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Date:   Wed, 19 Feb 2020 11:31:39 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>, Tejun Heo <tj@kernel.org>,
        Roman Gushchin <guro@fb.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH] mm: memcontrol: asynchronous reclaim for memory.high
Message-Id: <20200219113139.ee60838bc7eb35747eb330fa@linux-foundation.org>
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        <20200219183731.GC11847@dhcp22.suse.cz>
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On Wed, 19 Feb 2020 19:37:31 +0100 Michal Hocko <mhocko@kernel.org> wrote:

> On Wed 19-02-20 13:12:19, Johannes Weiner wrote:
> > We have received regression reports from users whose workloads moved
> > into containers and subsequently encountered new latencies. For some
> > users these were a nuisance, but for some it meant missing their SLA
> > response times. We tracked those delays down to cgroup limits, which
> > inject direct reclaim stalls into the workload where previously all
> > reclaim was handled my kswapd.
> 
> I am curious why is this unexpected when the high limit is explicitly
> documented as a throttling mechanism.

Yes, this sounds like a feature-not-a-bug.

But what was the nature of these stalls?  If they were "stuck in D
state waiting for something" then that's throttling.  If they were
"unexpected bursts of in-kernel CPU activity" then I see a better case.