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

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Date:	Thu, 14 Jul 2016 10:51:37 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, 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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
 locking changes

On Thu, Jul 14, 2016 at 10:41 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 07/14, John Stultz wrote:
>>
>> I'm not supposed to be applying this on-top of
>> Paul's change, right?
>
> Right, unless I am totally confused,
>
>> > Just in case, could you try the patch below? Of course, without other
>> > optimizations from Peter, this change makes cgroup_threadgroup_rwsem
>> > much worse than a plain rw_semaphore.
>> >
>> > Oleg.
>> >
>> > --- x/kernel/cgroup.c
>> > +++ x/kernel/cgroup.c
>> > @@ -5605,6 +5605,8 @@ int __init cgroup_init(void)
>> >         BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files));
>> >         BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files));
>> >
>> > +       rcu_sync_enter(&cgroup_threadgroup_rwsem.rss);
>> > +
>>
>>
>> So adding this does make a huge difference ontop of Peter's patch.
>
> Ah, sorry for confusion. I meant, you could try this one-liner without
> any other changes.

So the one-liner without other changes helps at a similar level as
Paul's change. From some simple testing I've got a 3.5ms spike, but
otherwise the values are under 200us.

> But we will need the "slow mode optimization" part from Peter's patch
> anyway, otherwise percpu_rw_semaphore simply makes no sense for
> cgroup_threadgroup_rwsem.

Yea, with Peter's patch it is further improved.

thanks
-john