| Message ID | 20211108114951.92862-1-xuanzhuo@linux.alibaba.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | virtio support cache indirect desc | expand |
On Mon, Nov 08, 2021 at 07:49:48PM +0800, Xuan Zhuo wrote: > If the VIRTIO_RING_F_INDIRECT_DESC negotiation succeeds, and the number > of sgs used for sending packets is greater than 1. We must constantly > call __kmalloc/kfree to allocate/release desc. > > In the case of extremely fast package delivery, the overhead cannot be > ignored: > > 27.46% [kernel] [k] virtqueue_add > 16.66% [kernel] [k] detach_buf_split > 16.51% [kernel] [k] virtnet_xsk_xmit > 14.04% [kernel] [k] virtqueue_add_outbuf > 5.18% [kernel] [k] __kmalloc > 4.08% [kernel] [k] kfree > 2.80% [kernel] [k] virtqueue_get_buf_ctx > 2.22% [kernel] [k] xsk_tx_peek_desc > 2.08% [kernel] [k] memset_erms > 0.83% [kernel] [k] virtqueue_kick_prepare > 0.76% [kernel] [k] virtnet_xsk_run > 0.62% [kernel] [k] __free_old_xmit_ptr > 0.60% [kernel] [k] vring_map_one_sg > 0.53% [kernel] [k] native_apic_mem_write > 0.46% [kernel] [k] sg_next > 0.43% [kernel] [k] sg_init_table > 0.41% [kernel] [k] kmalloc_slab > > This patch adds a cache function to virtio to cache these allocated indirect > desc instead of constantly allocating and releasing desc. Hmm a bunch of comments got ignored. See e.g. https://lore.kernel.org/r/20211027043851-mutt-send-email-mst%40kernel.org if they aren't relevant add code comments or commit log text explaining the design choice please. > v4: > 1. Only allow desc cache when VIRTIO_RING_F_INDIRECT_DESC negotiation is successful > 2. The desc cache threshold can be set for each virtqueue > > v3: > pre-allocate per buffer indirect descriptors array > > v2: > use struct list_head to cache the desc > > Xuan Zhuo (3): > virtio: cache indirect desc for split > virtio: cache indirect desc for packed > virtio-net: enable virtio desc cache > > drivers/net/virtio_net.c | 12 ++- > drivers/virtio/virtio_ring.c | 152 +++++++++++++++++++++++++++++++---- > include/linux/virtio.h | 17 ++++ > 3 files changed, 163 insertions(+), 18 deletions(-) > > -- > 2.31.0
On Mon, Nov 08, 2021 at 07:49:48PM +0800, Xuan Zhuo wrote: > If the VIRTIO_RING_F_INDIRECT_DESC negotiation succeeds, and the number > of sgs used for sending packets is greater than 1. We must constantly > call __kmalloc/kfree to allocate/release desc. > > In the case of extremely fast package delivery, the overhead cannot be > ignored: > > 27.46% [kernel] [k] virtqueue_add > 16.66% [kernel] [k] detach_buf_split > 16.51% [kernel] [k] virtnet_xsk_xmit > 14.04% [kernel] [k] virtqueue_add_outbuf > 5.18% [kernel] [k] __kmalloc > 4.08% [kernel] [k] kfree > 2.80% [kernel] [k] virtqueue_get_buf_ctx > 2.22% [kernel] [k] xsk_tx_peek_desc > 2.08% [kernel] [k] memset_erms > 0.83% [kernel] [k] virtqueue_kick_prepare > 0.76% [kernel] [k] virtnet_xsk_run > 0.62% [kernel] [k] __free_old_xmit_ptr > 0.60% [kernel] [k] vring_map_one_sg > 0.53% [kernel] [k] native_apic_mem_write > 0.46% [kernel] [k] sg_next > 0.43% [kernel] [k] sg_init_table > 0.41% [kernel] [k] kmalloc_slab > > This patch adds a cache function to virtio to cache these allocated indirect > desc instead of constantly allocating and releasing desc. > > v4: > 1. Only allow desc cache when VIRTIO_RING_F_INDIRECT_DESC negotiation is successful > 2. The desc cache threshold can be set for each virtqueue > > v3: > pre-allocate per buffer indirect descriptors array So I'm not sure why we are doing that. Did it improve anything? > v2: > use struct list_head to cache the desc > > Xuan Zhuo (3): > virtio: cache indirect desc for split > virtio: cache indirect desc for packed > virtio-net: enable virtio desc cache > > drivers/net/virtio_net.c | 12 ++- > drivers/virtio/virtio_ring.c | 152 +++++++++++++++++++++++++++++++---- > include/linux/virtio.h | 17 ++++ > 3 files changed, 163 insertions(+), 18 deletions(-) > > -- > 2.31.0
