Re: [PATCH v2 1/9] sched/uclamp: Fix relationship between uclamp and migration margin

[Valentin Schneider <vschneid@redhat.com>]

On 05/11/22 19:24, Qais Yousef wrote:
> On 11/04/22 17:35, Valentin Schneider wrote:
>
>> > +	/*
>> > +	 * We must use capacity_orig_of() for comparing against uclamp_min and
>> > +	 * uclamp_max. We only care about capacity pressure (by using
>> > +	 * capacity_of()) for comparing against the real util.
>> > +	 *
>> > +	 * If a task is boosted to 1024 for example, we don't want a tiny
>> > +	 * pressure to skew the check whether it fits a CPU or not.
>> > +	 *
>> > +	 * Similarly if a task is capped to capacity_orig_of(little_cpu), it
>> > +	 * should fit a little cpu even if there's some pressure.
>> > +	 *
>> > +	 * Only exception is for thermal pressure since it has a direct impact
>> > +	 * on available OPP of the system.
>> > +	 *
>> > +	 * We honour it for uclamp_min only as a drop in performance level
>> > +	 * could result in not getting the requested minimum performance level.
>> > +	 *
>>
>> Why specifically care about OPPs here? Per our CPU capacity model, a task
>> alone on a CPUx throttled to f=fmax/2 and a task coscheduled on a CPUy with
>> RT/DL tasks and/or IRQs such that cpu_capacity(CPUy) = 50% are both getting
>> (roughly) the same performance level.
>
> Depends how you define performance level. What you call performance level,
> I think is better called bandwidth. Uclamp is a performance and not a bandwidth
> hint.
>
> If a 10% task:
>
>       p->util_avg = 10% * 1024
>
> is requesting max performance level
>
>       p->uclamp_min = 1024
>
> This will translate to running at highest frequency and in case of big.LITTLE
> system, the biggest CPU too.
>
> RT/DL pressure has no impact in the task being able to achieve this; that is
> running at max frequency and biggest cpu.
>
> If the cpu has no bandwidth to fit this task, then our usual comparison of
> util_avg with capacity_of() should fail as usual.
>

Ok so we *do* have this with how the fitting criteria are combined (I
didn't get that when I first scanned through the code); thanks for
elaborating on that.

> In the example above, the RT/DL pressure has to be pretty high for the 10% task
> not to fit from bandwidth point of view. Which has nothing to do with
> uclamp_min.  Only thermal pressure which drops OPPs can actually affect the
> uclamp_min hint/request.
>
> That is, when the task runs it will run at maximum frequency regardless of the
> RT/DL pressure. The fact that the bandwidth of the CPU can be stolen has
> nothing to do with uclamp_min hint.
>
>
> Thanks!
>
> --
> Qais Yousef