linux-kernel - Re: [PATCH 2/4] sched: cpufreq: Fix apply_dvfs

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Date:   Tue, 12 Sep 2023 15:40:00 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qyousef@...alina.io>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Lukasz Luba <lukasz.luba@....com>
Subject: Re: [PATCH 2/4] sched: cpufreq: Fix apply_dvfs_headroom() escaping
 uclamp constraints

On 10/09/2023 19:46, Qais Yousef wrote:
> On 09/08/23 16:30, Vincent Guittot wrote:
> 

[...]

>>>> above 512 whatever the current (720)  formula or your proposal (608).
>>>> In the case of uclamp, it should be applied after having been scaled
>>>> by irq time.
>>>
>>> I lost you a bit here. I'm not sure how you reached the 720 and 608 numbers.
>>
>> My bad, I finally decided to use an irq pressure of 128 in my
>> calculation but forgot to change the value in my email
>>
>>>
>>> So the way I'm proposing it here
>>>
>>>         util = cfs + rt + dvfs_headroom(cfs+rt) = 800 + 0.25 * 800 = 1000
>>>         util = uclamp_rq_util_with(rq, util, NULL) = 512
>>>         util = scale_rq_capacity(512, 256, 1024) = 0.75 * 512 = 384
>>>         util += dvfs_headroom(irq) = 384 + 256 + 0.25 * 256 = 704
>>>         util += dvfs_headroom(dl_bw) = 704
>>>
>>>>
>>>> So we should have reported utilization of 720 with a bandwidth
>>>> requirement of 512 and then cpufreq can applies its headroom if needed
>>>
>>> The key part I'm changing is this
>>>
>>>         util = cfs + rt + dvfs_headroom(cfs+rt) = 800 + 0.25 * 800 = 1000
>>>         util = uclamp_rq_util_with(rq, util, NULL) = 512
>>>
>>> Before we had (assume irq, rt and dl are 0 for simplicity and a single task is
>>> running)
>>>
>>>         util = cfs = 800
>>>         util = uclamp_rq_util_with(rq, util, NULL) = 512
>>>         util = dvfs_headroom(util) = 512 * 0.25 * 512 = 640
>>>
>>> So we are running higher than we are allowed to. So applying the headroom
>>> after taking uclamp constraints into account is the problem.

I'm not sure I understood all the example math in this thread correctly:

Examples:

irq = 128 or 256

util = 800 uclamp = 512


--- current code:

((util_cfs + util_rt) * ((max - irq) / max) + irq + dl_bw) * scale

<- uclamped(cfs+rt) ->

<--               scale_irq_capacity()                  -->|<-- map_util_perf() 
                                                               / (headroom())  

irq = 128: (512 * (1024 - 128) / 1024 + 128 + 0) * 1.25 = 576 * 1.25 = 720

irq = 256: (512 * (1024 - 256) / 1024 + 256 + 0) * 1.25 = 640 * 1.25 = 800


--- new approach:

irq = 128: (512 * (1024 - 128) / 1024 + 128 + 0.25 * 128)            = 608

irq = 256: (512 * (1024 - 256) / 1024 + 256 + 0.25 * 256)            = 704

            <->
            uclamped(cfs+rt+headroom(cfs+rt))

            <- scale_irq_capacity() ->

            <--               headroom(irq) ?        -->


Is the correct?

[...]