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

[Qais Yousef <qyousef@layalina.io>]

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.

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