. It is selected
>>>>>>>> when we enable the logger (during migration) and cleared when we
>>>>>>>> disable the logger. If the vhost logger device goes away for some
>>>>>>>> reason, the logger will be re-selected from the rest of vhost
>>>>>>>> devices.
>>>>>>>>
>>>>>>>> After making mem-section logger a singleton instance, constant cost
>>>>>>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
>>>>>>>> queues or how many vhost devices are configured:
>>>>>>>>
>>>>>>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
>>>>>>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
>>>>>>>>
>>>>>>>> Co-developed-by: Joao Martins <[e-mail]>
>>>>>>>> Signed-off-by: Joao Martins <[e-mail]>
>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>>>
>>>>>>>> ---
>>>>>>>> v3 -> v4:
>>>>>>>>       - add comment to clarify effect on cache locality and
>>>>>>>>         performance
>>>>>>>>
>>>>>>>> v2 -> v3:
>>>>>>>>       - add after-fix benchmark to commit log
>>>>>>>>       - rename vhost_log_dev_enabled to vhost_dev_should_log
>>>>>>>>       - remove unneeded comparisons for backend_type
>>>>>>>>       - use QLIST array instead of single flat list to store vhost
>>>>>>>>         logger devices
>>>>>>>>       - simplify logger election logic
>>>>>>>> ---
>>>>>>>>      hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
>>>>>>>>      include/hw/virtio/vhost.h |  1 +
>>>>>>>>      2 files changed, 62 insertions(+), 6 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>>>>>>>> index 612f4db..58522f1 100644
>>>>>>>> --- a/hw/virtio/vhost.c
>>>>>>>> +++ b/hw/virtio/vhost.c
>>>>>>>> @@ -45,6 +45,7 @@
>>>>>>>>
>>>>>>>>      static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
>>>>>>>>      static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
>>>>>>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
>>>>>>>>
>>>>>>>>      /* Memslots used by backends that support private memslots (without an fd). */
>>>>>>>>      static unsigned int used_memslots;
>>>>>>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
>>>>>>>>          }
>>>>>>>>      }
>>>>>>>>
>>>>>>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
>>>>>>>> +{
>>>>>>>> +    assert(dev->vhost_ops);
>>>>>>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
>>>>>>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
>>>>>>>> +
>>>>>>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
>>>>>>> A dumb question, why not simple check
>>>>>>>
>>>>>>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
>>>>>> Because we are not sure if the logger comes from vhost_log_shm[] or
>>>>>> vhost_log[]. Don't want to complicate the check here by calling into
>>>>>> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
>>>>> It has very low overhead, isn't it?
>>>> Whether this has low overhead will have to depend on the specific
>>>> backend's implementation for .vhost_requires_shm_log(), which the common
>>>> vhost layer should not assume upon or rely on the current implementation.
>>>>
>>>>> static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
>>>>> {
>>>>>        return dev->vhost_ops->vhost_requires_shm_log &&
>>>>>               dev->vhost_ops->vhost_requires_shm_log(dev);
>>>>> }
>>> For example, if I understand the code correctly, the log type won't be
>>> changed during runtime, so we can endup with a boolean to record that
>>> instead of a query ops?
>> Right now the log type won't change during runtime, but I am not sure if
>> this may prohibit future revisit to allow change at the runtime,
> We can be bothered when we have such a request then.
>
>> then
>> there'll be complex code involvled to maintain the state.
>>
>> Other than this, I think it's insufficient to just check the shm log
>> v.s. normal log. The logger device requires to identify a leading logger
>> device that gets elected in vhost_dev_elect_mem_logger(), as all the
>> dev->log points to the same logger that is refenerce counted, that we
>> have to add extra field and complex logic to maintain the election
>> status.
> One thing I don't understand here (and in the changelog) is why do we
> need an election here?

vhost_sync_dirty_bitmap() not just scans the guest memory sections but 
the specific one for virtqueues (used rings) also. To save more CPU 
cycles to the best extend, the guest memory must be scanned only once in 
each log iteration, though the logging for used rings would still have 
to use the specific vhost instance, so all vhost_device instance still 
keeps the dev->log pointer to the shared log as-is. Generally the shared 
memory logger can be picked from an arbitrary vhost_device instance, but 
to keep the code simple, performant and predictable, logger selection is 
made on the control path at the vhost add/remove time rather than be 
determined at the dirty log collection runtime, the latter of which is 
in the hotpath.

>
>> I thought that Eugenio's previous suggestion tried to simplify
>> the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that
>> gets expanded to use the lh_first field for the QLIST would simply
>> satisfy the basic need. Why extra logic to make the check ever more
>> complex, is there any benefit by adding more fields to the vhost_dev?
> I don't get here, the idea is to just pick one shared log which should
> be much more simpler than what is proposed here.
The code you showed earlier won't work as all vhost_device instance 
points to the same dev->log device...

Regards,
-Siwei
>
> Thanks
>
>>
>> Thanks,
>> -Siwei
>>
>>>>> And it helps to simplify the logic.
>>>> Generally yes, but when it comes to hot path operations the performance
>>>> consideration could override this principle. I think there's no harm to
>>>> check against logger device cached in vhost layer itself, and the
>>>> current patch does not create a lot of complexity or performance side
>>>> effect (actually I think the conditional should be very straightforward
>>>> to turn into just a couple of assembly compare and branch instructions
>>>> rather than indirection through another jmp call).
>>> Thanks
>>>
>>>> -Siwei
>>>>
>>>>> Thanks
>>>>>
>>>>>> -Siwei
>>>>>>> ?
>>>>>>>
>>>>>>> Thanks
>>>>>>>

