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

[John Stultz]

On Wed, Jul 13, 2016 at 2:01 PM, Dmitry Shmidt <dimitrysh@xxxxxxxxxx> wrote:
> On Wed, Jul 13, 2016 at 1:52 PM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> On Wed, Jul 13, 2016 at 10:26:57PM +0200, Peter Zijlstra wrote:
>>> On Wed, Jul 13, 2016 at 04:18:23PM -0400, Tejun Heo wrote:
>>> > Hello, John.
>>> >
>>> > On Wed, Jul 13, 2016 at 01:13:11PM -0700, John Stultz wrote:
>>> > > On Wed, Jul 13, 2016 at 11:33 AM, Tejun Heo <tj@xxxxxxxxxx> wrote:
>>> > > > On Wed, Jul 13, 2016 at 02:21:02PM -0400, Tejun Heo wrote:
>>> > > >> One interesting thing to try would be replacing it with a regular
>>> > > >> non-percpu rwsem and see how it behaves.  That should easily tell us
>>> > > >> whether this is from actual contention or artifacts from percpu_rwsem
>>> > > >> implementation.
>>> > > >
>>> > > > So, something like the following.  Can you please see whether this
>>> > > > makes any difference?
>>> > >
>>> > > Yea. So this brings it down for me closer to what we're seeing with
>>> > > the Dmitry's patch reverting the two problematic commits, usually
>>> > > 10-50us with one early spike at 18ms.
>>> >
>>> > So, it's a percpu rwsem issue then.  I haven't really followed the
>>> > perpcpu rwsem changes closely.  Oleg, are multi-milisec delay expected
>>> > on down write expected with the current implementation of
>>> > percpu_rwsem?
>>>
>>> There is a synchronize_sched() in there, so sorta. That thing is heavily
>>> geared towards readers, as is the only 'sane' choice for global locks.
>>
>> Then one diagnostic step to take would be to replace that
>> synchronize_sched() with synchronize_sched_expedited(), and see if that
>> gets rid of the delays.
>>
>> Not a particularly real-time-friendly fix, but certainly a good check
>> on our various assumptions.
>
> All delays <200 us, but one that is 3 ms.

Yep. I'm seeing the same here too with Paul's change.

thanks
-john