linux-kernel - Re: [PATCH v2 2/3] mm: memcontrol: flush percpu slab vmstats on kmem offlining

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Message-ID: <20190819224645.GA9473@tower.dhcp.thefacebook.com>
Date:   Mon, 19 Aug 2019 22:46:50 +0000
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH v2 2/3] mm: memcontrol: flush percpu slab vmstats on kmem
 offlining

On Mon, Aug 19, 2019 at 03:27:44PM -0700, Andrew Morton wrote:
> On Mon, 19 Aug 2019 13:23:37 -0700 Roman Gushchin <guro@...com> wrote:
> 
> > I've noticed that the "slab" value in memory.stat is sometimes 0,
> > even if some children memory cgroups have a non-zero "slab" value.
> > The following investigation showed that this is the result
> > of the kmem_cache reparenting in combination with the per-cpu
> > batching of slab vmstats.
> > 
> > At the offlining some vmstat value may leave in the percpu cache,
> > not being propagated upwards by the cgroup hierarchy. It means
> > that stats on ancestor levels are lower than actual. Later when
> > slab pages are released, the precise number of pages is substracted
> > on the parent level, making the value negative. We don't show negative
> > values, 0 is printed instead.
> > 
> > To fix this issue, let's flush percpu slab memcg and lruvec stats
> > on memcg offlining. This guarantees that numbers on all ancestor
> > levels are accurate and match the actual number of outstanding
> > slab pages.
> > 
> > Fixes: fb2f2b0adb98 ("mm: memcg/slab: reparent memcg kmem_caches on cgroup removal")
> > Signed-off-by: Roman Gushchin <guro@...com>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Michal Hocko <mhocko@...nel.org>
> > Cc: Vladimir Davydov <vdavydov.dev@...il.com>
> 
> [1/3] and [3/3] have cc:stable.  [2/3] does not.  However [3/3] does
> not correctly apply without [2/3] having being applied.

Right, [2/3] is required by slab kmem reparenting, which appeared in 5.3.

I can rearrange [2/3] and [3/3] so that first two patches will have
cc table and apply correctly. Let me do this, I'll send v3 shortly.

Thanks!