Jason Wang March 22, 2024, 5:08 a.m. UTC | #9

On Fri, Mar 22, 2024 at 5:43 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 3/20/2024 8:56 PM, Jason Wang wrote:
> > On Thu, Mar 21, 2024 at 5:03 AM Si-Wei Liu <[e-mail]> wrote:
> >>
> >>
> >> On 3/19/2024 8:27 PM, Jason Wang wrote:
> >>> On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>
> >>>> On 3/17/2024 8:22 PM, Jason Wang wrote:
> >>>>> On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>> On 3/14/2024 9:03 PM, Jason Wang wrote:
> >>>>>>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>>>> On setups with one or more virtio-net devices with vhost on,
> >>>>>>>> dirty tracking iteration increases cost the bigger the number
> >>>>>>>> amount of queues are set up e.g. on idle guests migration the
> >>>>>>>> following is observed with virtio-net with vhost=on:
> >>>>>>>>
> >>>>>>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
> >>>>>>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
> >>>>>>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
> >>>>>>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
> >>>>>>>>
> >>>>>>>> With high memory rates the symptom is lack of convergence as soon
> >>>>>>>> as it has a vhost device with a sufficiently high number of queues,
> >>>>>>>> the sufficient number of vhost devices.
> >>>>>>>>
> >>>>>>>> On every migration iteration (every 100msecs) it will redundantly
> >>>>>>>> query the *shared log* the number of queues configured with vhost
> >>>>>>>> that exist in the guest. For the virtqueue data, this is necessary,
> >>>>>>>> but not for the memory sections which are the same. So essentially
> >>>>>>>> we end up scanning the dirty log too often.
> >>>>>>>>
> >>>>>>>> To fix that, select a vhost device responsible for scanning the
> >>>>>>>> log with regards to memory sections dirty tracking. It is selected
> >>>>>>>> when we enable the logger (during migration) and cleared when we
> >>>>>>>> disable the logger. If the vhost logger device goes away for some
> >>>>>>>> reason, the logger will be re-selected from the rest of vhost
> >>>>>>>> devices.
> >>>>>>>>
> >>>>>>>> After making mem-section logger a singleton instance, constant cost
> >>>>>>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
> >>>>>>>> queues or how many vhost devices are configured:
> >>>>>>>>
> >>>>>>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
> >>>>>>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
> >>>>>>>>
> >>>>>>>> Co-developed-by: Joao Martins <[e-mail]>
> >>>>>>>> Signed-off-by: Joao Martins <[e-mail]>
> >>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
> >>>>>>>>
> >>>>>>>> ---
> >>>>>>>> v3 -> v4:
> >>>>>>>>       - add comment to clarify effect on cache locality and
> >>>>>>>>         performance
> >>>>>>>>
> >>>>>>>> v2 -> v3:
> >>>>>>>>       - add after-fix benchmark to commit log
> >>>>>>>>       - rename vhost_log_dev_enabled to vhost_dev_should_log
> >>>>>>>>       - remove unneeded comparisons for backend_type
> >>>>>>>>       - use QLIST array instead of single flat list to store vhost
> >>>>>>>>         logger devices
> >>>>>>>>       - simplify logger election logic
> >>>>>>>> ---
> >>>>>>>>      hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
> >>>>>>>>      include/hw/virtio/vhost.h |  1 +
> >>>>>>>>      2 files changed, 62 insertions(+), 6 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >>>>>>>> index 612f4db..58522f1 100644
> >>>>>>>> --- a/hw/virtio/vhost.c
> >>>>>>>> +++ b/hw/virtio/vhost.c
> >>>>>>>> @@ -45,6 +45,7 @@
> >>>>>>>>
> >>>>>>>>      static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
> >>>>>>>>      static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
> >>>>>>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
> >>>>>>>>
> >>>>>>>>      /* Memslots used by backends that support private memslots (without an fd). */
> >>>>>>>>      static unsigned int used_memslots;
> >>>>>>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
> >>>>>>>>          }
> >>>>>>>>      }
> >>>>>>>>
> >>>>>>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
> >>>>>>>> +{
> >>>>>>>> +    assert(dev->vhost_ops);
> >>>>>>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
> >>>>>>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
> >>>>>>>> +
> >>>>>>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
> >>>>>>> A dumb question, why not simple check
> >>>>>>>
> >>>>>>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
> >>>>>> Because we are not sure if the logger comes from vhost_log_shm[] or
> >>>>>> vhost_log[]. Don't want to complicate the check here by calling into
> >>>>>> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
> >>>>> It has very low overhead, isn't it?
> >>>> Whether this has low overhead will have to depend on the specific
> >>>> backend's implementation for .vhost_requires_shm_log(), which the common
> >>>> vhost layer should not assume upon or rely on the current implementation.
> >>>>
> >>>>> static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
> >>>>> {
> >>>>>        return dev->vhost_ops->vhost_requires_shm_log &&
> >>>>>               dev->vhost_ops->vhost_requires_shm_log(dev);
> >>>>> }
> >>> For example, if I understand the code correctly, the log type won't be
> >>> changed during runtime, so we can endup with a boolean to record that
> >>> instead of a query ops?
> >> Right now the log type won't change during runtime, but I am not sure if
> >> this may prohibit future revisit to allow change at the runtime,
> > We can be bothered when we have such a request then.
> >
> >> then
> >> there'll be complex code involvled to maintain the state.
> >>
> >> Other than this, I think it's insufficient to just check the shm log
> >> v.s. normal log. The logger device requires to identify a leading logger
> >> device that gets elected in vhost_dev_elect_mem_logger(), as all the
> >> dev->log points to the same logger that is refenerce counted, that we
> >> have to add extra field and complex logic to maintain the election
> >> status.
> > One thing I don't understand here (and in the changelog) is why do we
> > need an election here?
>
> vhost_sync_dirty_bitmap() not just scans the guest memory sections but
> the specific one for virtqueues (used rings) also. To save more CPU
> cycles to the best extend, the guest memory must be scanned only once in
> each log iteration, though the logging for used rings would still have
> to use the specific vhost instance, so all vhost_device instance still
> keeps the dev->log pointer to the shared log as-is. Generally the shared
> memory logger can be picked from an arbitrary vhost_device instance, but
> to keep the code simple, performant and predictable

This is the point, I don't see why election is simpler than picking an
arbitrary shared log in this case.

> , logger selection is
> made on the control path at the vhost add/remove time rather than be
> determined at the dirty log collection runtime, the latter of which is
> in the hotpath.
>
> >
> >> I thought that Eugenio's previous suggestion tried to simplify
> >> the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that
> >> gets expanded to use the lh_first field for the QLIST would simply
> >> satisfy the basic need. Why extra logic to make the check ever more
> >> complex, is there any benefit by adding more fields to the vhost_dev?
> > I don't get here, the idea is to just pick one shared log which should
> > be much more simpler than what is proposed here.
> The code you showed earlier won't work as all vhost_device instance
> points to the same dev->log device...

This part I don't understand.

Thanks

>
> Regards,
> -Siwei
> >
> > Thanks
> >
> >>
> >> Thanks,
> >> -Siwei
> >>
> >>>>> And it helps to simplify the logic.
> >>>> Generally yes, but when it comes to hot path operations the performance
> >>>> consideration could override this principle. I think there's no harm to
> >>>> check against logger device cached in vhost layer itself, and the
> >>>> current patch does not create a lot of complexity or performance side
> >>>> effect (actually I think the conditional should be very straightforward
> >>>> to turn into just a couple of assembly compare and branch instructions
> >>>> rather than indirection through another jmp call).
> >>> Thanks
> >>>
> >>>> -Siwei
> >>>>
> >>>>> Thanks
> >>>>>
> >>>>>> -Siwei
> >>>>>>> ?
> >>>>>>>
> >>>>>>> Thanks
> >>>>>>>
>

