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=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable 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 E67DAC19425 for ; Wed, 9 Dec 2020 15:43:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BB47723B31 for ; Wed, 9 Dec 2020 15:43:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730428AbgLIPmx (ORCPT ); Wed, 9 Dec 2020 10:42:53 -0500 Received: from relay.sw.ru ([185.231.240.75]:45694 "EHLO relay3.sw.ru" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726431AbgLIPmq (ORCPT ); Wed, 9 Dec 2020 10:42:46 -0500 Received: from [192.168.15.177] by relay3.sw.ru with esmtp (Exim 4.94) (envelope-from ) id 1kn1b1-00CQdb-IE; Wed, 09 Dec 2020 18:41:39 +0300 Subject: Re: [PATCH 6/9] mm: vmscan: use per memcg nr_deferred of shrinker To: Yang Shi Cc: Roman Gushchin , Shakeel Butt , Dave Chinner , Johannes Weiner , Michal Hocko , Andrew Morton , Linux MM , Linux FS-devel Mailing List , Linux Kernel Mailing List References: <20201202182725.265020-1-shy828301@gmail.com> <20201202182725.265020-7-shy828301@gmail.com> <49464720-675d-5144-043c-eba6852a9c06@virtuozzo.com> From: Kirill Tkhai Message-ID: Date: Wed, 9 Dec 2020 18:41:49 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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...". Kirill