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

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.

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