Si-Wei Liu March 22, 2024, 9:13 p.m. UTC | #10

On 3/21/2024 10:08 PM, Jason Wang wrote:
> On Fri, Mar 22, 2024 at 5:43 AM Si-Wei Liu <[e-mail]> wrote:
>>
>>
>> On 3/20/2024 8:56 PM, Jason Wang wrote:
>>> On Thu, Mar 21, 2024 at 5:03 AM Si-Wei Liu <[e-mail]> wrote:
>>>>
>>>> On 3/19/2024 8:27 PM, Jason Wang wrote:
>>>>> On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>> On 3/17/2024 8:22 PM, Jason Wang wrote:
>>>>>>> On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>>>> On 3/14/2024 9:03 PM, Jason Wang wrote:
>>>>>>>>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>>>>>> On setups with one or more virtio-net devices with vhost on,
>>>>>>>>>> dirty tracking iteration increases cost the bigger the number
>>>>>>>>>> amount of queues are set up e.g. on idle guests migration the
>>>>>>>>>> following is observed with virtio-net with vhost=on:
>>>>>>>>>>
>>>>>>>>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
>>>>>>>>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
>>>>>>>>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
>>>>>>>>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
>>>>>>>>>>
>>>>>>>>>> With high memory rates the symptom is lack of convergence as soon
>>>>>>>>>> as it has a vhost device with a sufficiently high number of queues,
>>>>>>>>>> the sufficient number of vhost devices.
>>>>>>>>>>
>>>>>>>>>> On every migration iteration (every 100msecs) it will redundantly
>>>>>>>>>> query the *shared log* the number of queues configured with vhost
>>>>>>>>>> that exist in the guest. For the virtqueue data, this is necessary,
>>>>>>>>>> but not for the memory sections which are the same. So essentially
>>>>>>>>>> we end up scanning the dirty log too often.
>>>>>>>>>>
>>>>>>>>>> To fix that, select a vhost device responsible for scanning the
>>>>>>>>>> log with regards to memory sections dirty tracking. It is selected
>>>>>>>>>> when we enable the logger (during migration) and cleared when we
>>>>>>>>>> disable the logger. If the vhost logger device goes away for some
>>>>>>>>>> reason, the logger will be re-selected from the rest of vhost
>>>>>>>>>> devices.
>>>>>>>>>>
>>>>>>>>>> After making mem-section logger a singleton instance, constant cost
>>>>>>>>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
>>>>>>>>>> queues or how many vhost devices are configured:
>>>>>>>>>>
>>>>>>>>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
>>>>>>>>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
>>>>>>>>>>
>>>>>>>>>> Co-developed-by: Joao Martins <[e-mail]>
>>>>>>>>>> Signed-off-by: Joao Martins <[e-mail]>
>>>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>>>>>
>>>>>>>>>> ---
>>>>>>>>>> v3 -> v4:
>>>>>>>>>>        - add comment to clarify effect on cache locality and
>>>>>>>>>>          performance
>>>>>>>>>>
>>>>>>>>>> v2 -> v3:
>>>>>>>>>>        - add after-fix benchmark to commit log
>>>>>>>>>>        - rename vhost_log_dev_enabled to vhost_dev_should_log
>>>>>>>>>>        - remove unneeded comparisons for backend_type
>>>>>>>>>>        - use QLIST array instead of single flat list to store vhost
>>>>>>>>>>          logger devices
>>>>>>>>>>        - simplify logger election logic
>>>>>>>>>> ---
>>>>>>>>>>       hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
>>>>>>>>>>       include/hw/virtio/vhost.h |  1 +
>>>>>>>>>>       2 files changed, 62 insertions(+), 6 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>>>>>>>>>> index 612f4db..58522f1 100644
>>>>>>>>>> --- a/hw/virtio/vhost.c
>>>>>>>>>> +++ b/hw/virtio/vhost.c
>>>>>>>>>> @@ -45,6 +45,7 @@
>>>>>>>>>>
>>>>>>>>>>       static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
>>>>>>>>>>       static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
>>>>>>>>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
>>>>>>>>>>
>>>>>>>>>>       /* Memslots used by backends that support private memslots (without an fd). */
>>>>>>>>>>       static unsigned int used_memslots;
>>>>>>>>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
>>>>>>>>>>           }
>>>>>>>>>>       }
>>>>>>>>>>
>>>>>>>>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
>>>>>>>>>> +{
>>>>>>>>>> +    assert(dev->vhost_ops);
>>>>>>>>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
>>>>>>>>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
>>>>>>>>>> +
>>>>>>>>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
>>>>>>>>> A dumb question, why not simple check
>>>>>>>>>
>>>>>>>>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
>>>>>>>> Because we are not sure if the logger comes from vhost_log_shm[] or
>>>>>>>> vhost_log[]. Don't want to complicate the check here by calling into
>>>>>>>> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
>>>>>>> It has very low overhead, isn't it?
>>>>>> Whether this has low overhead will have to depend on the specific
>>>>>> backend's implementation for .vhost_requires_shm_log(), which the common
>>>>>> vhost layer should not assume upon or rely on the current implementation.
>>>>>>
>>>>>>> static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
>>>>>>> {
>>>>>>>         return dev->vhost_ops->vhost_requires_shm_log &&
>>>>>>>                dev->vhost_ops->vhost_requires_shm_log(dev);
>>>>>>> }
>>>>> For example, if I understand the code correctly, the log type won't be
>>>>> changed during runtime, so we can endup with a boolean to record that
>>>>> instead of a query ops?
>>>> Right now the log type won't change during runtime, but I am not sure if
>>>> this may prohibit future revisit to allow change at the runtime,
>>> We can be bothered when we have such a request then.
>>>
>>>> then
>>>> there'll be complex code involvled to maintain the state.
>>>>
>>>> Other than this, I think it's insufficient to just check the shm log
>>>> v.s. normal log. The logger device requires to identify a leading logger
>>>> device that gets elected in vhost_dev_elect_mem_logger(), as all the
>>>> dev->log points to the same logger that is refenerce counted, that we
>>>> have to add extra field and complex logic to maintain the election
>>>> status.
>>> One thing I don't understand here (and in the changelog) is why do we
>>> need an election here?
>> vhost_sync_dirty_bitmap() not just scans the guest memory sections but
>> the specific one for virtqueues (used rings) also. To save more CPU
>> cycles to the best extend, the guest memory must be scanned only once in
>> each log iteration, though the logging for used rings would still have
>> to use the specific vhost instance, so all vhost_device instance still
>> keeps the dev->log pointer to the shared log as-is. Generally the shared
>> memory logger can be picked from an arbitrary vhost_device instance, but
>> to keep the code simple, performant and predictable
> This is the point, I don't see why election is simpler than picking an
> arbitrary shared log in this case.
Maybe I missed your point, but I am confused and fail to understand why 
electing a fixed vhost_dev is not as simple? Regardless of the 
specifics, I think the point is one _fixed_ vhost_dev has to be picked 
amongst all the instances per backend type in charge of logging guest 
memory, no matter if it's at the start on the list or not.

>
>> , logger selection is
>> made on the control path at the vhost add/remove time rather than be
>> determined at the dirty log collection runtime, the latter of which is
>> in the hotpath.
>>
>>>> I thought that Eugenio's previous suggestion tried to simplify
>>>> the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that
>>>> gets expanded to use the lh_first field for the QLIST would simply
>>>> satisfy the basic need. Why extra logic to make the check ever more
>>>> complex, is there any benefit by adding more fields to the vhost_dev?
>>> I don't get here, the idea is to just pick one shared log which should
>>> be much more simpler than what is proposed here.
>> The code you showed earlier won't work as all vhost_device instance
>> points to the same dev->log device...
> This part I don't understand.

vhost_log_get() has the following:

     log = share ? vhost_log_shm[backend_type] : vhost_log[backend_type];

     if (!log || log->size != size) {
         log = vhost_log_alloc(size, share);
         if (share) {
             vhost_log_shm[backend_type] = log;
         } else {
             vhost_log[backend_type] = log;
         }
     } else {
         ++log->refcnt;
     }

So for a specific backend type, vhost_log_get() would return the same 
logger device (stored at either vhost_log_shm[] or vhost_log[]) to all 
vhost_dev instances, and the check you suggested earlier:

dev->log == vhost_log_shm[dev->vhost_ops->backend_type]

will always hold true if the vhost_dev instance (representing the 
specific virtqueue) is active.

Regards,
-Siwei
>
> Thanks
>
>> Regards,
>> -Siwei
>>> Thanks
>>>
>>>> Thanks,
>>>> -Siwei
>>>>
>>>>>>> And it helps to simplify the logic.
>>>>>> Generally yes, but when it comes to hot path operations the performance
>>>>>> consideration could override this principle. I think there's no harm to
>>>>>> check against logger device cached in vhost layer itself, and the
>>>>>> current patch does not create a lot of complexity or performance side
>>>>>> effect (actually I think the conditional should be very straightforward
>>>>>> to turn into just a couple of assembly compare and branch instructions
>>>>>> rather than indirection through another jmp call).
>>>>> Thanks
>>>>>
>>>>>> -Siwei
>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>>>> -Siwei
>>>>>>>>> ?
>>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>>>

