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

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Message-ID: <20180615190724.GX1351649@devbig577.frc2.facebook.com>
Date:   Fri, 15 Jun 2018 12:07:24 -0700
From:   Tejun Heo <tj@...nel.org>
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

Hello, Ivan.

On Fri, Jun 15, 2018 at 08:40:02PM +0300, Ivan Zahariev wrote:
> The lazy pids accounting + modern fast CPUs makes the "pids.current"
> metric practically unusable for resource limiting in our case. For a
> test, when we started and ended one single process very quickly, we
> saw "pids.current" equal up to 185 (while the correct value at all
> time is either 0 or 1). If we want that a "cgroup" can spawn maximum
> 50 processes, we should use some high value like 300 for "pids.max",
> in order to compensate the pids uncharge lag (and this depends on
> the speed of the CPU and how busy the system is).

Yeah, that actually makes a lot of sense.  We can't keep everything
synchronous for obvious performance reasons but we definitely can wait
for RCU grace period before failing.  Forking might become a bit
slower while pids are draining but shouldn't fail and that shouldn't
incur any performance overhead in normal conditions when pids aren't
constrained.

Thanks.

-- 
tejun