On Mon, Nov 08, 2021 at 10:47:40PM +0800, Xuan Zhuo wrote: > On Mon, 8 Nov 2021 08:49:27 -0500, Michael S. Tsirkin <mst@redhat.com> wrote: > > > > Hmm a bunch of comments got ignored. See e.g. > > https://lore.kernel.org/r/20211027043851-mutt-send-email-mst%40kernel.org > > if they aren't relevant add code comments or commit log text explaining the > > design choice please. > > I should have responded to related questions, I am guessing whether some emails > have been lost. > > I have sorted out the following 6 questions, if there are any missing questions, > please let me know. > > 1. use list_head > In the earliest version, I used pointers directly. You suggest that I use > llist_head, but considering that llist_head has atomic operations. There is no > competition problem here, so I used list_head. > > In fact, I did not increase the allocated space for list_head. > > use as desc array: | vring_desc | vring_desc | vring_desc | vring_desc | > use as queue item: | list_head ........................................| the concern is that you touch many cache lines when removing an entry. I suggest something like: llist: add a non-atomic list_del_first One has to know what one's doing, but if one has locked the list preventing all accesses, then it's ok to just pop off an entry without atomics. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> --- diff --git a/include/linux/llist.h b/include/linux/llist.h index 24f207b0190b..13a47dddb12b 100644 --- a/include/linux/llist.h +++ b/include/linux/llist.h @@ -247,6 +247,17 @@ static inline struct llist_node *__llist_del_all(struct llist_head *head) extern struct llist_node *llist_del_first(struct llist_head *head); +static inline struct llist_node *__llist_del_first(struct llist_head *head) +{ + struct llist_node *first = head->first; + + if (!first) + return NULL; + + head->first = first->next; + return first; +} + struct llist_node *llist_reverse_order(struct llist_node *head); #endif /* LLIST_H */ ----- > 2. > > > + if (vq->use_desc_cache && total_sg <= VIRT_QUEUE_CACHE_DESC_NUM) { > > > + if (vq->desc_cache_chain) { > > > + desc = vq->desc_cache_chain; > > > + vq->desc_cache_chain = (void *)desc->addr; > > > + goto got; > > > + } > > > + n = VIRT_QUEUE_CACHE_DESC_NUM; > > > > Hmm. This will allocate more entries than actually used. Why do it? > > > This is because the size of each cache item is fixed, and the logic has been > modified in the latest code. I think this problem no longer exists. > > > 3. > > What bothers me here is what happens if cache gets > > filled on one numa node, then used on another? > > I'm thinking about another question, how did the cross-numa appear here, and > virtio desc queue also has the problem of cross-numa. So is it necessary for us > to deal with the cross-numa scene? It's true that desc queue might be cross numa, and people are looking for ways to improve that. Not a reason to make things worse ... > Indirect desc is used as virtio desc, so as long as it is in the same numa as > virito desc. So we can allocate indirect desc cache at the same time when > allocating virtio desc queue. Using it from current node like we do now seems better. > 4. > > So e.g. for rx, we are wasting memory since indirect isn't used. > > In the current version, desc cache is set up based on pre-queue. > > So if the desc cache is not used, we don't need to set the desc cache. > > For example, virtio-net, as long as the tx queue and the rx queue in big packet > mode enable desc cache. I liked how in older versions adding indrect enabled it implicitly though without need to hack drivers. > 5. > > Would a better API be a cache size in bytes? This controls how much > > memory is spent after all. > > My design is to set a threshold. When total_sg is greater than this threshold, > it will fall back to kmalloc/kfree. When total_sg is less than or equal to > this threshold, use the allocated cache. > I know. My question is this, do devices know what a good threshold is? If yes how do they know? > 6. kmem_cache_* > > I have tested these, the performance is not as good as the method used in this > patch. Do you mean kmem_cache_alloc_bulk/kmem_cache_free_bulk? You mentioned just kmem_cache_alloc previously. > > Thanks.