Jason Wang March 25, 2024, 6:13 a.m. UTC | #11

On Sat, Mar 23, 2024 at 5:14 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 3/21/2024 10:08 PM, Jason Wang wrote:
> > On Fri, Mar 22, 2024 at 5:43 AM Si-Wei Liu <[e-mail]> wrote:
> >>
> >>
> >> On 3/20/2024 8:56 PM, Jason Wang wrote:
> >>> On Thu, Mar 21, 2024 at 5:03 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>
> >>>> On 3/19/2024 8:27 PM, Jason Wang wrote:
> >>>>> On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>> On 3/17/2024 8:22 PM, Jason Wang wrote:
> >>>>>>> On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>>>> On 3/14/2024 9:03 PM, Jason Wang wrote:
> >>>>>>>>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>>>>>> On setups with one or more virtio-net devices with vhost on,
> >>>>>>>>>> dirty tracking iteration increases cost the bigger the number
> >>>>>>>>>> amount of queues are set up e.g. on idle guests migration the
> >>>>>>>>>> following is observed with virtio-net with vhost=on:
> >>>>>>>>>>
> >>>>>>>>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
> >>>>>>>>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
> >>>>>>>>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
> >>>>>>>>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
> >>>>>>>>>>
> >>>>>>>>>> With high memory rates the symptom is lack of convergence as soon
> >>>>>>>>>> as it has a vhost device with a sufficiently high number of queues,
> >>>>>>>>>> the sufficient number of vhost devices.
> >>>>>>>>>>
> >>>>>>>>>> On every migration iteration (every 100msecs) it will redundantly
> >>>>>>>>>> query the *shared log* the number of queues configured with vhost
> >>>>>>>>>> that exist in the guest. For the virtqueue data, this is necessary,
> >>>>>>>>>> but not for the memory sections which are the same. So essentially
> >>>>>>>>>> we end up scanning the dirty log too often.
> >>>>>>>>>>
> >>>>>>>>>> To fix that, select a vhost device responsible for scanning the
> >>>>>>>>>> log with regards to memory sections dirty tracking. It is selected
> >>>>>>>>>> when we enable the logger (during migration) and cleared when we
> >>>>>>>>>> disable the logger. If the vhost logger device goes away for some
> >>>>>>>>>> reason, the logger will be re-selected from the rest of vhost
> >>>>>>>>>> devices.
> >>>>>>>>>>
> >>>>>>>>>> After making mem-section logger a singleton instance, constant cost
> >>>>>>>>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
> >>>>>>>>>> queues or how many vhost devices are configured:
> >>>>>>>>>>
> >>>>>>>>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
> >>>>>>>>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
> >>>>>>>>>>
> >>>>>>>>>> Co-developed-by: Joao Martins <[e-mail]>
> >>>>>>>>>> Signed-off-by: Joao Martins <[e-mail]>
> >>>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
> >>>>>>>>>>
> >>>>>>>>>> ---
> >>>>>>>>>> v3 -> v4:
> >>>>>>>>>>        - add comment to clarify effect on cache locality and
> >>>>>>>>>>          performance
> >>>>>>>>>>
> >>>>>>>>>> v2 -> v3:
> >>>>>>>>>>        - add after-fix benchmark to commit log
> >>>>>>>>>>        - rename vhost_log_dev_enabled to vhost_dev_should_log
> >>>>>>>>>>        - remove unneeded comparisons for backend_type
> >>>>>>>>>>        - use QLIST array instead of single flat list to store vhost
> >>>>>>>>>>          logger devices
> >>>>>>>>>>        - simplify logger election logic
> >>>>>>>>>> ---
> >>>>>>>>>>       hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
> >>>>>>>>>>       include/hw/virtio/vhost.h |  1 +
> >>>>>>>>>>       2 files changed, 62 insertions(+), 6 deletions(-)
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >>>>>>>>>> index 612f4db..58522f1 100644
> >>>>>>>>>> --- a/hw/virtio/vhost.c
> >>>>>>>>>> +++ b/hw/virtio/vhost.c
> >>>>>>>>>> @@ -45,6 +45,7 @@
> >>>>>>>>>>
> >>>>>>>>>>       static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
> >>>>>>>>>>       static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
> >>>>>>>>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
> >>>>>>>>>>
> >>>>>>>>>>       /* Memslots used by backends that support private memslots (without an fd). */
> >>>>>>>>>>       static unsigned int used_memslots;
> >>>>>>>>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
> >>>>>>>>>>           }
> >>>>>>>>>>       }
> >>>>>>>>>>
> >>>>>>>>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
> >>>>>>>>>> +{
> >>>>>>>>>> +    assert(dev->vhost_ops);
> >>>>>>>>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
> >>>>>>>>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
> >>>>>>>>>> +
> >>>>>>>>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
> >>>>>>>>> A dumb question, why not simple check
> >>>>>>>>>
> >>>>>>>>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
> >>>>>>>> Because we are not sure if the logger comes from vhost_log_shm[] or
> >>>>>>>> vhost_log[]. Don't want to complicate the check here by calling into
> >>>>>>>> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
> >>>>>>> It has very low overhead, isn't it?
> >>>>>> Whether this has low overhead will have to depend on the specific
> >>>>>> backend's implementation for .vhost_requires_shm_log(), which the common
> >>>>>> vhost layer should not assume upon or rely on the current implementation.
> >>>>>>
> >>>>>>> static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
> >>>>>>> {
> >>>>>>>         return dev->vhost_ops->vhost_requires_shm_log &&
> >>>>>>>                dev->vhost_ops->vhost_requires_shm_log(dev);
> >>>>>>> }
> >>>>> For example, if I understand the code correctly, the log type won't be
> >>>>> changed during runtime, so we can endup with a boolean to record that
> >>>>> instead of a query ops?
> >>>> Right now the log type won't change during runtime, but I am not sure if
> >>>> this may prohibit future revisit to allow change at the runtime,
> >>> We can be bothered when we have such a request then.
> >>>
> >>>> then
> >>>> there'll be complex code involvled to maintain the state.
> >>>>
> >>>> Other than this, I think it's insufficient to just check the shm log
> >>>> v.s. normal log. The logger device requires to identify a leading logger
> >>>> device that gets elected in vhost_dev_elect_mem_logger(), as all the
> >>>> dev->log points to the same logger that is refenerce counted, that we
> >>>> have to add extra field and complex logic to maintain the election
> >>>> status.
> >>> One thing I don't understand here (and in the changelog) is why do we
> >>> need an election here?
> >> vhost_sync_dirty_bitmap() not just scans the guest memory sections but
> >> the specific one for virtqueues (used rings) also. To save more CPU
> >> cycles to the best extend, the guest memory must be scanned only once in
> >> each log iteration, though the logging for used rings would still have
> >> to use the specific vhost instance, so all vhost_device instance still
> >> keeps the dev->log pointer to the shared log as-is. Generally the shared
> >> memory logger can be picked from an arbitrary vhost_device instance, but
> >> to keep the code simple, performant and predictable
> > This is the point, I don't see why election is simpler than picking an
> > arbitrary shared log in this case.
> Maybe I missed your point, but I am confused and fail to understand why
> electing a fixed vhost_dev is not as simple? Regardless of the
> specifics, I think the point is one _fixed_ vhost_dev has to be picked
> amongst all the instances per backend type in charge of logging guest
> memory, no matter if it's at the start on the list or not.

See below.

>
> >
> >> , logger selection is
> >> made on the control path at the vhost add/remove time rather than be
> >> determined at the dirty log collection runtime, the latter of which is
> >> in the hotpath.
> >>
> >>>> I thought that Eugenio's previous suggestion tried to simplify
> >>>> the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that
> >>>> gets expanded to use the lh_first field for the QLIST would simply
> >>>> satisfy the basic need. Why extra logic to make the check ever more
> >>>> complex, is there any benefit by adding more fields to the vhost_dev?
> >>> I don't get here, the idea is to just pick one shared log which should
> >>> be much more simpler than what is proposed here.
> >> The code you showed earlier won't work as all vhost_device instance
> >> points to the same dev->log device...
> > This part I don't understand.
>
> vhost_log_get() has the following:
>
>      log = share ? vhost_log_shm[backend_type] : vhost_log[backend_type];
>
>      if (!log || log->size != size) {
>          log = vhost_log_alloc(size, share);
>          if (share) {
>              vhost_log_shm[backend_type] = log;
>          } else {
>              vhost_log[backend_type] = log;
>          }
>      } else {
>          ++log->refcnt;
>      }
>
> So for a specific backend type, vhost_log_get() would return the same
> logger device (stored at either vhost_log_shm[] or vhost_log[]) to all
> vhost_dev instances, and the check you suggested earlier:
>
> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
>
> will always hold true if the vhost_dev instance (representing the
> specific virtqueue) is active.

