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

From: Tejun Heo <tj@kernel.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	John Stultz <john.stultz@linaro.org>,
	Ingo Molnar <mingo@redhat.com>,
	lkml <linux-kernel@vger.kernel.org>,
	Dmitry Shmidt <dimitrysh@google.com>,
	Rom Lemarchand <romlem@google.com>,
	Colin Cross <ccross@google.com>, Todd Kjos <tkjos@google.com>,
	Oleg Nesterov <oleg@redhat.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes
Date: Wed, 13 Jul 2016 18:01:28 -0400	[thread overview]
Message-ID: <20160713220128.GA15005@htj.duckdns.org> (raw)
In-Reply-To: <20160713211841.GQ7094@linux.vnet.ibm.com>

Hello, Paul.

On Wed, Jul 13, 2016 at 02:18:41PM -0700, Paul E. McKenney wrote:
> On Wed, Jul 13, 2016 at 05:05:26PM -0400, Tejun Heo wrote:
> > On Wed, Jul 13, 2016 at 02:03:15PM -0700, Paul E. McKenney wrote:
> > > Take the patch that I just sent out and make the choice of normal
> > > vs. expedited depend on CONFIG_PREEMPT_RT or whatever the -rt guys are
> > > calling it these days.  Is there a low-latency Kconfig option other
> > > than CONFIG_NO_HZ_FULL?
> > 
> > Sounds like a plan to me.
> 
> I like the way we like each other's idea.  Mutually assured laziness?  ;-)

Heh, indeed. :)

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).  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.

Thanks.

-- 
tejun