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.
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.
Thanx, Paul
