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

[Dmitry Shmidt]

On Wed, Jul 13, 2016 at 7:42 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Jul 13, 2016 at 10:21:12AM +0200, Peter Zijlstra wrote:
>> On Tue, Jul 12, 2016 at 05:00:04PM -0700, John Stultz wrote:
>> > Hey Tejun,
>> >
>> >   So Dmitry Shmidt recently noticed that with 4.4 based systems we're
>> > seeing quite a bit of performance overhead from
>> > __cgroup_procs_write().
>> >
>> > With 4.4 tree as it stands, we're seeing __cgroup_procs_write() quite
>> > often take 10s of miliseconds to execute (with max times up in the
>> > 80ms range).
>> >
>> > While with 4.1 it was quite often in the single usec range, and max
>> > time values still in in sub-milisecond range.
>> >
>> > The majority of these performance regressions seem to come from the
>> > locking changes in:
>> >
>> > 3014dde762f6 ("cgroup: simplify threadgroup locking")
>> > and
>> > 1ed1328792ff  ("sched, cgroup: replace signal_struct->group_rwsem with
>> > a global percpu_rwsem")
>> >
>> > Dmitry has found that by reverting these two changes (which don't
>> > revert easiliy), we can get back down to tens 10-100 usec range for
>> > most calls, with max values occasionally spiking to ~18ms.
>> >
>> > Those two commits do talk about performance regressions, that were
>> > supposedly alleviated by percpu_rwsem changes, but I'm not sure we are
>> > seeing this.
>>
>> Do you have 'funny' RCU options that quickly force a grace period when
>> you go idle or something?
>>
>> But yes, it does not surprise me to find this commit is causing
>> problems.
>
> Hmmm...  Looks like RCU is present both before and after.  But please
> do send along your .config.
Attached

> Speaking of .config, is CONFIG_PREEMPT=y?  If so, does the workload
> feature preemption and migration?  If that is the case, you might be
> seeing contention on the per-CPU cgroup_threadgroup_rwsem, given that
> the second patch seems to be adding acquisitions.
CONFIG_PREEMPT=y is set.
We see this issue during the boot, so it supposes to be enough CPU load to
cause preemption and migration.

>                                                         Thanx, Paul
>

Attachment: .config
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