Re: [Patch v8 4/7] sched/fair: Enable periodic update of average thermal pressure

[Vincent Guittot <vincent.guittot@linaro.org>]

On Fri, 24 Jan 2020 at 16:37, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>
> On 17/01/2020 16:39, Vincent Guittot wrote:
> > On Fri, 17 Jan 2020 at 15:55, Peter Zijlstra <peterz@infradead.org> wrote:
> >>
> >> On Fri, Jan 17, 2020 at 02:22:51PM +0100, Vincent Guittot wrote:
> >>> On Thu, 16 Jan 2020 at 16:15, Peter Zijlstra <peterz@infradead.org> wrote:
> >>
> >>>>
> >>>> That there indentation trainwreck is a reason to rename the function.
> >>>>
> >>>>         decayed = update_rt_rq_load_avg(now, rq, curr_class == &rt_sched_class) |
> >>>>                   update_dl_rq_load_avg(now, rq, curr_class == &dl_sched_class) |
> >>>>                   update_thermal_load_avg(rq_clock_task(rq), rq, thermal_pressure) |
> >>>>                   update_irq_load_avg(rq, 0);
> >>>>
> >>>> Is much better.
> >>>>
> >>>> But now that you made me look at that, I noticed it's using a different
> >>>> clock -- it is _NOT_ using now/rq_clock_pelt(), which means it'll not be
> >>>> in sync with the other averages.
> >>>>
> >>>> Is there a good reason for that?
> >>>
> >>> We don't need to apply frequency and cpu capacity invariance on the
> >>> thermal capping signal which is  what rq_clock_pelt does
> >>
> >> Hmm, I suppose that is true, and that really could've done with a
> >> comment. Now clock_pelt is sort-of in sync with clock_task, but won't it
> >> still give weird artifacts by having it on a slightly different basis?
> >
> > No we should not. Weird artifacts happens when we
> > add/subtract/propagate signals between each other and then apply pelt
> > algorithm on the results. In the case of thermal signal, we only add
> > it to others to update cpu_capacity but pelt algo is then not applied
> > on it. The error because of some signals being at segment boundaries
> > whereas others are not, is limited to 2% and doesn't accumulate over
> > time.
> >
> >>
> >> Anyway, looking at this, would it make sense to remove the @now argument
> >> from update_*_load_avg()? All those functions already take @rq, and
> >> rq_clock_*() are fairly trivial inlines.
> >
> > TBH I was thinking of doing the opposite for update_irq_load_avg which
> > hides the clock that is used for irq_avg. This helps to easily
> > identify which signals use the exact same clock and can be mixed to
> > create a new pelt signal and which can't
>
> The 'now' argument is one thing but why not:
>
> -int update_thermal_load_avg(u64 now, struct rq *rq, u64 capacity)
> +int update_thermal_load_avg(u64 now, struct rq *rq)
>  {
> +       u64 capacity = arch_cpu_thermal_pressure(cpu_of(rq));
> +
>         if (___update_load_sum(now, &rq->avg_thermal,
>
> This would make the call-sites __update_blocked_others() and
> task_tick(_fair)() cleaner.

I prefer to keep the capacity as argument. This is more aligned with
others that provides the value of the signal to apply

>
> I guess the argument is not to pollute pelt.c. But it already contains

you've got it. I don't want to pollute the pelt.c file with things not
related to pelt but thermal as an example.

> arch_scale_[freq|cpu]_capacity() for irq.
>
>