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 19:05:50 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>, John Stultz <john.stultz@...aro.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 07/14, Peter Zijlstra wrote:
>
> On Thu, Jul 14, 2016 at 04:58:44PM +0200, Oleg Nesterov wrote:
> >
> > But note that we do not need RCU_NONE. All we need is the trivial change
> > below.
>
> Hurm, maybe. So having that unbalanced keeps us in GP_PASSED state and
> since we'll never drop gp_count back to 0 nothing will ever happen.

Yes, exactly. Of course, this is a bit ugly, I'll try to send some
rcu-sync cleanups/simplifications later.

Perhaps we can even add a "cgroup migration is not that cold as it was
supposed" boot option for this rcu_sync_enter() for now, not sure.

> > Damn, I am trying to find my old rcu-sync patches which I didn't
> > send, but can't... OK, this almost off-topic right now, just this "enter"
> > is ugly and we can't switch the slow/fast modes dynamically.
> >
> > The rest of you patch is "optimize the slow path" and we already discussed
> > it before, I personally like it. Perhaps you can redo it without RCU_NONE
> > part?
>
> Indeed, I rebased that patch on top of the current tree and had to add
> support for down_trylock() but otherwise much the same thing.
>
> I can send it out again.

Great, thanks! This should fix the problem.

Oleg.