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

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Message-ID: <20160714064956.GC30909@twins.programming.kicks-ass.net>
Date:	Thu, 14 Jul 2016 08:49:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
 locking changes

On Wed, Jul 13, 2016 at 06:01:28PM -0400, Tejun Heo wrote:

> Technically, I think the lglock approach would be better here given
> the combination of requirements; however, it's quite a bit more code
> which would likely require some sophistications down the line (like
> blocking new readers first at the start of down_write). 

So the immediate problem with lg style locks is that the 'local' lock
will not stay local since these are preemptible locks we can get
migrations etc..

All fixable, but still.

> If we have to
> go there, we'll go there but for now I think it'd be simpler to
> conditionally switch to the expedited operations.  It can be a config
> option which is selected by !RT as you suggested.  If anyone hits an
> actual issue with that, we can go for the lglock thing.

So the main objection I have is that this isn't a fundamental fix, this
only cures things because Android only runs on small machines.

If someone with a big computer tries to do the same things we're up some
creek without no paddle. There's just no way we can make a global writer
'fast'.