On Wed, Nov 10, 2021 at 07:53:49AM -0500, Michael S. Tsirkin wrote: > On Mon, Nov 08, 2021 at 10:47:40PM +0800, Xuan Zhuo wrote: > > On Mon, 8 Nov 2021 08:49:27 -0500, Michael S. Tsirkin <mst@redhat.com> wrote: > > > > > > Hmm a bunch of comments got ignored. See e.g. > > > https://lore.kernel.org/r/20211027043851-mutt-send-email-mst%40kernel.org > > > if they aren't relevant add code comments or commit log text explaining the > > > design choice please. > > > > I should have responded to related questions, I am guessing whether some emails > > have been lost. > > > > I have sorted out the following 6 questions, if there are any missing questions, > > please let me know. > > > > 1. use list_head > > In the earliest version, I used pointers directly. You suggest that I use > > llist_head, but considering that llist_head has atomic operations. There is no > > competition problem here, so I used list_head. > > > > In fact, I did not increase the allocated space for list_head. > > > > use as desc array: | vring_desc | vring_desc | vring_desc | vring_desc | > > use as queue item: | list_head ........................................| > > the concern is that you touch many cache lines when removing an entry. > > I suggest something like: > > llist: add a non-atomic list_del_first > > One has to know what one's doing, but if one has locked the list > preventing all accesses, then it's ok to just pop off an entry without > atomics. > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > > --- > > diff --git a/include/linux/llist.h b/include/linux/llist.h > index 24f207b0190b..13a47dddb12b 100644 > --- a/include/linux/llist.h > +++ b/include/linux/llist.h > @@ -247,6 +247,17 @@ static inline struct llist_node *__llist_del_all(struct llist_head *head) > > extern struct llist_node *llist_del_first(struct llist_head *head); > > +static inline struct llist_node *__llist_del_first(struct llist_head *head) > +{ > + struct llist_node *first = head->first; > + > + if (!first) > + return NULL; > + > + head->first = first->next; > + return first; > +} > + > struct llist_node *llist_reverse_order(struct llist_node *head); > > #endif /* LLIST_H */ > > > ----- > > > > 2. > > > > + if (vq->use_desc_cache && total_sg <= VIRT_QUEUE_CACHE_DESC_NUM) { > > > > + if (vq->desc_cache_chain) { > > > > + desc = vq->desc_cache_chain; > > > > + vq->desc_cache_chain = (void *)desc->addr; > > > > + goto got; > > > > + } > > > > + n = VIRT_QUEUE_CACHE_DESC_NUM; > > > > > > Hmm. This will allocate more entries than actually used. Why do it? > > > > > > This is because the size of each cache item is fixed, and the logic has been > > modified in the latest code. I think this problem no longer exists. > > > > > > 3. > > > What bothers me here is what happens if cache gets > > > filled on one numa node, then used on another? > > > > I'm thinking about another question, how did the cross-numa appear here, and > > virtio desc queue also has the problem of cross-numa. So is it necessary for us > > to deal with the cross-numa scene? > > It's true that desc queue might be cross numa, and people are looking > for ways to improve that. Not a reason to make things worse ... > To add to that, given it's a concern, how about actually testing performance for this config? > > Indirect desc is used as virtio desc, so as long as it is in the same numa as > > virito desc. So we can allocate indirect desc cache at the same time when > > allocating virtio desc queue. > > Using it from current node like we do now seems better. > > > 4. > > > So e.g. for rx, we are wasting memory since indirect isn't used. > > > > In the current version, desc cache is set up based on pre-queue. > > > > So if the desc cache is not used, we don't need to set the desc cache. > > > > For example, virtio-net, as long as the tx queue and the rx queue in big packet > > mode enable desc cache. > > > I liked how in older versions adding indrect enabled it implicitly > though without need to hack drivers. > > > 5. > > > Would a better API be a cache size in bytes? This controls how much > > > memory is spent after all. > > > > My design is to set a threshold. When total_sg is greater than this threshold, > > it will fall back to kmalloc/kfree. When total_sg is less than or equal to > > this threshold, use the allocated cache. > > > > I know. My question is this, do devices know what a good threshold is? > If yes how do they know? > > > 6. kmem_cache_* > > > > I have tested these, the performance is not as good as the method used in this > > patch. > > Do you mean kmem_cache_alloc_bulk/kmem_cache_free_bulk? > You mentioned just kmem_cache_alloc previously. > > > > > Thanks.
