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

[Valentin Schneider <vschneid@redhat.com>]

I'm only seeing this now that it has hit tip/sched/core and I've had a
stroll through fair.c, apologies for this being late.

On 04/08/22 15:36, Qais Yousef wrote:
> +static inline int util_fits_cpu(unsigned long util,
> +				unsigned long uclamp_min,
> +				unsigned long uclamp_max,
> +				int cpu)
> +{
> +	unsigned long capacity_orig, capacity_orig_thermal;
> +	unsigned long capacity = capacity_of(cpu);
> +	bool fits, uclamp_max_fits;
> +
> +	/*
> +	 * Check if the real util fits without any uclamp boost/cap applied.
> +	 */
> +	fits = fits_capacity(util, capacity);
> +
> +	if (!uclamp_is_used())
> +		return fits;
> +
> +	/*
> +	 * 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.