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@xxxxxxxxxx> 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@xxxxxxxxxx> --- 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. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
