Re: [PATCH 3/3] sched/fair: schedutil: explicit update only when required

[Peter Zijlstra]

On Thu, May 10, 2018 at 04:05:53PM +0100, Patrick Bellasi wrote:
> All the above considered, let's make schedutil updates more explicit in
> fair.c by removing the cfs_rq_util_change() wrapper function in favour
> of the existing cpufreq_update_util() public API.
> This can be done by calling cpufreq_update_util() explicitly in the few
> call sites where it really makes sense and when all the (potentially)
> required cfs_rq's information have been updated.

Aside from having to redraw my ever stale diagrams _again_ I don't think
I object too much here. As you write tracking the exact point where we
did do the update was fairly tedious.

> @@ -5397,9 +5366,27 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  		update_cfs_group(se);
>  	}
>  
> +	/* The task is visible from the root cfs_rq */
> +	if (!se) {
> +		unsigned int flags = 0;

That one shadows the @flags argument. Some checker is bound to complain
about it.

> +
>  		add_nr_running(rq, 1);
>  
> +		if (p->in_iowait)
> +			flags |= SCHED_CPUFREQ_IOWAIT;
> +
> +		/*
> +		 * !last_update_time means we've passed through
> +		 * migrate_task_rq_fair() indicating we migrated.
> +		 *
> +		 * IOW we're enqueueing a task on a new CPU.
> +		 */
> +		if (!p->se.avg.last_update_time)
> +			flags |= SCHED_CPUFREQ_MIGRATION;
> +
> +		cpufreq_update_util(rq, flags);
> +	}
> +
>  	hrtick_update(rq);
>  }