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Tsirkin" To: Xuan Zhuo Subject: Re: [PATCH v4 0/3] virtio support cache indirect desc Message-ID: <20211110075412-mutt-send-email-mst@kernel.org> References: <20211108084732-mutt-send-email-mst@kernel.org> <1636382860.765897-1-xuanzhuo@linux.alibaba.com> <20211110074326-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: <20211110074326-mutt-send-email-mst@kernel.org> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: netdev@vger.kernel.org, "David S. Miller" , Jakub Kicinski , virtualization@lists.linux-foundation.org X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" 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 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 > > --- > > 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. _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 53729C433EF for ; Wed, 10 Nov 2021 12:54:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2C4D66115B for ; Wed, 10 Nov 2021 12:54:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231336AbhKJM5l (ORCPT ); Wed, 10 Nov 2021 07:57:41 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:43019 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231162AbhKJM5l (ORCPT ); Wed, 10 Nov 2021 07:57:41 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1636548893; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=z4n9H695I56p3uFL/xN9H+OG/zilvbb6UbzgWj/opIc=; b=QH4jiq2SrVUlG8S4LwZ9y6IjFFzS7m9RproP5uavjpRqOy6AORCCD71A+ZPkFsLUicS9wd E/5/24iJHk5uPnCrEmlvtf6I+wKiUB+9FeXHDT3ngS82TR+N9ZCyD8h/3kjJCi9NtfpfhF 6RT8KCirO9j6y7R/E0GCZRfQbACn1m0= Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-403-ymMU1VNPMb-oG2OESB-qWg-1; Wed, 10 Nov 2021 07:54:52 -0500 X-MC-Unique: ymMU1VNPMb-oG2OESB-qWg-1 Received: by mail-ed1-f70.google.com with SMTP id y12-20020a056402270c00b003e28de6e995so2226807edd.11 for ; Wed, 10 Nov 2021 04:54:52 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=z4n9H695I56p3uFL/xN9H+OG/zilvbb6UbzgWj/opIc=; b=hxTE6wpwWb3Vkamr5opTkLXvz774Yd0ws57uabKcFOsm3CulmvzVxiAVoenxkqNYsu GTarpEtiKsFX3MXYXv2lPMjbpZKxygeRqj9L1NeGY8Jjblv4tcJRELV7LwpUGTE/gsAA sz6+tt5pjk66O3VdoXEBpvImyg9WrJB6jcLOLQut6cZ9ZP/EVJOg4WEEMNE7Nc99HHFu I8Ve3MtsyKSMolx3r9CVR4bSqVMRO92wLRzN6p/+j8VNoBsbujKikYXYKuHUn/cZkag6 l/GApKmqioy4/8dOU3bzOJoK944IXNkpSfww9s8K0OZ0q9QU4tAyoJzAi53LDlThEcNI D9aA== X-Gm-Message-State: AOAM530y1zoPa/nal488X4xIgW/OdnvSCqFJlSf7NFS/caCShgtJ0yBT +2XV6GK6FyThvXppmPv6F80MlNh05fMt73WRR6yH1sFNoIo12V/41LH7yJni2mf/ZEnFofv8HG4 1Dv9ey3OGl7LEKxLT X-Received: by 2002:aa7:cd99:: with SMTP id x25mr21043883edv.249.1636548891080; Wed, 10 Nov 2021 04:54:51 -0800 (PST) X-Google-Smtp-Source: ABdhPJw50M4PLj6z/Dq+XVHSq0+qJ6iYuzTlHJFOEl/jotgYfNBocUvMNCYW+Use7QYwyxpd7H5SiQ== X-Received: by 2002:aa7:cd99:: with SMTP id x25mr21043861edv.249.1636548890882; Wed, 10 Nov 2021 04:54:50 -0800 (PST) Received: from redhat.com ([2.55.133.41]) by smtp.gmail.com with ESMTPSA id r19sm12535963edt.54.2021.11.10.04.54.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 10 Nov 2021 04:54:50 -0800 (PST) Date: Wed, 10 Nov 2021 07:54:46 -0500 From: "Michael S. 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: <20211110075412-mutt-send-email-mst@kernel.org> References: <20211108084732-mutt-send-email-mst@kernel.org> <1636382860.765897-1-xuanzhuo@linux.alibaba.com> <20211110074326-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211110074326-mutt-send-email-mst@kernel.org> Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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 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 > > --- > > 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.