Re: [PATCH 1/1] sched/cputime: Mitigate performance regression in times()/clock_gettime()

[Stanislaw Gruszka]

On Mon, Aug 15, 2016 at 05:13:30PM +0800, Wanpeng Li wrote:
> 2016-08-12 20:10 GMT+08:00 Stanislaw Gruszka <sgruszka@xxxxxxxxxx>:
> > Hi
> >
> > On Wed, Aug 10, 2016 at 01:26:41PM +0200, Ingo Molnar wrote:
> >> Nice detective work! I'm wondering, where do we stand if compared with a
> >> pre-6e998916dfe3 kernel?
> >>
> >> I admit this is a difficult question: 6e998916dfe3 does not revert cleanly and I
> >> suspect v3.17 does not run easily on a recent distro. Could you attempt to revert
> >> the bad effects of 6e998916dfe3 perhaps, just to get numbers - i.e. don't try to
> >> make the result correct, just see what the performance gap is, roughly.
> >>
> >> If there's still a significant gap then it might make sense to optimize this some
> >> more.
> >
> > I measured (partial) revert performance on 4.7 using mmtest instructions
> > from Giovanni and also tested some other possible fix (draft version):
> >
> > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> > index 75f98c5..54fdf6d 100644
> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -294,6 +294,8 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
> >         unsigned int seq, nextseq;
> >         unsigned long flags;
> >
> > +       (void) task_sched_runtime(tsk);
> > +
> >         rcu_read_lock();
> >         /* Attempt a lockless read on the first round. */
> >         nextseq = 0;
> > @@ -308,7 +310,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
> >                         task_cputime(t, &utime, &stime);
> >                         times->utime += utime;
> >                         times->stime += stime;
> > -                       times->sum_exec_runtime += task_sched_runtime(t);
> > +                       times->sum_exec_runtime += t->se.sum_exec_runtime;
> 
> If this will not have updated stats for other threads?

No, until tick/sched() on CPUs running threads.

Stanislaw