From: Shakeel Butt <shakeelb@google.com>
To: Roman Gushchin <guro@fb.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
Date: Wed, 29 Aug 2018 14:30:16 -0700 [thread overview]
Message-ID: <CALvZod5_kMtsiQqdEmktL3zMEf_3LL+_1khdr+TST2vFTChiVA@mail.gmail.com> (raw)
In-Reply-To: <20180829212422.GA13097@castle>
On Wed, Aug 29, 2018 at 2:24 PM Roman Gushchin <guro@fb.com> wrote:
>
> 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>
> >
> > BTW this makes a very good use-case for optimizing kmem uncharging
> > similar to what you did for skmem uncharging.
>
> The only thing I'm slightly worried here is that it can make
> reclaiming of memory cgroups harder. Probably, it's still ok,
> but let me first finish the work I'm doing on optimizing the
> whole memcg reclaim process, and then return to this case.
>
Yes, maybe we can disable that optimization for offlined memcgs.
Anyways, we can discuss this later as you have suggested.
Shakeel
next prev parent reply other threads:[~2018-08-29 21:30 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-21 21:35 [PATCH v2 1/3] mm: rework memcg kernel stack accounting Roman Gushchin
2018-08-21 21:35 ` [PATCH v2 2/3] mm: drain memcg stocks on css offlining Roman Gushchin
2018-08-21 21:35 ` [PATCH v2 3/3] mm: don't miss the last page because of round-off error Roman Gushchin
2018-08-21 22:10 ` [PATCH v2 1/3] mm: rework memcg kernel stack accounting Shakeel Butt
2018-08-21 22:15 ` Roman Gushchin
2018-08-29 21:24 ` Roman Gushchin
2018-08-29 21:30 ` Shakeel Butt [this message]
2018-08-22 14:12 ` Michal Hocko
2018-08-23 16:23 ` Roman Gushchin
2018-08-24 7:52 ` Michal Hocko
2018-08-24 12:50 ` Johannes Weiner
2018-08-24 15:42 ` Roman Gushchin
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