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=-15.3 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,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 D2BC9C433FE for ; Thu, 10 Dec 2020 15:19:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 91CCF2310E for ; Thu, 10 Dec 2020 15:19:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391653AbgLJPTP (ORCPT ); Thu, 10 Dec 2020 10:19:15 -0500 Received: from relay.sw.ru ([185.231.240.75]:58460 "EHLO relay3.sw.ru" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726090AbgLJPTD (ORCPT ); Thu, 10 Dec 2020 10:19:03 -0500 Received: from [192.168.15.68] by relay3.sw.ru with esmtp (Exim 4.94) (envelope-from ) id 1knNhZ-00CZ4E-Tz; Thu, 10 Dec 2020 18:17:54 +0300 Subject: Re: [PATCH 6/9] mm: vmscan: use per memcg nr_deferred of shrinker To: Johannes Weiner , Yang Shi Cc: Roman Gushchin , Shakeel Butt , Dave Chinner , 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> <20201210151331.GD264602@cmpxchg.org> From: Kirill Tkhai Message-ID: <6ffd6aa1-2c55-f4d3-a60a-56786d40531a@virtuozzo.com> Date: Thu, 10 Dec 2020 18:17:54 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <20201210151331.GD264602@cmpxchg.org> 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 10.12.2020 18:13, Johannes Weiner wrote: > On Wed, Dec 09, 2020 at 09:32:37AM -0800, Yang Shi wrote: >> 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(); > > Is there a reason why the shrinker allocations aren't done in > .css_alloc()? That would take care of all necessary ordering: The reason is that allocations have to be made in a place, where mem-cgroup_iter() can't miss it, since memcg_expand_shrinker_maps() shouldn't miss allocated shrinker maps. > > #0 > css = ->css_alloc() > list_add_tail_rcu(css, parent->children) > rcu_assign_pointer() > ->css_online(css) > css->flags |= CSS_ONLINE > > #1 > memcg = mem_cgroup_iter() > list_entry_rcu() > rcu_dereference() > shrink_slab(.., memcg) > > RCU ensures that once the cgroup shows up in the reclaim cgroup it's > also fully allocated. > >> /* 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; > > ...then we should be able to delete this online check here entirely: > > A not-yet online cgroup is guaranteed to have a shrinker map, just > with no bits set. The shrinker code handles that just fine. > > An offlined cgroup will eventually have an empty bitmap as the called > shrinkers return SHRINK_EMPTY. This could also be shortcut by clearing > the bit in memcg_drain_list_lru_node() the same way we set it in the > parent when we move all objects upward, but seems correct as-is. >