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Date: Thu, 14 Jul 2016 14:04:28 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        John Stultz <john.stultz@linaro.org>, Ingo Molnar <mingo@redhat.com>,
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        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
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On Thu, Jul 14, 2016 at 07:35:05AM -0400, Tejun Heo wrote:
> Hello,
> 
> On Wed, Jul 13, 2016 at 04:04:04PM -0700, Paul E. McKenney wrote:
> > commit b4edebb8f5664a3a51be1e3ff3d7f1cb2d3d5c88
> > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Date:   Wed Jul 13 15:13:31 2016 -0700
> > 
> >     rcu: Provide RCUSYNC_EXPEDITE option for rcusync.expedited default
> >     
> >     This commit provides an RCUSYNC_EXPEDITE Kconfig option that specifies
> >     the default value for the rcusync.expedited kernel parameter.  This
> >     makes it easier to use rcusync.expedited functionality in cases where
> >     specifying kernel boot parameters should be avoided.
> >     
> >     Reported-by: John Stultz <john.stultz@linaro.org>
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >     Tested-by: John Stultz <john.stultz@linaro.org>
> >     Acked-by: John Stultz <john.stultz@linaro.org>
> >     Cc: <stable@vger.kernel.org> # 4.4.x-
> 
> I think it probably makes sense to make this the default on !RT at
> least with a separate patch w/o stable cc'd.  While most use cases
> will be fine with the latency on write path, it also means that the
> reader side is blocked for the duration which can hurt.  rwsem implies
> a lot more readers and thus more read lock operations than writes.
> It's weird to trade off higher latency for lower cpu usage when it
> would also slow down all readers.

NAK, no expedited muck by default. There's more than just RT that
doesn't like IPI sprays.