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

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