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 X-Spam-Level: X-Spam-Status: No, score=-12.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 328B7C4361B for ; Wed, 9 Dec 2020 17:32:53 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 9DF1223CD1 for ; Wed, 9 Dec 2020 17:32:52 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9DF1223CD1 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 110598D002D; Wed, 9 Dec 2020 12:32:52 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 0C2E68D002C; Wed, 9 Dec 2020 12:32:52 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F413D8D002D; Wed, 9 Dec 2020 12:32:51 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0075.hostedemail.com [216.40.44.75]) by kanga.kvack.org (Postfix) with ESMTP id DFBE88D002C for ; Wed, 9 Dec 2020 12:32:51 -0500 (EST) Received: from smtpin20.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 989523629 for ; Wed, 9 Dec 2020 17:32:51 +0000 (UTC) X-FDA: 77574438942.20.egg78_1607a15273f1 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin20.hostedemail.com (Postfix) with ESMTP id 7B5F9180C07AF for ; Wed, 9 Dec 2020 17:32:51 +0000 (UTC) X-HE-Tag: egg78_1607a15273f1 X-Filterd-Recvd-Size: 9891 Received: from mail-ej1-f65.google.com (mail-ej1-f65.google.com [209.85.218.65]) by imf06.hostedemail.com (Postfix) with ESMTP for ; Wed, 9 Dec 2020 17:32:50 +0000 (UTC) Received: by mail-ej1-f65.google.com with SMTP id n26so3300281eju.6 for ; Wed, 09 Dec 2020 09:32:50 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=pCTu9SYS4xAxD6fbbi1HzsiCoR2Dn8r2H8c6zyF7DTI=; b=RDQPsXg/3vW9HZUFWux/wmqLbdK5YLbvHnMQAGT/VPDCsX77rEmD/49Xpf0BPbqWzi veyHP8O/LbBWcN3gnRO4YJ3eknxH+VVlPCV1PzsyE03xgqqfjhx1ZhBzpmwLn2MEuzJR w6DX5SND8/JKdEQSkaoVLxeno798Xz9gWj8HqY04jEtUwvjSvskoIxxlAiyIoQd/KwKI RP/N+hVKcoyo2L/rHovy8nJTtvekg/emT/+ofSDko9yblBzKHuJX4XVmm5Hg3n58ypPW tKktiwWBpf+1iPDoC11RAeVBugE6YARtx/3tbk9fVZpbR2nSi8LsSyZpAum4Qu+P971b 5fNA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=pCTu9SYS4xAxD6fbbi1HzsiCoR2Dn8r2H8c6zyF7DTI=; b=oNW23S63ZUgUc50LoYWT31ScQwxgYY9RwgFNX3bbU1AKACtf7DnIFbfp4VOOpuDYCs NaIpeNRUMFyCYJfGXR5C+9jHSFi3hDKAlcwbYZtWkSj5pew3kr9MLU5AgiOjmCpVINuH H5ZSVRmYw3TEErHTy/cOXKPsIIBi/ijsqiTQ9rNfmC5GOgxlm/CvxSotw/CHFyT369kX iK2/j9xRCk8iMK5zkHLYnamFeaFCzqhPORET8gZVA831QYvgSetHyqAC3rIJbjrazalN ZASJT/VnNhwe4Xer4vNwnWGvnAoDDMI8fehvZoao2zClQ+0sEdVmcNzrKEy9f82PmxiY MRCA== X-Gm-Message-State: AOAM532kwGqOwIQxKbXspzCuBr5Rc7DuPORd6xKsFMb3vIv4etAXC+N4 CGj5A1ZottKcm9rGlEh6sweMLpFe96DjrUZmP4k= X-Google-Smtp-Source: ABdhPJyX22yOKJwAo5RsbC9l4EZnf1ndp1QDw/Wv2fCvk/9ohTuzEt/5+LZi6rr83b5nFfCsUevVc9AV8nE5HdfzJbs= X-Received: by 2002:a17:907:20a4:: with SMTP id pw4mr2907593ejb.499.1607535169602; Wed, 09 Dec 2020 09:32:49 -0800 (PST) MIME-Version: 1.0 References: <20201202182725.265020-1-shy828301@gmail.com> <20201202182725.265020-7-shy828301@gmail.com> <49464720-675d-5144-043c-eba6852a9c06@virtuozzo.com> In-Reply-To: From: Yang Shi Date: Wed, 9 Dec 2020 09:32:37 -0800 Message-ID: Subject: Re: [PATCH 6/9] mm: vmscan: use per memcg nr_deferred of shrinker To: Kirill Tkhai Cc: Roman Gushchin , Shakeel Butt , Dave Chinner , Johannes Weiner , Michal Hocko , Andrew Morton , Linux MM , Linux FS-devel Mailing List , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Dec 9, 2020 at 7:42 AM Kirill Tkhai wrote: > > On 08.12.2020 20:13, Yang Shi wrote: > > On Thu, Dec 3, 2020 at 3:40 AM Kirill Tkhai wrote: > >> > >> On 02.12.2020 21:27, Yang Shi wrote: > >>> Use per memcg's nr_deferred for memcg aware shrinkers. The shrinker's nr_deferred > >>> will be used in the following cases: > >>> 1. Non memcg aware shrinkers > >>> 2. !CONFIG_MEMCG > >>> 3. memcg is disabled by boot parameter > >>> > >>> Signed-off-by: Yang Shi > >>> --- > >>> mm/vmscan.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++++---- > >>> 1 file changed, 82 insertions(+), 6 deletions(-) > >>> > >>> diff --git a/mm/vmscan.c b/mm/vmscan.c > >>> index cba0bc8d4661..d569fdcaba79 100644 > >>> --- a/mm/vmscan.c > >>> +++ b/mm/vmscan.c > >>> @@ -203,6 +203,12 @@ static DECLARE_RWSEM(shrinker_rwsem); > >>> static DEFINE_IDR(shrinker_idr); > >>> static int shrinker_nr_max; > >>> > >>> +static inline bool is_deferred_memcg_aware(struct shrinker *shrinker) > >>> +{ > >>> + return (shrinker->flags & SHRINKER_MEMCG_AWARE) && > >>> + !mem_cgroup_disabled(); > >>> +} > >>> + > >>> static int prealloc_memcg_shrinker(struct shrinker *shrinker) > >>> { > >>> int id, ret = -ENOMEM; > >>> @@ -271,7 +277,58 @@ static bool writeback_throttling_sane(struct scan_control *sc) > >>> #endif > >>> return false; > >>> } > >>> + > >>> +static inline long count_nr_deferred(struct shrinker *shrinker, > >>> + struct shrink_control *sc) > >>> +{ > >>> + bool per_memcg_deferred = is_deferred_memcg_aware(shrinker) && sc->memcg; > >>> + struct memcg_shrinker_deferred *deferred; > >>> + struct mem_cgroup *memcg = sc->memcg; > >>> + int nid = sc->nid; > >>> + int id = shrinker->id; > >>> + long nr; > >>> + > >>> + if (!