| Message ID | 20220113145642.205388-1-sgarzare@redhat.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [RFC] vhost: cache avail index in vhost_enable_notify() | expand |
| Context | Check | Description |
|---|---|---|
| netdev/tree_selection | success | Not a local patch |
On Thu, Jan 13, 2022 at 03:56:42PM +0100, Stefano Garzarella wrote: > In vhost_enable_notify() we enable the notifications and we read > the avail index to check if new buffers have become available in > the meantime. In this case, the device would go to re-read avail > index to access the descriptor. > > As we already do in other place, we can cache the value in `avail_idx` > and compare it with `last_avail_idx` to check if there are new > buffers available. > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> I guess we can ... but what's the point? > --- > drivers/vhost/vhost.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index 59edb5a1ffe2..07363dff559e 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) > &vq->avail->idx, r); > return false; > } > + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > > - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; > + return vq->avail_idx != vq->last_avail_idx; > } > EXPORT_SYMBOL_GPL(vhost_enable_notify); > > -- > 2.31.1
On Thu, Jan 13, 2022 at 10:19:46AM -0500, Michael S. Tsirkin wrote: >On Thu, Jan 13, 2022 at 03:56:42PM +0100, Stefano Garzarella wrote: >> In vhost_enable_notify() we enable the notifications and we read >> the avail index to check if new buffers have become available in >> the meantime. In this case, the device would go to re-read avail >> index to access the descriptor. >> >> As we already do in other place, we can cache the value in `avail_idx` >> and compare it with `last_avail_idx` to check if there are new >> buffers available. >> >> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> > >I guess we can ... but what's the point? > That without this patch if avail index is new, then device when will call vhost_get_vq_desc() will find old value in cache and will read it again. With this patch we also do the same path and update the cache every time we read avail index. I marked it RFC because I don't know if it's worth it :-) Stefano
On Thu, Jan 13, 2022 at 04:44:47PM +0100, Stefano Garzarella wrote: > On Thu, Jan 13, 2022 at 10:19:46AM -0500, Michael S. Tsirkin wrote: > > On Thu, Jan 13, 2022 at 03:56:42PM +0100, Stefano Garzarella wrote: > > > In vhost_enable_notify() we enable the notifications and we read > > > the avail index to check if new buffers have become available in > > > the meantime. In this case, the device would go to re-read avail > > > index to access the descriptor. > > > > > > As we already do in other place, we can cache the value in `avail_idx` > > > and compare it with `last_avail_idx` to check if there are new > > > buffers available. > > > > > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> > > > > I guess we can ... but what's the point? > > > > That without this patch if avail index is new, then device when will call > vhost_get_vq_desc() will find old value in cache and will read it again. > > With this patch we also do the same path and update the cache every time we > read avail index. > > I marked it RFC because I don't know if it's worth it :-) > > Stefano Pls include info like this in commit log. Thanks!
On Thu, Jan 13, 2022 at 10:57 PM Stefano Garzarella <sgarzare@redhat.com> wrote: > > In vhost_enable_notify() we enable the notifications and we read > the avail index to check if new buffers have become available in > the meantime. In this case, the device would go to re-read avail > index to access the descriptor. > > As we already do in other place, we can cache the value in `avail_idx` > and compare it with `last_avail_idx` to check if there are new > buffers available. > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> Patch looks fine but I guess we won't get performance improvement since it doesn't save any userspace/VM memory access? Thanks > --- > drivers/vhost/vhost.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index 59edb5a1ffe2..07363dff559e 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) > &vq->avail->idx, r); > return false; > } > + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > > - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; > + return vq->avail_idx != vq->last_avail_idx; > } > EXPORT_SYMBOL_GPL(vhost_enable_notify); > > -- > 2.31.1 >
On Fri, Jan 14, 2022 at 02:18:01PM +0800, Jason Wang wrote: >On Thu, Jan 13, 2022 at 10:57 PM Stefano Garzarella <sgarzare@redhat.com> wrote: >> >> In vhost_enable_notify() we enable the notifications and we read >> the avail index to check if new buffers have become available in >> the meantime. In this case, the device would go to re-read avail >> index to access the descriptor. >> >> As we already do in other place, we can cache the value in `avail_idx` >> and compare it with `last_avail_idx` to check if there are new >> buffers available. >> >> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> > >Patch looks fine but I guess we won't get performance improvement >since it doesn't save any userspace/VM memory access? It should save the memory access when vhost_enable_notify() find something new in the VQ, so in this path: vhost_enable_notify() <- VM memory access for avail index == true vhost_disable_notify() ... vhost_get_vq_desc() <- VM memory access for avail index with the patch applied, this access is avoided since avail index is cached In any case, I don't expect this to be a very common path, indeed we usually use unlikely() for this path: if (unlikely(vhost_enable_notify(dev, vq))) { vhost_disable_notify(dev, vq); continue; } So I don't expect a significant performance increase. v1 coming with a better commit description. Thanks, Stefano
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 59edb5a1ffe2..07363dff559e 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) &vq->avail->idx, r); return false; } + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; + return vq->avail_idx != vq->last_avail_idx; } EXPORT_SYMBOL_GPL(vhost_enable_notify);
In vhost_enable_notify() we enable the notifications and we read the avail index to check if new buffers have become available in the meantime. In this case, the device would go to re-read avail index to access the descriptor. As we already do in other place, we can cache the value in `avail_idx` and compare it with `last_avail_idx` to check if there are new buffers available. Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> --- drivers/vhost/vhost.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)