Right, so the point is see if we can find something simpler to avoid
the QLIST as it involves vhost_dev which seems unnecessary.

Does something like a counter work?

vhost_sync_dirty_bitmap() {
      rounds ++;
      vhost_dev_sync_region(rounds);
}

vhost_dev_sync_region(rounds) {
      if (dev->log->rounds == rounds)
           return;
      else
           dev->log->rounds;
}

(pseudo codes, just used to demonstrate the idea).

Thanks

>
> Regards,
> -Siwei
> >
> > Thanks
> >
> >> Regards,
> >> -Siwei
> >>> Thanks
> >>>
> >>>> Thanks,
> >>>> -Siwei
> >>>>
> >>>>>>> And it helps to simplify the logic.
> >>>>>> Generally yes, but when it comes to hot path operations the performance
> >>>>>> consideration could override this principle. I think there's no harm to
> >>>>>> check against logger device cached in vhost layer itself, and the
> >>>>>> current patch does not create a lot of complexity or performance side
> >>>>>> effect (actually I think the conditional should be very straightforward
> >>>>>> to turn into just a couple of assembly compare and branch instructions
> >>>>>> rather than indirection through another jmp call).
> >>>>> Thanks
> >>>>>
> >>>>>> -Siwei
> >>>>>>
> >>>>>>> Thanks
> >>>>>>>
> >>>>>>>> -Siwei
> >>>>>>>>> ?
> >>>>>>>>>
> >>>>>>>>> Thanks
> >>>>>>>>>
>

Si-Wei Liu March 25, 2024, 11:20 p.m. UTC | #12

On 3/24/2024 11:13 PM, Jason Wang wrote:
> On Sat, Mar 23, 2024 at 5:14 AM Si-Wei Liu <[e-mail]> wrote:
>>
>>
>> On 3/21/2024 10:08 PM, Jason Wang wrote:
>>> On Fri, Mar 22, 2024 at 5:43 AM Si-Wei Liu <[e-mail]> wrote:
>>>>
>>>> On 3/20/2024 8:56 PM, Jason Wang wrote:
>>>>> On Thu, Mar 21, 2024 at 5:03 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>> On 3/19/2024 8:27 PM, Jason Wang wrote:
>>>>>>> On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>>>> On 3/17/2024 8:22 PM, Jason Wang wrote:
>>>>>>>>> On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>>>>>> On 3/14/2024 9:03 PM, Jason Wang wrote:
>>>>>>>>>>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>>>>>>>> On setups with one or more virtio-net devices with vhost on,
>>>>>>>>>>>> dirty tracking iteration increases cost the bigger the number
>>>>>>>>>>>> amount of queues are set up e.g. on idle guests migration the
>>>>>>>>>>>> following is observed with virtio-net with vhost=on:
>>>>>>>>>>>>
>>>>>>>>>>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
>>>>>>>>>>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
>>>>>>>>>>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
>>>>>>>>>>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
>>>>>>>>>>>>
>>>>>>>>>>>> With high memory rates the symptom is lack of convergence as soon
>>>>>>>>>>>> as it has a vhost device with a sufficiently high number of queues,
>>>>>>>>>>>> the sufficient number of vhost devices.
>>>>>>>>>>>>
>>>>>>>>>>>> On every migration iteration (every 100msecs) it will redundantly
>>>>>>>>>>>> query the *shared log* the number of queues configured with vhost
>>>>>>>>>>>> that exist in the guest. For the virtqueue data, this is necessary,
>>>>>>>>>>>> but not for the memory sections which are the same. So essentially
>>>>>>>>>>>> we end up scanning the dirty log too often.
>>>>>>>>>>>>
>>>>>>>>>>>> To fix that, select a vhost device responsible for scanning the
>>>>>>>>>>>> log with regards to memory sections dirty tracking. It is selected
>>>>>>>>>>>> when we enable the logger (during migration) and cleared when we
>>>>>>>>>>>> disable the logger. If the vhost logger device goes away for some
>>>>>>>>>>>> reason, the logger will be re-selected from the rest of vhost
>>>>>>>>>>>> devices.
>>>>>>>>>>>>
>>>>>>>>>>>> After making mem-section logger a singleton instance, constant cost
>>>>>>>>>>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
>>>>>>>>>>>> queues or how many vhost devices are configured:
>>>>>>>>>>>>
>>>>>>>>>>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
>>>>>>>>>>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
>>>>>>>>>>>>
>>>>>>>>>>>> Co-developed-by: Joao Martins <[e-mail]>
>>>>>>>>>>>> Signed-off-by: Joao Martins <[e-mail]>
>>>>>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>>>>>>>
>>>>>>>>>>>> ---
>>>>>>>>>>>> v3 -> v4:
>>>>>>>>>>>>         - add comment to clarify effect on cache locality and
>>>>>>>>>>>>           performance
>>>>>>>>>>>>
>>>>>>>>>>>> v2 -> v3:
>>>>>>>>>>>>         - add after-fix benchmark to commit log
>>>>>>>>>>>>         - rename vhost_log_dev_enabled to vhost_dev_should_log
>>>>>>>>>>>>         - remove unneeded comparisons for backend_type
>>>>>>>>>>>>         - use QLIST array instead of single flat list to store vhost
>>>>>>>>>>>>           logger devices
>>>>>>>>>>>>         - simplify logger election logic
>>>>>>>>>>>> ---
>>>>>>>>>>>>        hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
>>>>>>>>>>>>        include/hw/virtio/vhost.h |  1 +
>>>>>>>>>>>>        2 files changed, 62 insertions(+), 6 deletions(-)
>>>>>>>>>>>>
>>>>>>>>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>>>>>>>>>>>> index 612f4db..58522f1 100644
>>>>>>>>>>>> --- a/hw/virtio/vhost.c
>>>>>>>>>>>> +++ b/hw/virtio/vhost.c
>>>>>>>>>>>> @@ -45,6 +45,7 @@
>>>>>>>>>>>>
>>>>>>>>>>>>        static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
>>>>>>>>>>>>        static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
>>>>>>>>>>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
>>>>>>>>>>>>
>>>>>>>>>>>>        /* Memslots used by backends that support private memslots (without an fd). */
>>>>>>>>>>>>        static unsigned int used_memslots;
>>>>>>>>>>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
>>>>>>>>>>>>            }
>>>>>>>>>>>>        }
>>>>>>>>>>>>
>>>>>>>>>>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
>>>>>>>>>>>> +{
>>>>>>>>>>>> +    assert(dev->vhost_ops);
>>>>>>>>>>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
>>>>>>>>>>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
>>>>>>>>>>>> +
>>>>>>>>>>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
>>>>>>>>>>> A dumb question, why not simple check
>>>>>>>>>>>
>>>>>>>>>>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
>>>>>>>>>> Because we are not sure if the logger comes from vhost_log_shm[] or
>>>>>>>>>> vhost_log[]. Don't want to complicate the check here by calling into
>>>>>>>>>> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
>>>>>>>>> It has very low overhead, isn't it?
>>>>>>>> Whether this has low overhead will have to depend on the specific
>>>>>>>> backend's implementation for .vhost_requires_shm_log(), which the common
>>>>>>>> vhost layer should not assume upon or rely on the current implementation.
>>>>>>>>
>>>>>>>>> static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
>>>>>>>>> {
>>>>>>>>>          return dev->vhost_ops->vhost_requires_shm_log &&
>>>>>>>>>                 dev->vhost_ops->vhost_requires_shm_log(dev);
>>>>>>>>> }
>>>>>>> For example, if I understand the code correctly, the log type won't be
>>>>>>> changed during runtime, so we can endup with a boolean to record that
>>>>>>> instead of a query ops?
>>>>>> Right now the log type won't change during runtime, but I am not sure if
>>>>>> this may prohibit future revisit to allow change at the runtime,
>>>>> We can be bothered when we have such a request then.
>>>>>
>>>>>> then
>>>>>> there'll be complex code involvled to maintain the state.
>>>>>>
>>>>>> Other than this, I think it's insufficient to just check the shm log
>>>>>> v.s. normal log. The logger device requires to identify a leading logger
>>>>>> device that gets elected in vhost_dev_elect_mem_logger(), as all the
>>>>>> dev->log points to the same logger that is refenerce counted, that we
>>>>>> have to add extra field and complex logic to maintain the election
>>>>>> status.
>>>>> One thing I don't understand here (and in the changelog) is why do we
>>>>> need an election here?
>>>> vhost_sync_dirty_bitmap() not just scans the guest memory sections but
>>>> the specific one for virtqueues (used rings) also. To save more CPU
>>>> cycles to the best extend, the guest memory must be scanned only once in
>>>> each log iteration, though the logging for used rings would still have
>>>> to use the specific vhost instance, so all vhost_device instance still
>>>> keeps the dev->log pointer to the shared log as-is. Generally the shared
>>>> memory logger can be picked from an arbitrary vhost_device instance, but
>>>> to keep the code simple, performant and predictable
>>> This is the point, I don't see why election is simpler than picking an
>>> arbitrary shared log in this case.
>> Maybe I missed your point, but I am confused and fail to understand why
>> electing a fixed vhost_dev is not as simple? Regardless of the
>> specifics, I think the point is one _fixed_ vhost_dev has to be picked
>> amongst all the instances per backend type in charge of logging guest
>> memory, no matter if it's at the start on the list or not.
> See below.
>
>>>> , logger selection is
>>>> made on the control path at the vhost add/remove time rather than be
>>>> determined at the dirty log collection runtime, the latter of which is
>>>> in the hotpath.
>>>>
>>>>>> I thought that Eugenio's previous suggestion tried to simplify
>>>>>> the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that
>>>>>> gets expanded to use the lh_first field for the QLIST would simply
>>>>>> satisfy the basic need. Why extra logic to make the check ever more
>>>>>> complex, is there any benefit by adding more fields to the vhost_dev?
>>>>> I don't get here, the idea is to just pick one shared log which should
>>>>> be much more simpler than what is proposed here.
>>>> The code you showed earlier won't work as all vhost_device instance
>>>> points to the same dev->log device...
>>> This part I don't understand.
>> vhost_log_get() has the following:
>>
>>       log = share ? vhost_log_shm[backend_type] : vhost_log[backend_type];
>>
>>       if (!log || log->size != size) {
>>           log = vhost_log_alloc(size, share);
>>           if (share) {
>>               vhost_log_shm[backend_type] = log;
>>           } else {
>>               vhost_log[backend_type] = log;
>>           }
>>       } else {
>>           ++log->refcnt;
>>       }
>>
>> So for a specific backend type, vhost_log_get() would return the same
>> logger device (stored at either vhost_log_shm[] or vhost_log[]) to all
>> vhost_dev instances, and the check you suggested earlier:
>>
>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
>>
>> will always hold true if the vhost_dev instance (representing the
>> specific virtqueue) is active.
> Right, so the point is see if we can find something simpler to avoid
> the QLIST as it involves vhost_dev which seems unnecessary.
To make it independent of the specific vhost_dev, it would require the 
framework (migration dirty logger) to pass down "dirty_sync_count" like 
information to the vhost layer through memory listener .log_sync 
interface. I'm not sure if this change is worth the effort, as this 
patch is meant to fix a long standing bug I suppose we need to find out 
a way that is applicable to back-port to past -stable's.

