Re: [PATCH v5 06/15] sched/core: uclamp: enforce last task UCLAMP_MAX

[Peter Zijlstra]

On Mon, Oct 29, 2018 at 06:33:01PM +0000, Patrick Bellasi wrote:
> When a util_max clamped task sleeps, its clamp constraints are removed
> from the CPU. However, the blocked utilization on that CPU can still be
> higher than the max clamp value enforced while that task was running.
> 
> The release of a util_max clamp when a CPU is going to be idle could
> thus allow unwanted CPU frequency increases while tasks are not
> running. This can happen, for example, when a frequency update is
> triggered from another CPU of the same frequency domain.
> In this case, when we aggregate the utilization of all the CPUs in a
> shared frequency domain, schedutil can still see the full not clamped
> blocked utilization of all the CPUs and thus, eventually, increase the
> frequency.

> @@ -810,6 +811,28 @@ static inline void uclamp_cpu_update(struct rq *rq, unsigned int clamp_id)
>  		if (max_value >= SCHED_CAPACITY_SCALE)
>  			break;
>  	}
> +
> +	/*
> +	 * Just for the UCLAMP_MAX value, in case there are no RUNNABLE
> +	 * task, we want to keep the CPU clamped to the last task's clamp
> +	 * value. This is to avoid frequency spikes to MAX when one CPU, with
> +	 * an high blocked utilization, sleeps and another CPU, in the same
> +	 * frequency domain, do not see anymore the clamp on the first CPU.
> +	 *
> +	 * The UCLAMP_FLAG_IDLE is set whenever we detect, from the above
> +	 * loop, that there are no more RUNNABLE taks on that CPU.
> +	 * In this case we enforce the CPU util_max to that of the last
> +	 * dequeued task.
> +	 */
> +	if (max_value < 0) {
> +		if (clamp_id == UCLAMP_MAX) {
> +			rq->uclamp.flags |= UCLAMP_FLAG_IDLE;
> +			max_value = last_clamp_value;
> +		} else {
> +			max_value = uclamp_none(UCLAMP_MIN);
> +		}
> +	}
> +
>  	rq->uclamp.value[clamp_id] = max_value;
>  }

*groan*, so it could be jet-lag, but I find the comment really hard to
understand.

Would not something like:

	/*
	 * Avoid blocked utilization pushing up the frequency when we go
	 * idle (which drops the max-clamp) by retaining the last known
	 * max-clamp.
	 */

Be more clear?