Re: [RFC][PATCH] sched: attach extra runtime to the right avg

[Peter Zijlstra <peterz@infradead.org>]

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.

Right..

> 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'.

Well, its a 0-sum. It doesn't drift unbounded. The difference will grow
up to 1023, at which point we'll account for it whole and we're back to
0.

The problem is that there's two states: running, blocked. And the
current scheme does not differentiate. We'll accrue the sub-block and
spill it into whatever state gets lucky.

Now, on average you'd hope that that works out and both running and
blocked get an equal number of spills pro-rata.

But apparently this isn't quite working out for Josef.

> 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.


> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 008c514dc241..b03703cd7989 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2965,7 +2965,7 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
>  	if (!delta)
>  		return 0;
>  
> -	sa->last_update_time += delta << 10;
> +	sa->last_update_time = now & ~1023ULL;
>  

So if we have a task that always runs <1024ns it should still get blocks
of runtime because the difference between now and now&~1023 can be !0
and spill.

I'm just not immediately seeing how its different from the 0-sum we had.
It should be identical since delta*1024 would equally land us on those
same edges (there's an offset in the differential form between the two,
but since we start with last_update_time=0, the resulting edges are the
same afaict).


*confused*