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

[Dmitry Shmidt]

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.

>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> index be922c9f3d37..211acddc7e21 100644
> --- a/kernel/rcu/sync.c
> +++ b/kernel/rcu/sync.c
> @@ -38,19 +38,19 @@ static const struct {
>  #endif
>  } gp_ops[] = {
>         [RCU_SYNC] = {
> -               .sync = synchronize_rcu,
> +               .sync = synchronize_rcu_expedited,
>                 .call = call_rcu,
>                 .wait = rcu_barrier,
>                 __INIT_HELD(rcu_read_lock_held)
>         },
>         [RCU_SCHED_SYNC] = {
> -               .sync = synchronize_sched,
> +               .sync = synchronize_sched_expedited,
>                 .call = call_rcu_sched,
>                 .wait = rcu_barrier_sched,
>                 __INIT_HELD(rcu_read_lock_sched_held)
>         },
>         [RCU_BH_SYNC] = {
> -               .sync = synchronize_rcu_bh,
> +               .sync = synchronize_rcu_bh_expedited,
>                 .call = call_rcu_bh,
>                 .wait = rcu_barrier_bh,
>                 __INIT_HELD(rcu_read_lock_bh_held)
>