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

From: Thara Gopinath <thara.gopinath@linaro.org>
To: Qais Yousef <qais.yousef@arm.com>, Peter Zijlstra <peterz@infradead.org>
Cc: 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
Date: Thu, 31 Oct 2019 12:03:08 -0400	[thread overview]
Message-ID: <5DBB05BC.40502@linaro.org> (raw)
In-Reply-To: <20191031105342.b3sl5xhysldfla3g@e107158-lin.cambridge.arm.com>

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.

-- 
Warm Regards
Thara