(shrinker->flags & SHRINKER_NUMA_AWARE)) > >>> + nid = 0; > >>> + > >>> + if (per_memcg_deferred) { > >>> + deferred = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_deferred, > >>> + true); > >> > >> My comment is about both 5/9 and 6/9 patches. > > > > Sorry for the late reply, I don't know why Gmail filtered this out to spam. > > > >> > >> shrink_slab_memcg() races with mem_cgroup_css_online(). A visibility of CSS_ONLINE flag > >> in shrink_slab_memcg()->mem_cgroup_online() does not guarantee that you will see > >> memcg->nodeinfo[nid]->shrinker_deferred != NULL in count_nr_deferred(). This may occur > >> because of processor reordering on !x86 (there is no a common lock or memory barriers). > >> > >> Regarding to shrinker_map this is not a problem due to map check in shrink_slab_memcg(). > >> The map can't be NULL there. > >> > >> Regarding to shrinker_deferred you should prove either this is not a problem too, > >> or to add proper synchronization (maybe, based on barriers) or to add some similar check > >> (maybe, in shrink_slab_memcg() too). > > > > It seems shrink_slab_memcg() might see shrinker_deferred as NULL > > either due to the same reason. I don't think there is a guarantee it > > won't happen. > > > > We just need guarantee CSS_ONLINE is seen after shrinker_maps and > > shrinker_deferred are allocated, so I'm supposed barriers before > > "css->flags |= CSS_ONLINE" should work. > > > > So the below patch may be ok: > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index df128cab900f..9f7fb0450d69 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -5539,6 +5539,12 @@ static int mem_cgroup_css_online(struct > > cgroup_subsys_state *css) > > return -ENOMEM; > > } > > > > > > + /* > > + * Barrier for CSS_ONLINE, so that shrink_slab_memcg() sees > > shirnker_maps > > + * and shrinker_deferred before CSS_ONLINE. > > + */ > > + smp_mb(); > > + > > /* Online state pins memcg ID, memcg ID pins CSS */ > > refcount_set(&memcg->id.ref, 1); > > css_get(css); > > smp barriers synchronize data access from different cpus. They should go in a pair. > In case of you add the smp barrier into mem_cgroup_css_online(), we should also > add one more smp barrier in another place, which we want to synchonize with this. > Also, every place should contain a comment referring to its pair: "Pairs with...". Thanks, I think you are correct. Looked into it further, it seems the race pattern looks like: CPU A CPU B store shrinker_maps pointer load CSS_ONLINE store CSS_ONLINE load shrinker_maps pointer By checking the memory-barriers document, it seems we need write barrier/read barrier pair as below: CPU A CPU B store shrinker_maps pointer load CSS_ONLINE store CSS_ONLINE load shrinker_maps pointer So, the patch should look like: diff --git a/mm/memcontrol.c b/mm/memcontrol.c index df128cab900f..489c0a84f82b 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5539,6 +5539,13 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css) return -ENOMEM; } + /* + * Barrier for CSS_ONLINE, so that shrink_slab_memcg() sees shirnker_maps + * and shrinker_deferred before CSS_ONLINE. It pairs with the read barrier + * in shrink_slab_memcg(). + */ + smp_wmb(); + /* Online state pins memcg ID, memcg ID pins CSS */ refcount_set(&memcg->id.ref, 1); css_get(css); diff --git a/mm/vmscan.c b/mm/vmscan.c index 9d2a6485e982..fc9bda576d98 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -603,13 +603,15 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, if (!mem_cgroup_online(memcg)) return 0; + /* Pairs with write barrier in mem_cgroup_css_online */ + smp_rmb(); + if (!down_read_trylock(&shrinker_rwsem)) return 0; + /* Once memcg is online it can't be NULL */ map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map, true); - if (unlikely(!map)) - goto unlock; for_each_set_bit(i, map->map, shrinker_nr_max) { struct shrink_control sc = { Does this seem correct? > > Kirill >