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SubjectRe: [RFC][PATCH] sched: attach extra runtime to the right avg
On Sun, Jul 02, 2017 at 11:37:18AM +0200, Ingo Molnar wrote:
> * josef@toxicpanda.com <josef@toxicpanda.com> wrote:
>
> > From: Josef Bacik <jbacik@fb.com>
> >
> > We only track the load avg of a se in 1024 ns chunks, so in order to
> > make up for the loss of the < 1024 ns part of a run/sleep delta we only
> > add the time we processed to the se->avg.last_update_time. The problem
> > is there is no way to know if this extra time was while we were asleep
> > or while we were running. Instead keep track of the remainder and apply
> > it in the appropriate place. If the remainder was while we were
> > running, add it to the delta the next time we update the load avg while
> > running, and the same for sleeping. This (coupled with other fixes)
> > mostly fixes the regression to my workload introduced by Peter's
> > experimental runnable load propagation patches.
> >
> > Signed-off-by: Josef Bacik <jbacik@fb.com>
>
> > @@ -2897,12 +2904,16 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
> > * Use 1024ns as the unit of measurement since it's a reasonable
> > * approximation of 1us and fast to compute.
> > */
> > + remainder = delta & (1023UL);
> > + sa->last_update_time = now;
> > + if (running)
> > + sa->run_remainder = remainder;
> > + else
> > + sa->sleep_remainder = remainder;
> > delta >>= 10;
> > if (!delta)
> > return 0;
> >
> > - sa->last_update_time += delta << 10;
> > -
>
> So I'm wondering, this chunk changes how sa->last_update_time is maintained in
> ___update_load_avg(): the new code takes a precise timestamp, but the old code was
> not taking an imprecise timestamp, but was updating it via deltas - where each
> delta was rounded down to the nearest 1024 nsecs boundary.
>
> That, if this is the main code path that updates ->last_update_time, creates a
> constant drift of rounding error that skews ->last_update_time into larger and
> larger distances from the real 'now' - ever increasing the value of 'delta'.
>
> An intermediate approach to improve that skew would be something like below. It
> doesn't track the remainder like your patch does, but doesn't lose precision
> either, just rounds down 'now' to the nearest 1024 boundary.
>
> Does this fix the regression you observed as well? Totally untested.
>

Yup this fixes my problem as well, I'm good with this if you prefer this
approach. Thanks,

Josef

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