linux-kernel - Re: Cgroups "pids" controller does not update "pids.current" count immediately

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Message-ID: <20180614172757.ovu6sbolcgc4cdjr@gordon>
Date:   Fri, 15 Jun 2018 03:27:57 +1000
From:   Aleksa Sarai <asarai@...e.de>
To:     Ivan Zahariev <famzah@...soft.com>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Cgroups "pids" controller does not update "pids.current" count
 immediately

On 2018-06-15, Aleksa Sarai <asarai@...e.de> wrote:
> > I've tested this on 4.14.27 and 4.4.0-124-generic Ubuntu.
> > 
> > If I start a couple of processes which exit very quickly (like a simple Bash
> > script with many commands in it), the reported value in "pids.current" is
> > not updated immediately when processes exit. This leads to too many
> > processes incorrectly accounted in "pids.current" which hits the "pids.max"
> > prematurely.
> 
> One possible reason for this might be related to zombie processes.
> cgroup.procs doesn't include any zombie processes (tasks are removed
> when they exit(2)), but the pids controller does track zombies (tasks
> are removed when the 'struct task' is put'd). This could explain why
> there's a discrepancy which clears itself up after a short period of
> time -- though I am not sure that your reproducer will actually produce
> zombies (I only took a quick look at it).

Scratch that -- it can happen even without zombies. Basically it just
depends on when the 'task struct' is freed (which could happen
arbitrarily later than the process exit(2)-ed).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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