>
> Does something like a counter work?
It won't. It seems the "rounds" counter is still per vhost_dev instance, 
but we need it per migration log_sync iteration across all vhost_dev 
instance of same backend type. Maybe I miss something, but I don't see 
how easily it would be to come up with proper accounting for "rounds", 
if not going through the list of vhost_dev instances at the run time 
(which is what I tried to avoid).


Thanks,
-Siwei
>
> vhost_sync_dirty_bitmap() {
>        rounds ++;
>        vhost_dev_sync_region(rounds);
> }
>
> vhost_dev_sync_region(rounds) {
>        if (dev->log->rounds == rounds)
>             return;
>        else
>             dev->log->rounds;
> }
>
> (pseudo codes, just used to demonstrate the idea).
>
> Thanks
>
>> Regards,
>> -Siwei
>>> Thanks
>>>
>>>> Regards,
>>>> -Siwei
>>>>> Thanks
>>>>>
>>>>>> Thanks,
>>>>>> -Siwei
>>>>>>
>>>>>>>>> And it helps to simplify the logic.
>>>>>>>> Generally yes, but when it comes to hot path operations the performance
>>>>>>>> consideration could override this principle. I think there's no harm to
>>>>>>>> check against logger device cached in vhost layer itself, and the
>>>>>>>> current patch does not create a lot of complexity or performance side
>>>>>>>> effect (actually I think the conditional should be very straightforward
>>>>>>>> to turn into just a couple of assembly compare and branch instructions
>>>>>>>> rather than indirection through another jmp call).
>>>>>>> Thanks
>>>>>>>
>>>>>>>> -Siwei
>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>>>
>>>>>>>>>> -Siwei
>>>>>>>>>>> ?
>>>>>>>>>>>
>>>>>>>>>>> Thanks
>>>>>>>>>>>

