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Date:   Tue, 21 Aug 2018 15:15:32 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Shakeel Butt <shakeelb@google.com>
CC:     Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        <kernel-team@fb.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, <luto@kernel.org>,
        Konstantin Khlebnikov <koct9i@gmail.com>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v2 1/3] mm: rework memcg kernel stack accounting
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On Tue, Aug 21, 2018 at 03:10:52PM -0700, Shakeel Butt wrote:
> On Tue, Aug 21, 2018 at 2:36 PM Roman Gushchin <guro@fb.com> wrote:
> >
> > If CONFIG_VMAP_STACK is set, kernel stacks are allocated
> > using __vmalloc_node_range() with __GFP_ACCOUNT. So kernel
> > stack pages are charged against corresponding memory cgroups
> > on allocation and uncharged on releasing them.
> >
> > The problem is that we do cache kernel stacks in small
> > per-cpu caches and do reuse them for new tasks, which can
> > belong to different memory cgroups.
> >
> > Each stack page still holds a reference to the original cgroup,
> > so the cgroup can't be released until the vmap area is released.
> >
> > To make this happen we need more than two subsequent exits
> > without forks in between on the current cpu, which makes it
> > very unlikely to happen. As a result, I saw a significant number
> > of dying cgroups (in theory, up to 2 * number_of_cpu +
> > number_of_tasks), which can't be released even by significant
> > memory pressure.
> >
> > As a cgroup structure can take a significant amount of memory
> > (first of all, per-cpu data like memcg statistics), it leads
> > to a noticeable waste of memory.
> >
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> 
> Reviewed-by: Shakeel Butt <shakeelb@google.com>

Thanks!

> 
> BTW this makes a very good use-case for optimizing kmem uncharging
> similar to what you did for skmem uncharging.

Good point!
Let me prepare the patch.