linux-kernel - Re: [Patch v5 4/6] sched/fair: update cpu

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Message-ID: <20191106174135.gsmnwpwxfarywded@e107158-lin.cambridge.arm.com>
Date:   Wed, 6 Nov 2019 17:41:35 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Thara Gopinath <thara.gopinath@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, ionela.voinescu@....com,
        vincent.guittot@...aro.org, rui.zhang@...el.com,
        edubezval@...il.com, qperret@...gle.com,
        linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        javi.merino@...nel.org, daniel.lezcano@...aro.org
Subject: Re: [Patch v5 4/6] sched/fair: update cpu_capcity to reflect thermal
 pressure

On 11/06/19 12:31, Thara Gopinath wrote:
> On 11/06/2019 11:56 AM, Qais Yousef wrote:
> > On 11/05/19 13:49, 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.
> >>
> >> Other signals that are deducted from cpu_capacity to reflect the actual
> >> capacity available are rt and dl util_avg. util_avg tracks a binary signal
> >> and uses the weights 1024 and 0. Whereas thermal pressure is tracked
> >> using load_avg. load_avg uses the actual "delta" capacity as the weight.
> > 
> > I think you intended to put this as comment...
> > 
> >>
> >> Signed-off-by: Thara Gopinath <thara.gopinath@...aro.org>
> >> ---
> >>  kernel/sched/fair.c | 1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 9fb0494..5f6c371 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -7738,6 +7738,7 @@ 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);
> > 
> > ... here?
> 
> I did not!  But I can.
> > 
> > I find the explanation hard to parse too. Do you think you can rephrase it?
> > Something based on what you wrote here would be more understandable IMHO:
> > https://lore.kernel.org/lkml/5DBB05BC.40502@linaro.org/
> I will try to rephrase it! I am sorry that you found it hard to parse.
> Honestly, I cannot copy paste the code snippet I pointed out to you here
> in comment.(And I think that is the reason you found it easier to
> understand) But I will try my best to put it in words.

No worries. The problem could be me :-)

But a comment in the code is very important as util_avg + load_avg is confusing
without a comment. I wouldn't expect both signal to be compatible but the
thermal one is special. A comment explaining why it's special is all we need.


Thanks

--
Qais Yousef