Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 09, 2014 at 12:07:08PM -0700, bsegall@google.com wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> 
> > On Wed, Jul 09, 2014 at 09:07:53AM +0800, Yuyang Du wrote:
> >> That is chalenging... Can someone (Peter) grant us a lock of the remote rq? :)
> >
> > Nope :-).. we got rid of that lock for a good reason.
> >
> > Also, this is one area where I feel performance really trumps
> > correctness, we can fudge the blocked load a little. So the
> > sched_clock_cpu() difference is a strict upper bound on the
> > rq_clock_task() difference (and under 'normal' circumstances shouldn't
> > be much off).
> 
> Well, unless IRQ_TIME_ACCOUNTING or such is on, in which case you lose.
> Or am I misunderstanding the suggestion?

If its on its still an upper bound, and typically the difference is not
too large I think.

Since clock_task is the regular clock minus some local amount, the
difference between two regular clock reads is always a strict upper
bound on clock_task differences.

> Actually the simplest thing
> would probably be to grab last_update_time (which on 32-bit could be
> done with the _copy hack) and use that. Then I think the accuracy is
> only worse than current in that you can lose runnable load as well as
> blocked load, and that it isn't as easily corrected - currently if the
> blocked tasks wake up they'll add the correct numbers to
> runnable_load_avg, even if blocked_load_avg is screwed up and hit zero.
> This code would have to wait until it stabilized again.

The problem with that is that last_update_time is measured in
clock_task, and you cannot transfer these values between CPUs.
clock_task can drift unbounded between CPUs.