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Date:   Fri, 12 Apr 2019 20:15:38 +0000
From:   Roman Gushchin <guro@...com>
To:     Shakeel Butt <shakeelb@...gle.com>
CC:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH 3/4] mm: memcontrol: fix recursive statistics correctness
 & scalabilty

On Fri, Apr 12, 2019 at 12:55:10PM -0700, Shakeel Butt wrote:
> On Fri, Apr 12, 2019 at 8:15 AM Johannes Weiner <hannes@...xchg.org> wrote:
> >
> > Right now, when somebody needs to know the recursive memory statistics
> > and events of a cgroup subtree, they need to walk the entire subtree
> > and sum up the counters manually.
> >
> > There are two issues with this:
> >
> > 1. When a cgroup gets deleted, its stats are lost. The state counters
> > should all be 0 at that point, of course, but the events are not. When
> > this happens, the event counters, which are supposed to be monotonic,
> > can go backwards in the parent cgroups.
> >
> 
> We also faced this exact same issue as well and had the similar solution.
> 
> > 2. During regular operation, we always have a certain number of lazily
> > freed cgroups sitting around that have been deleted, have no tasks,
> > but have a few cache pages remaining. These groups' statistics do not
> > change until we eventually hit memory pressure, but somebody watching,
> > say, memory.stat on an ancestor has to iterate those every time.
> >
> > This patch addresses both issues by introducing recursive counters at
> > each level that are propagated from the write side when stats change.
> >
> > Upward propagation happens when the per-cpu caches spill over into the
> > local atomic counter. This is the same thing we do during charge and
> > uncharge, except that the latter uses atomic RMWs, which are more
> > expensive; stat changes happen at around the same rate. In a sparse
> > file test (page faults and reclaim at maximum CPU speed) with 5 cgroup
> > nesting levels, perf shows __mod_memcg_page state at ~1%.
> >
> 
> (Unrelated to this patchset) I think there should also a way to get
> the exact memcg stats. As the machines are getting bigger (more cpus
> and larger basic page size) the accuracy of stats are getting worse.
> Internally we have an additional interface memory.stat_exact for that.
> However I am not sure in the upstream kernel will an additional
> interface is better or something like /proc/sys/vm/stat_refresh which
> sync all per-cpu stats.

I was thinking about eventually consistent counters: sync them periodically
from a worker thread. It should keep the cost of reading small, but
should increase the accuracy. Will it work for you?

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