linux-kernel - Re: [PATCH 1/2] mm: memcontrol: flush percpu vmstats before releasing memcg

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Date:   Tue, 13 Aug 2019 14:27:52 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Roman Gushchin <guro@...com>
Cc:     <linux-mm@...ck.org>, Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH 1/2] mm: memcontrol: flush percpu vmstats before
 releasing memcg

On Mon, 12 Aug 2019 15:29:10 -0700 Roman Gushchin <guro@...com> wrote:

> Percpu caching of local vmstats with the conditional propagation
> by the cgroup tree leads to an accumulation of errors on non-leaf
> levels.
> 
> Let's imagine two nested memory cgroups A and A/B. Say, a process
> belonging to A/B allocates 100 pagecache pages on the CPU 0.
> The percpu cache will spill 3 times, so that 32*3=96 pages will be
> accounted to A/B and A atomic vmstat counters, 4 pages will remain
> in the percpu cache.
> 
> Imagine A/B is nearby memory.max, so that every following allocation
> triggers a direct reclaim on the local CPU. Say, each such attempt
> will free 16 pages on a new cpu. That means every percpu cache will
> have -16 pages, except the first one, which will have 4 - 16 = -12.
> A/B and A atomic counters will not be touched at all.
> 
> Now a user removes A/B. All percpu caches are freed and corresponding
> vmstat numbers are forgotten. A has 96 pages more than expected.
> 
> As memory cgroups are created and destroyed, errors do accumulate.
> Even 1-2 pages differences can accumulate into large numbers.
> 
> To fix this issue let's accumulate and propagate percpu vmstat
> values before releasing the memory cgroup. At this point these
> numbers are stable and cannot be changed.
> 
> Since on cpu hotplug we do flush percpu vmstats anyway, we can
> iterate only over online cpus.
> 
> Fixes: 42a300353577 ("mm: memcontrol: fix recursive statistics correctness & scalabilty")

Is this not serious enough for a cc:stable?