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Tsirkin" To: Xuan Zhuo Cc: virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, Jason Wang , "David S. Miller" , Jakub Kicinski Subject: Re: [PATCH v4 0/3] virtio support cache indirect desc Message-ID: <20211111100044-mutt-send-email-mst@kernel.org> References: <20211110074326-mutt-send-email-mst@kernel.org> <1636613527.8447719-1-xuanzhuo@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1636613527.8447719-1-xuanzhuo@linux.alibaba.com> Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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 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 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 > > > > --- > > > > 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. > >