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Date:   Thu, 31 Oct 2019 10:53:43 +0000
From:   Qais Yousef <qais.yousef@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thara Gopinath <thara.gopinath@linaro.org>, mingo@redhat.com,
        ionela.voinescu@arm.com, vincent.guittot@linaro.org,
        rui.zhang@intel.com, edubezval@gmail.com, qperret@google.com,
        linux-kernel@vger.kernel.org, amit.kachhap@gmail.com,
        javi.merino@kernel.org, daniel.lezcano@linaro.org
Subject: Re: [Patch v4 4/6] sched/fair: update cpu_capcity to reflect thermal
 pressure
Message-ID: <20191031105342.b3sl5xhysldfla3g@e107158-lin.cambridge.arm.com>
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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?

If I managed to get it right somehow. It'd be nice if we can do inverse
conversion on delta_capacity so that avg_thermal.{load_avg, util_avg} meaning
is consistent across the board. But I don't feel strongly about it if this gets
documented properly.

Thanks

--
Qais Yousef