Jason Wang March 26, 2024, 4:36 a.m. UTC | #13

On Tue, Mar 26, 2024 at 7:21 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 3/24/2024 11:13 PM, Jason Wang wrote:
> > On Sat, Mar 23, 2024 at 5:14 AM Si-Wei Liu <[e-mail]> wrote:
> >>
> >>
> >> On 3/21/2024 10:08 PM, Jason Wang wrote:
> >>> On Fri, Mar 22, 2024 at 5:43 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>
> >>>> On 3/20/2024 8:56 PM, Jason Wang wrote:
> >>>>> On Thu, Mar 21, 2024 at 5:03 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>> On 3/19/2024 8:27 PM, Jason Wang wrote:
> >>>>>>> On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>>>> On 3/17/2024 8:22 PM, Jason Wang wrote:
> >>>>>>>>> On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>>>>>> On 3/14/2024 9:03 PM, Jason Wang wrote:
> >>>>>>>>>>> On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>>>>>>>> On setups with one or more virtio-net devices with vhost on,
> >>>>>>>>>>>> dirty tracking iteration increases cost the bigger the number
> >>>>>>>>>>>> amount of queues are set up e.g. on idle guests migration the
> >>>>>>>>>>>> following is observed with virtio-net with vhost=on:
> >>>>>>>>>>>>
> >>>>>>>>>>>> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
> >>>>>>>>>>>> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
> >>>>>>>>>>>> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
> >>>>>>>>>>>> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14
> >>>>>>>>>>>>
> >>>>>>>>>>>> With high memory rates the symptom is lack of convergence as soon
> >>>>>>>>>>>> as it has a vhost device with a sufficiently high number of queues,
> >>>>>>>>>>>> the sufficient number of vhost devices.
> >>>>>>>>>>>>
> >>>>>>>>>>>> On every migration iteration (every 100msecs) it will redundantly
> >>>>>>>>>>>> query the *shared log* the number of queues configured with vhost
> >>>>>>>>>>>> that exist in the guest. For the virtqueue data, this is necessary,
> >>>>>>>>>>>> but not for the memory sections which are the same. So essentially
> >>>>>>>>>>>> we end up scanning the dirty log too often.
> >>>>>>>>>>>>
> >>>>>>>>>>>> To fix that, select a vhost device responsible for scanning the
> >>>>>>>>>>>> log with regards to memory sections dirty tracking. It is selected
> >>>>>>>>>>>> when we enable the logger (during migration) and cleared when we
> >>>>>>>>>>>> disable the logger. If the vhost logger device goes away for some
> >>>>>>>>>>>> reason, the logger will be re-selected from the rest of vhost
> >>>>>>>>>>>> devices.
> >>>>>>>>>>>>
> >>>>>>>>>>>> After making mem-section logger a singleton instance, constant cost
> >>>>>>>>>>>> of 7%-9% (like the 1 queue report) will be seen, no matter how many
> >>>>>>>>>>>> queues or how many vhost devices are configured:
> >>>>>>>>>>>>
> >>>>>>>>>>>> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
> >>>>>>>>>>>> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
> >>>>>>>>>>>>
> >>>>>>>>>>>> Co-developed-by: Joao Martins <[e-mail]>
> >>>>>>>>>>>> Signed-off-by: Joao Martins <[e-mail]>
> >>>>>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
> >>>>>>>>>>>>
> >>>>>>>>>>>> ---
> >>>>>>>>>>>> v3 -> v4:
> >>>>>>>>>>>>         - add comment to clarify effect on cache locality and
> >>>>>>>>>>>>           performance
> >>>>>>>>>>>>
> >>>>>>>>>>>> v2 -> v3:
> >>>>>>>>>>>>         - add after-fix benchmark to commit log
> >>>>>>>>>>>>         - rename vhost_log_dev_enabled to vhost_dev_should_log
> >>>>>>>>>>>>         - remove unneeded comparisons for backend_type
> >>>>>>>>>>>>         - use QLIST array instead of single flat list to store vhost
> >>>>>>>>>>>>           logger devices
> >>>>>>>>>>>>         - simplify logger election logic
> >>>>>>>>>>>> ---
> >>>>>>>>>>>>        hw/virtio/vhost.c         | 67 ++++++++++++++++++++++++++++++++++++++++++-----
> >>>>>>>>>>>>        include/hw/virtio/vhost.h |  1 +
> >>>>>>>>>>>>        2 files changed, 62 insertions(+), 6 deletions(-)
> >>>>>>>>>>>>
> >>>>>>>>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >>>>>>>>>>>> index 612f4db..58522f1 100644
> >>>>>>>>>>>> --- a/hw/virtio/vhost.c
> >>>>>>>>>>>> +++ b/hw/virtio/vhost.c
> >>>>>>>>>>>> @@ -45,6 +45,7 @@
> >>>>>>>>>>>>
> >>>>>>>>>>>>        static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
> >>>>>>>>>>>>        static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
> >>>>>>>>>>>> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
> >>>>>>>>>>>>
> >>>>>>>>>>>>        /* Memslots used by backends that support private memslots (without an fd). */
> >>>>>>>>>>>>        static unsigned int used_memslots;
> >>>>>>>>>>>> @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
> >>>>>>>>>>>>            }
> >>>>>>>>>>>>        }
> >>>>>>>>>>>>
> >>>>>>>>>>>> +static inline bool vhost_dev_should_log(struct vhost_dev *dev)
> >>>>>>>>>>>> +{
> >>>>>>>>>>>> +    assert(dev->vhost_ops);
> >>>>>>>>>>>> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
> >>>>>>>>>>>> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
> >>>>>>>>>>>> +
> >>>>>>>>>>>> +    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
> >>>>>>>>>>> A dumb question, why not simple check
> >>>>>>>>>>>
> >>>>>>>>>>> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
> >>>>>>>>>> Because we are not sure if the logger comes from vhost_log_shm[] or
> >>>>>>>>>> vhost_log[]. Don't want to complicate the check here by calling into
> >>>>>>>>>> vhost_dev_log_is_shared() everytime when the .log_sync() is called.
> >>>>>>>>> It has very low overhead, isn't it?
> >>>>>>>> Whether this has low overhead will have to depend on the specific
> >>>>>>>> backend's implementation for .vhost_requires_shm_log(), which the common
> >>>>>>>> vhost layer should not assume upon or rely on the current implementation.
> >>>>>>>>
> >>>>>>>>> static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
> >>>>>>>>> {
> >>>>>>>>>          return dev->vhost_ops->vhost_requires_shm_log &&
> >>>>>>>>>                 dev->vhost_ops->vhost_requires_shm_log(dev);
> >>>>>>>>> }
> >>>>>>> For example, if I understand the code correctly, the log type won't be
> >>>>>>> changed during runtime, so we can endup with a boolean to record that
> >>>>>>> instead of a query ops?
> >>>>>> Right now the log type won't change during runtime, but I am not sure if
> >>>>>> this may prohibit future revisit to allow change at the runtime,
> >>>>> We can be bothered when we have such a request then.
> >>>>>
> >>>>>> then
> >>>>>> there'll be complex code involvled to maintain the state.
> >>>>>>
> >>>>>> Other than this, I think it's insufficient to just check the shm log
> >>>>>> v.s. normal log. The logger device requires to identify a leading logger
> >>>>>> device that gets elected in vhost_dev_elect_mem_logger(), as all the
> >>>>>> dev->log points to the same logger that is refenerce counted, that we
> >>>>>> have to add extra field and complex logic to maintain the election
> >>>>>> status.
> >>>>> One thing I don't understand here (and in the changelog) is why do we
> >>>>> need an election here?
> >>>> vhost_sync_dirty_bitmap() not just scans the guest memory sections but
> >>>> the specific one for virtqueues (used rings) also. To save more CPU
> >>>> cycles to the best extend, the guest memory must be scanned only once in
> >>>> each log iteration, though the logging for used rings would still have
> >>>> to use the specific vhost instance, so all vhost_device instance still
> >>>> keeps the dev->log pointer to the shared log as-is. Generally the shared
> >>>> memory logger can be picked from an arbitrary vhost_device instance, but
> >>>> to keep the code simple, performant and predictable
> >>> This is the point, I don't see why election is simpler than picking an
> >>> arbitrary shared log in this case.
> >> Maybe I missed your point, but I am confused and fail to understand why
> >> electing a fixed vhost_dev is not as simple? Regardless of the
> >> specifics, I think the point is one _fixed_ vhost_dev has to be picked
> >> amongst all the instances per backend type in charge of logging guest
> >> memory, no matter if it's at the start on the list or not.
> > See below.
> >
> >>>> , logger selection is
> >>>> made on the control path at the vhost add/remove time rather than be
> >>>> determined at the dirty log collection runtime, the latter of which is
> >>>> in the hotpath.
> >>>>
> >>>>>> I thought that Eugenio's previous suggestion tried to simplify
> >>>>>> the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that
> >>>>>> gets expanded to use the lh_first field for the QLIST would simply
> >>>>>> satisfy the basic need. Why extra logic to make the check ever more
> >>>>>> complex, is there any benefit by adding more fields to the vhost_dev?
> >>>>> I don't get here, the idea is to just pick one shared log which should
> >>>>> be much more simpler than what is proposed here.
> >>>> The code you showed earlier won't work as all vhost_device instance
> >>>> points to the same dev->log device...
> >>> This part I don't understand.
> >> vhost_log_get() has the following:
> >>
> >>       log = share ? vhost_log_shm[backend_type] : vhost_log[backend_type];
> >>
> >>       if (!log || log->size != size) {
> >>           log = vhost_log_alloc(size, share);
> >>           if (share) {
> >>               vhost_log_shm[backend_type] = log;
> >>           } else {
> >>               vhost_log[backend_type] = log;
> >>           }
> >>       } else {
> >>           ++log->refcnt;
> >>       }
> >>
> >> So for a specific backend type, vhost_log_get() would return the same
> >> logger device (stored at either vhost_log_shm[] or vhost_log[]) to all
> >> vhost_dev instances, and the check you suggested earlier:
> >>
> >> dev->log == vhost_log_shm[dev->vhost_ops->backend_type]
> >>
> >> will always hold true if the vhost_dev instance (representing the
> >> specific virtqueue) is active.
> > Right, so the point is see if we can find something simpler to avoid
> > the QLIST as it involves vhost_dev which seems unnecessary.
> To make it independent of the specific vhost_dev, it would require the
> framework (migration dirty logger) to pass down "dirty_sync_count" like
> information to the vhost layer through memory listener .log_sync
> interface. I'm not sure if this change is worth the effort, as this
> patch is meant to fix a long standing bug

I may miss something but it looks to be a performance optimization.

