Date:   Mon, 07 Nov 2022 18:58:23 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org, Xuewen Yan <xuewen.yan94@...il.com>,
        Lukasz Luba <lukasz.luba@....com>, Wei Wang <wvw@...gle.com>,
        Jonathan JMChen <Jonathan.JMChen@...iatek.com>,
        Hank <han.lin@...iatek.com>
Subject: Re: [PATCH v2 9/9] sched/fair: Consider capacity inversion in
 util_fits_cpu()

On 05/11/22 20:41, Qais Yousef wrote:
> On 11/04/22 17:35, Valentin Schneider wrote:
>> On 04/08/22 15:36, Qais Yousef wrote:
>> > We do consider thermal pressure in util_fits_cpu() for uclamp_min only.
>> > With the exception of the biggest cores which by definition are the max
>> > performance point of the system and all tasks by definition should fit.
>> >
>> > Even under thermal pressure, the capacity of the biggest CPU is the
>> > highest in the system and should still fit every task. Except when it
>> > reaches capacity inversion point, then this is no longer true.
>> >
>> > We can handle this by using the inverted capacity as capacity_orig in
>> > util_fits_cpu(). Which not only addresses the problem above, but also
>> > ensure uclamp_max now considers the inverted capacity. Force fitting
>> > a task when a CPU is in this adverse state will contribute to making the
>> > thermal throttling last longer.
>> >
>> > Signed-off-by: Qais Yousef <qais.yousef@....com>
>> > ---
>> >  kernel/sched/fair.c | 14 +++++++++-----
>> >  1 file changed, 9 insertions(+), 5 deletions(-)
>> >
>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> > index cb32dc9a057f..77ae343e32a3 100644
>> > --- a/kernel/sched/fair.c
>> > +++ b/kernel/sched/fair.c
>> > @@ -4293,12 +4293,16 @@ static inline int util_fits_cpu(unsigned long util,
>> >        * For uclamp_max, we can tolerate a drop in performance level as the
>> >        * goal is to cap the task. So it's okay if it's getting less.
>> >        *
>> > -	 * In case of capacity inversion, which is not handled yet, we should
>> > -	 * honour the inverted capacity for both uclamp_min and uclamp_max all
>> > -	 * the time.
>> > +	 * In case of capacity inversion we should honour the inverted capacity
>> > +	 * for both uclamp_min and uclamp_max all the time.
>> >        */
>> > -	capacity_orig = capacity_orig_of(cpu);
>> > -	capacity_orig_thermal = capacity_orig - arch_scale_thermal_pressure(cpu);
>> > +	capacity_orig = cpu_in_capacity_inversion(cpu);
>> > +	if (capacity_orig) {
>> > +		capacity_orig_thermal = capacity_orig;
>> > +	} else {
>> > +		capacity_orig = capacity_orig_of(cpu);
>> > +		capacity_orig_thermal = capacity_orig - arch_scale_thermal_pressure(cpu);
>> > +	}
>> >
>>
>> IIUC the rq->cpu_capacity_inverted computation in update_cpu_capacity() can be
>> summarised as:
>>
>> - If there is a PD with equal cap_orig, but higher effective (orig - thermal)
>>   capacity
>>   OR
>>   there is a PD with pd_cap_orig > cpu_effective_cap:
>>   rq->cpu_capacity_inverted = capacity_orig - thermal_load_avg(rq)
>>
>> - Else:
>>   rq->cpu_capacity_inverted = 0
>>
>> Then, the code above uses either rq->cpu_capacity_inverted if it is
>> non-zero, otherwise:
>>
>>   capacity_orig - arch_scale_thermal_pressure(cpu);
>>
>> Why use average thermal pressure in one case, and use instantaneous
>> thermal pressure in the other?
>
> There was a big debate on [1] about using avg vs instantaneous.
>

Interesting thread, thanks for the link!

> I used avg for detecting inversion to be consistent with using average in in
> scale_rt_capacity(). I didn't want the inversion state to be flipping too
> quickly too.
>
> I used the instantaneous in the other check based on that discussion. It seemed
> using the average is hurtful when for example the medium drops an OPP and by
> not reacting quickly at wake up we lose the chance to place it on a big; which
> if my memory didn't fail me is what Xuewen was seeing.
>

OK So IIUC by using the inst. pressure you start excluding CPUs sooner, and
with the avg pressure you keep those CPUs out (if the pressure remained
long enough).

> [1] https://lore.kernel.org/lkml/24631a27-42d9-229f-d9b0-040ac993b749@arm.com/
>

>>
>> Can't we get rid of rq->cpu_capacity_inverted and replace this whole thing
>> with an unconditional
>>
>>   capacity_orig_thermal = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu));
>>
>> ?
>
> I can't see how we end up with equivalent behavior then. Or address the
> concerns raised by Xuewen and Lukasz on the RT thread in regards to avg vs
> instantaneous.
>
> Specifically, if we don't use the new rq->cpu_capacity_inverted we can't handle
> the case where the task is requesting to run at maximum performance but a small
> drop in thermal pressure means it won't fit anywhere. That PD is the best fit
> until it hits an inversion.
>
> Originally I wanted to defer handling thermal pressure into a different series.
> But Vincent thought it's better to handle it now. We want more data points from
> more systems tbh. But I think what we have now is still a good improvement over
> what we had before.
>
> Lukasz had a patch [2] which could allow making thermal_load_avg() more
> acceptable for systems that care about faster response times.
>
> [2] https://lore.kernel.org/lkml/20220429091245.12423-1-lukasz.luba@arm.com/
>
>
> Thanks
>
> --
> Qais Yousef

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