On Thu, Nov 11, 2021 at 02:52:07PM +0800, Xuan Zhuo wrote: > On Wed, 10 Nov 2021 07:53:44 -0500, Michael S. Tsirkin <mst@redhat.com> wrote: > > On Mon, Nov 08, 2021 at 10:47:40PM +0800, Xuan Zhuo wrote: > > > On Mon, 8 Nov 2021 08:49:27 -0500, Michael S. Tsirkin <mst@redhat.com> wrote: > > > > > > > > Hmm a bunch of comments got ignored. See e.g. > > > > https://lore.kernel.org/r/20211027043851-mutt-send-email-mst%40kernel.org > > > > if they aren't relevant add code comments or commit log text explaining the > > > > design choice please. > > > > > > I should have responded to related questions, I am guessing whether some emails > > > have been lost. > > > > > > I have sorted out the following 6 questions, if there are any missing questions, > > > please let me know. > > > > > > 1. use list_head > > > In the earliest version, I used pointers directly. You suggest that I use > > > llist_head, but considering that llist_head has atomic operations. There is no > > > competition problem here, so I used list_head. > > > > > > In fact, I did not increase the allocated space for list_head. > > > > > > use as desc array: | vring_desc | vring_desc | vring_desc | vring_desc | > > > use as queue item: | list_head ........................................| > > > > the concern is that you touch many cache lines when removing an entry. > > > > I suggest something like: > > > > llist: add a non-atomic list_del_first > > > > One has to know what one's doing, but if one has locked the list > > preventing all accesses, then it's ok to just pop off an entry without > > atomics. > > > > Oh, great, but my way of solving the problem is too conservative. > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > > > > --- > > > > diff --git a/include/linux/llist.h b/include/linux/llist.h > > index 24f207b0190b..13a47dddb12b 100644 > > --- a/include/linux/llist.h > > +++ b/include/linux/llist.h > > @@ -247,6 +247,17 @@ static inline struct llist_node *__llist_del_all(struct llist_head *head) > > > > extern struct llist_node *llist_del_first(struct llist_head *head); > > > > +static inline struct llist_node *__llist_del_first(struct llist_head *head) > > +{ > > + struct llist_node *first = head->first; > > + > > + if (!first) > > + return NULL; > > + > > + head->first = first->next; > > + return first; > > +} > > + > > struct llist_node *llist_reverse_order(struct llist_node *head); > > > > #endif /* LLIST_H */ > > > > > > ----- > > > > > > > 2. > > > > > + if (vq->use_desc_cache && total_sg <= VIRT_QUEUE_CACHE_DESC_NUM) { > > > > > + if (vq->desc_cache_chain) { > > > > > + desc = vq->desc_cache_chain; > > > > > + vq->desc_cache_chain = (void *)desc->addr; > > > > > + goto got; > > > > > + } > > > > > + n = VIRT_QUEUE_CACHE_DESC_NUM; > > > > > > > > Hmm. This will allocate more entries than actually used. Why do it? > > > > > > > > > This is because the size of each cache item is fixed, and the logic has been > > > modified in the latest code. I think this problem no longer exists. > > > > > > > > > 3. > > > > What bothers me here is what happens if cache gets > > > > filled on one numa node, then used on another? > > > > > > I'm thinking about another question, how did the cross-numa appear here, and > > > virtio desc queue also has the problem of cross-numa. So is it necessary for us > > > to deal with the cross-numa scene? > > > > It's true that desc queue might be cross numa, and people are looking > > for ways