linux-kernel - Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes

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Date:	Wed, 13 Jul 2016 10:21:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Dmitry Shmidt <dimitrysh@...gle.com>,
	Rom Lemarchand <romlem@...gle.com>,
	Colin Cross <ccross@...gle.com>, Todd Kjos <tkjos@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
 locking changes

On Tue, Jul 12, 2016 at 05:00:04PM -0700, John Stultz wrote:
> Hey Tejun,
> 
>   So Dmitry Shmidt recently noticed that with 4.4 based systems we're
> seeing quite a bit of performance overhead from
> __cgroup_procs_write().
> 
> With 4.4 tree as it stands, we're seeing __cgroup_procs_write() quite
> often take 10s of miliseconds to execute (with max times up in the
> 80ms range).
> 
> While with 4.1 it was quite often in the single usec range, and max
> time values still in in sub-milisecond range.
> 
> The majority of these performance regressions seem to come from the
> locking changes in:
> 
> 3014dde762f6 ("cgroup: simplify threadgroup locking")
> and
> 1ed1328792ff  ("sched, cgroup: replace signal_struct->group_rwsem with
> a global percpu_rwsem")
> 
> Dmitry has found that by reverting these two changes (which don't
> revert easiliy), we can get back down to tens 10-100 usec range for
> most calls, with max values occasionally spiking to ~18ms.
> 
> Those two commits do talk about performance regressions, that were
> supposedly alleviated by percpu_rwsem changes, but I'm not sure we are
> seeing this.

Do you have 'funny' RCU options that quickly force a grace period when
you go idle or something?

But yes, it does not surprise me to find this commit is causing
problems.