Re: [PATCH v4 1/3] sched/fair: add util_est on top of PELT

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 06, 2018 at 06:33:15PM +0000, Patrick Bellasi wrote:

> Good point, I was actually expecting this question and I should have
> added it to the cover letter, sorry.
> 
> The reasoning was: the task's estimated utilization is defined as the
> max between PELT and the "estimation". Where "estimation" is
> the max between EWMA and the last ENQUEUED utilization.
> 
> Thus I was envisioning these two calls:
> 
>    _task_util_est := max(EWMA, ENQUEUED)
>     task_util_est := max(util_avg, _task_util_est)
> 
> but since now we have clients only for the first API, I've not added
> the second one. Still I would prefer to keep the "_" to make it clear
> that's and util_est's internal signal, not the actual task's estimated
> utilization.
> 
> Does it make sense?
> 
> Do you prefer I just remove the "_" and we will refactor it once we
> should add a customer for the proper task's util_est?

Hurm... I was thinking:

  task_util_est := max(util_avg, EWMA)

But the above mixes ENQUEUED into it.. *confused*.

Also, I think I'm confused by the 'enqueued' name... it makes sense for
the cfs use-case, where we track the sum of all 'enqueued' tasks, but it
doesn't make sense for the task use-case where it tracks task_util() at
dequeue time.

> > > +/*
> > > + * Check if the specified (signed) value is within a specified margin,
> > > + * based on the observation that:
> > > + *     abs(x) < y := (unsigned)(x + y - 1) < (2 * y - 1)
> > > + */
> > > +static inline bool within_margin(long value, unsigned int margin)
> > 
> > This mixing of long and int is dodgy, do we want to consistently use int
> > here?
> 
> Right, perhaps better "unsigned int" for both params, isn't?

Can't, must be signed, since @value is supposed to be able to be
negative remember? ;-)


> > > +	WRITE_ONCE(cfs_rq->avg.util_est.enqueued, util_est);

> > > +	WRITE_ONCE(p->se.avg.util_est.enqueued, util_last);
> > 
> > Two stores to that word... can we fix that nicely?
> 
> Good point, the single word comes from the goal to fit into the same
> cache line of sched_avg.

Its the above two stores I confuzzled to be the same. So n/m all that.

> > > +SCHED_FEAT(UTIL_EST, false)
> > 
> > Since you couldn't measure it, do we wants it true?
> 
> I'm just a single tester so far, I would be more confident once
> someone volunteer to turn this on to give a better coverage.

Lets just enable it by default, that way its far more likely someone
will complain about it :-), disabling it again is a trivial matter when
needed.


Also, your patch 2/3 have insufficient READ_ONCE()s.