Re: [Patch v4 4/6] sched/fair: update cpu_capcity to reflect thermal pressure

Qais Yousef
2019-10-31T16:57:03Z

On 10/31/19 12:03, Thara Gopinath wrote:
> On 10/31/2019 06:53 AM, Qais Yousef wrote:
> > On 10/28/19 16:30, Peter Zijlstra wrote:
> >> On Wed, Oct 23, 2019 at 01:28:40PM +0100, Qais Yousef wrote:
> >>> On 10/22/19 16:34, Thara Gopinath wrote:
> >>>> cpu_capacity relflects the maximum available capacity of a cpu. Thermal
> >>>> pressure on a cpu means this maximum available capacity is reduced. This
> >>>> patch reduces the average thermal pressure for a cpu from its maximum
> >>>> available capacity so that cpu_capacity reflects the actual
> >>>> available capacity.
> >>>>
> >>>> Signed-off-by: Thara Gopinath <thara.gopinath@linaro.org>
> >>>> ---
> >>>> kernel/sched/fair.c | 1 +
> >>>> 1 file changed, 1 insertion(+)
> >>>>
> >>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>>> index 4f9c2cb..be3e802 100644
> >>>> --- a/kernel/sched/fair.c
> >>>> +++ b/kernel/sched/fair.c
> >>>> @@ -7727,6 +7727,7 @@ static unsigned long scale_rt_capacity(struct sched_domain *sd, int cpu)
> >>>>
> >>>> used = READ_ONCE(rq->avg_rt.util_avg);
> >>>> used += READ_ONCE(rq->avg_dl.util_avg);
> >>>> + used += READ_ONCE(rq->avg_thermal.load_avg);
> >>>
> >>> Maybe a naive question - but can we add util_avg with load_avg without
> >>> a conversion? I thought the 2 signals have different properties.
> >>
> >> Changelog of patch #1 explains, it's in that dense blob of text.
> >>
> >> But yes, you're quite right that that wants a comment here.
> >
> > Thanks for the pointer! A comment would be nice indeed.
> >
> > To make sure I got this correctly - it's because avg_thermal.load_avg
> > represents delta_capacity which is already a 'converted' form of load. So this
> > makes avg_thermal.load_avg a util_avg really. Correct?
> Hello Quais,
>
> Sorry for not replying to your earlier email. Thanks for the review.
> So if you look at the code, util_sum in calculated as a binary signal
> converted into capacity. Check out the the below snippet from accumulate_sum
>
> if (load)
> sa->load_sum += load * contrib;
> if (runnable)
> sa->runnable_load_sum += runnable * contrib;
> if (running)
> sa->util_sum += contrib << SCHED_CAPACITY_SHIFT;
>
> So the actual delta for the thermal pressure will never be considered
> if util_avg is used.
>
> I will update this patch with relevant comment.

Okay thanks for the explanation.

--
Qais Yousef