linux-kernel - Re: [Patch v4 4/6] sched/fair: update cpu

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Message-ID: <e875ef90-d561-4eee-4951-6556ac89c6a2@arm.com>
Date:   Thu, 31 Oct 2019 16:38:21 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qais.yousef@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Thara Gopinath <thara.gopinath@...aro.org>, mingo@...hat.com,
        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 v4 4/6] sched/fair: update cpu_capcity to reflect thermal
 pressure

On 31.10.19 11:53, 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@...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 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.

So why can't we use rq->avg_thermal.util_avg here? Since capacity is
closer to util than to load?

Is it because you want to use the influence of ___update_load_sum(...,
unsigned long load eq. per-cpu delta_capacity in your signal?

Why not call it this way then?

diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
index 38210691c615..d3035457483f 100644
--- a/kernel/sched/pelt.c
+++ b/kernel/sched/pelt.c
@@ -357,9 +357,9 @@ int update_thermal_load_avg(u64 now, struct rq *rq,
u64 capacity)
 {
        if (___update_load_sum(now, &rq->avg_thermal,
                               capacity,
-                              capacity,
-                              capacity)) {
-               ___update_load_avg(&rq->avg_thermal, 1, 1);
+                              0,
+                              0)) {
+               ___update_load_avg(&rq->avg_thermal, 1, 0);
                return 1;
        }