Re: [PATCH 2/4] sched: cpufreq: Fix apply_dvfs_headroom() escaping uclamp constraints

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 26/08/2023 22:08, Qais Yousef wrote:
> On 08/21/23 18:39, Dietmar Eggemann wrote:
>> On 20/08/2023 23:06, Qais Yousef wrote:
>>> DVFS headroom is applied after we calculate the effective_cpu_util()
>>> which is where we honour uclamp constraints. It makes more sense to
>>> apply the headroom there once and let all users naturally get the right
>>> thing without having to sprinkle the call around in various places.
>>
>> uclamp is applied in effective_cpu_util(..., FREQUENCY_UTIL, ...) which
>> IMHO currently has 2 power callers: (1) schedutil: sugov_get_util() and
>> (2) EAS: eenv_pd_max_util()
>>
>>> Before this fix running
>>>
>>> 	uclampset -M 800 cat /dev/zero > /dev/null
>>>
>>> Will cause the test system to run at max freq of 2.8GHz. After the fix
>>> it runs at 2.2GHz instead which is the correct value that matches the
>>> capacity of 800.
>>
>> IMHO, a system at util = 800 (w/o uclamp) would also run at 2.8Ghz since
>> we would call map_util_to_perf() on 800, no matter from where we call it.
> 
> Sorry, I would very strongly disagree here. What you're saying the effective
> range of uclamp_max is 800 and anything above that will always go to max. How
> can this be acceptable?

No that's not what I wanted to say here.

I wanted to highlight the different treatment of `(1) a task with
(natural) util = 800` and `(2) a task with uclamp_max = 800`.

(1) util = 800

util = (1.25 * 800 * (1024 - irq) / 1024 + ...

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


(2) uclamp_max = 800

util = (800 * (1024 - irq) / 1024 + ...

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

So for (1) the scheduler would ask for more than in (2).

That's essentially the same question which was raised here:

https://lkml.kernel.org/r/CAKfTPtDY48jpO+b-2KXawzxh-ty+FMKX6YUXioNR7kpgO=ua6Q@mail.gmail.com

>>> Note that similar problem exist for uclamp_min. If util was 50, and
>>> uclamp_min is 100. Since we apply_dvfs_headroom() after apply uclamp
>>> constraints, we'll end up with util of 125 instead of 100. IOW, we get
>>> boosted twice, first time by uclamp_min, and second time by dvfs
>>> headroom.
>>
>> I see what you want to change here but:
>>
>> So far we have `util -> uclamp -> map_util_to_perf()`
> 
> :-O
> 
> So when I set the system uclamp_max to 800 it will still run at max; and this
> is normal?!!

No that's an issue (A) as well. But the diff between (1) and (2) is IMHO a
new issue introduced by this patch-set.

>> which is fine when we see uclamp as an entity which constrains util, not
>> the util after being mapped to a capacity constraint.
> 
> -ENOPARSE.

What I meant is 'clamping the util' before scheduler hands over to
'cpufreq' is fine:

    util -> uclamp -> map_util_to_perf()
               
    scheduler    -->|<-- cpufreq

I do understand that you guys are already discussing a new
cpufreq_give_me_freq_for_this_utilization_ctx() between EM and CPUfreq
in the other thread of this patch to maybe sort out (A).

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