linux-kernel - Re: [PATCH v2 9/9] sched/fair: Consider capacity inversion in util_fits

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Date:   Wed, 9 Nov 2022 11:43:58 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qyousef@...alina.io>,
        Valentin Schneider <vschneid@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        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()

- Qais Yousef <qais.yousef@....com>

On 05/11/2022 21:41, Qais Yousef wrote:
> On 11/04/22 17:35, Valentin Schneider wrote:
>> On 04/08/22 15:36, Qais Yousef wrote:

[...]

>> 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.
> 
> 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.
> 
> [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.

I can't see the rationale in using:

!inversion: `cap_orig - instantaneous thermal pressure`

 inversion: `cap_orig - PELT thermal pressure`

I can see that there was a lot of discussion on this topic but hardly
any agreement IMHO.

AFAICS, the 2 capacity inversion patches just appeared in v2 and haven't
seen any review yet I'm afraid.


[...]