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 <m...@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 <m...@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 <m...@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. > > _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
