Messages in this thread Patch in this message | | | Date | Wed, 13 Jul 2016 13:52:11 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes |
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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@kernel.org> 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.
Thanx, Paul
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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)
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