> I suppose we need to find out
> a way that is applicable to back-port to past -stable's.
>
> >
> > Does something like a counter work?
> It won't. It seems the "rounds" counter is still per vhost_dev instance,
> but we need it per migration log_sync iteration across all vhost_dev
> instance of same backend type. Maybe I miss something, but I don't see
> how easily it would be to come up with proper accounting for "rounds",
> if not going through the list of vhost_dev instances at the run time
> (which is what I tried to avoid).

Ok, so I think I'm fine with this now.

So the root cause is the multiple listeners which we could tweak in the future.

Acked-by: Jason Wang <[e-mail]>

Thanks

>
>
> Thanks,
> -Siwei
> >
> > vhost_sync_dirty_bitmap() {
> >        rounds ++;
> >        vhost_dev_sync_region(rounds);
> > }
> >
> > vhost_dev_sync_region(rounds) {
> >        if (dev->log->rounds == rounds)
> >             return;
> >        else
> >             dev->log->rounds;
> > }
> >
> > (pseudo codes, just used to demonstrate the idea).
> >
> > Thanks
> >
> >> Regards,
> >> -Siwei
> >>> Thanks
> >>>
> >>>> Regards,
> >>>> -Siwei
> >>>>> Thanks
> >>>>>
> >>>>>> Thanks,
> >>>>>> -Siwei
> >>>>>>
> >>>>>>>>> And it helps to simplify the logic.
> >>>>>>>> Generally yes, but when it comes to hot path operations the performance
> >>>>>>>> consideration could override this principle. I think there's no harm to
> >>>>>>>> check against logger device cached in vhost layer itself, and the
> >>>>>>>> current patch does not create a lot of complexity or performance side
> >>>>>>>> effect (actually I think the conditional should be very straightforward
> >>>>>>>> to turn into just a couple of assembly compare and branch instructions
> >>>>>>>> rather than indirection through another jmp call).
> >>>>>>> Thanks
> >>>>>>>
> >>>>>>>> -Siwei
> >>>>>>>>
> >>>>>>>>> Thanks
> >>>>>>>>>
> >>>>>>>>>> -Siwei
> >>>>>>>>>>> ?
> >>>>>>>>>>>
> >>>>>>>>>>> Thanks
> >>>>>>>>>>>
>

Patch

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 612f4db..58522f1 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -45,6 +45,7 @@ 
 
 static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
 static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
+static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
 
 /* Memslots used by backends that support private memslots (without an fd). */
 static unsigned int used_memslots;
@@ -149,6 +150,47 @@  bool vhost_dev_has_iommu(struct vhost_dev *dev)
     }
 }
 
+static inline bool vhost_dev_should_log(struct vhost_dev *dev)
+{
+    assert(dev->vhost_ops);
+    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
+    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
+
+    return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
+}
+
+static inline void vhost_dev_elect_mem_logger(struct vhost_dev *hdev, bool add)
+{
+    VhostBackendType backend_type;
+
+    assert(hdev->vhost_ops);
+
+    backend_type = hdev->vhost_ops->backend_type;
+    assert(backend_type > VHOST_BACKEND_TYPE_NONE);
+    assert(backend_type < VHOST_BACKEND_TYPE_MAX);
+
+    if (add && !QLIST_IS_INSERTED(hdev, logdev_entry)) {
+        if (QLIST_EMPTY(&vhost_log_devs[backend_type])) {
+            QLIST_INSERT_HEAD(&vhost_log_devs[backend_type],
+                              hdev, logdev_entry);
+        } else {
+            /*
+             * The first vhost_device in the list is selected as the shared
+             * logger to scan memory sections. Put new entry next to the head
+             * to avoid inadvertent change to the underlying logger device.
+             * This is done in order to get better cache locality and to avoid
+             * performance churn on the hot path for log scanning. Even when
+             * new devices come and go quickly, it wouldn't end up changing
+             * the active leading logger device at all.
+             */
+            QLIST_INSERT_AFTER(QLIST_FIRST(&vhost_log_devs[backend_type]),
+                               hdev, logdev_entry);
+        }
+    } else if (!add && QLIST_IS_INSERTED(hdev, logdev_entry)) {
+        QLIST_REMOVE(hdev, logdev_entry);
+    }
+}
+
 static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
                                    MemoryRegionSection *section,
                                    hwaddr first,
@@ -166,12 +208,14 @@  static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
     start_addr = MAX(first, start_addr);
     end_addr = MIN(last, end_addr);
 
-    for (i = 0; i < dev->mem->nregions; ++i) {
-        struct vhost_memory_region *reg = dev->mem->regions + i;
-        vhost_dev_sync_region(dev, section, start_addr, end_addr,
-                              reg->guest_phys_addr,
-                              range_get_last(reg->guest_phys_addr,
-                                             reg->memory_size));
+    if (vhost_dev_should_log(dev)) {
+        for (i = 0; i < dev->mem->nregions; ++i) {
+            struct vhost_memory_region *reg = dev->mem->regions + i;
+            vhost_dev_sync_region(dev, section, start_addr, end_addr,
+                                  reg->guest_phys_addr,
+                                  range_get_last(reg->guest_phys_addr,
+                                                 reg->memory_size));
+        }
     }
     for (i = 0; i < dev->nvqs; ++i) {
         struct vhost_virtqueue *vq = dev->vqs + i;
@@ -383,6 +427,7 @@  static void vhost_log_put(struct vhost_dev *dev, bool sync)
         g_free(log);
     }
 
+    vhost_dev_elect_mem_logger(dev, false);
     dev->log = NULL;
     dev->log_size = 0;
 }
@@ -998,6 +1043,15 @@  static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
             goto err_vq;
         }
     }
+
+    /*
+     * At log start we select our vhost_device logger that will scan the
+     * memory sections and skip for the others. This is possible because
+     * the log is shared amongst all vhost devices for a given type of
+     * backend.
+     */
+    vhost_dev_elect_mem_logger(dev, enable_log);
+
     return 0;
 err_vq:
     for (; i >= 0; --i) {
@@ -2068,6 +2122,7 @@  int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings)
             VHOST_OPS_DEBUG(r, "vhost_set_log_base failed");
             goto fail_log;
         }
+        vhost_dev_elect_mem_logger(hdev, true);
     }
     if (vrings) {
         r = vhost_dev_set_vring_enable(hdev, true);
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 0247778..d75faf4 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -129,6 +129,7 @@  struct vhost_dev {
     void *opaque;
     struct vhost_log *log;
     QLIST_ENTRY(vhost_dev) entry;
+    QLIST_ENTRY(vhost_dev) logdev_entry;
     QLIST_HEAD(, vhost_iommu) iommu_list;
     IOMMUNotifier n;
     const VhostDevConfigOps *config_ops;