to improve that. Not a reason to make things worse ... > > > > I will test for it. > > > > > > Indirect desc is used as virtio desc, so as long as it is in the same numa as > > > virito desc. So we can allocate indirect desc cache at the same time when > > > allocating virtio desc queue. > > > > Using it from current node like we do now seems better. > > > > > 4. > > > > So e.g. for rx, we are wasting memory since indirect isn't used. > > > > > > In the current version, desc cache is set up based on pre-queue. > > > > > > So if the desc cache is not used, we don't need to set the desc cache. > > > > > > For example, virtio-net, as long as the tx queue and the rx queue in big packet > > > mode enable desc cache. > > > > > > I liked how in older versions adding indrect enabled it implicitly > > though without need to hack drivers. > > I see. > > > > > > 5. > > > > Would a better API be a cache size in bytes? This controls how much > > > > memory is spent after all. > > > > > > My design is to set a threshold. When total_sg is greater than this threshold, > > > it will fall back to kmalloc/kfree. When total_sg is less than or equal to > > > this threshold, use the allocated cache. > > > > > > > I know. My question is this, do devices know what a good threshold is? > > If yes how do they know? > > I think the driver knows the threshold, for example, MAX_SKB_FRAG + 2 is a > suitable threshold for virtio-net. > I guess... in that case I assume it's a good idea to have virtio core round the size up to whole cache lines, right? > > > > > 6. kmem_cache_* > > > > > > I have tested these, the performance is not as good as the method used in this > > > patch. > > > > Do you mean kmem_cache_alloc_bulk/kmem_cache_free_bulk? > > You mentioned just kmem_cache_alloc previously. > > > I will test for kmem_cache_alloc_bulk. > > Thanks. > > > > > > > > > Thanks. > >
If the VIRTIO_RING_F_INDIRECT_DESC negotiation succeeds, and the number of sgs used for sending packets is greater than 1. We must constantly call __kmalloc/kfree to allocate/release desc. In the case of extremely fast package delivery, the overhead cannot be ignored: 27.46% [kernel] [k] virtqueue_add 16.66% [kernel] [k] detach_buf_split 16.51% [kernel] [k] virtnet_xsk_xmit 14.04% [kernel] [k] virtqueue_add_outbuf 5.18% [kernel] [k] __kmalloc 4.08% [kernel] [k] kfree 2.80% [kernel] [k] virtqueue_get_buf_ctx 2.22% [kernel] [k] xsk_tx_peek_desc 2.08% [kernel] [k] memset_erms 0.83% [kernel] [k] virtqueue_kick_prepare 0.76% [kernel] [k] virtnet_xsk_run 0.62% [kernel] [k] __free_old_xmit_ptr 0.60% [kernel] [k] vring_map_one_sg 0.53% [kernel] [k] native_apic_mem_write 0.46% [kernel] [k] sg_next 0.43% [kernel] [k] sg_init_table 0.41% [kernel] [k] kmalloc_slab This patch adds a cache function to virtio to cache these allocated indirect desc instead of constantly allocating and releasing desc. v4: 1. Only allow desc cache when VIRTIO_RING_F_INDIRECT_DESC negotiation is successful 2. The desc cache threshold can be set for each virtqueue v3: pre-allocate per buffer indirect descriptors array v2: use struct list_head to cache the desc Xuan Zhuo (3): virtio: cache indirect desc for split virtio: cache indirect desc for packed virtio-net: enable virtio desc cache drivers/net/virtio_net.c | 12 ++- drivers/virtio/virtio_ring.c | 152 +++++++++++++++++++++++++++++++---- include/linux/virtio.h | 17 ++++ 3 files changed, 163 insertions(+), 18 deletions(-) -- 2.31.0