Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 20-Mar 09:43, Dietmar Eggemann wrote:
> From: Quentin Perret <quentin.perret@arm.com>

[...]

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 76bd46502486..65a1bead0773 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6513,6 +6513,60 @@ static unsigned long compute_energy(struct task_struct *p, int dst_cpu)
>  	return energy;
>  }
> 
> +static bool task_fits(struct task_struct *p, int cpu)
> +{
> +	unsigned long next_util = cpu_util_next(cpu, p, cpu);
> +
> +	return util_fits_capacity(next_util, capacity_orig_of(cpu));
                                             ^^^^^^^^^^^^^^^^^^^^^

Since here we are at scheduling CFS tasks, should we not better use
capacity_of() to account for RT/IRQ pressure ?

> +}
> +
> +static int find_energy_efficient_cpu(struct sched_domain *sd,
> +					struct task_struct *p, int prev_cpu)
> +{
> +	unsigned long cur_energy, prev_energy, best_energy;
> +	int cpu, best_cpu = prev_cpu;
> +
> +	if (!task_util(p))

We are still waking up a task... what if the task was previously
running on a big CPU which is now idle?

I understand that from a _relative_ energy_diff standpoint there is
not much to do for a 0 utilization task. However, for those tasks we
can still try to return the most energy efficient CPU among the ones
in their cpus_allowed mask.

It should be a relatively low overhead (maybe contained in a fallback
most_energy_efficient_cpu() kind of function) which allows, for
example on ARM big.LITTLE systems, to consolidate those tasks on
LITTLE CPUs instead for example keep running them on a big CPU.

> +		return prev_cpu;
> +
> +	/* Compute the energy impact of leaving the task on prev_cpu. */
> +	prev_energy = best_energy = compute_energy(p, prev_cpu);
> +
> +	/* Look for the CPU that minimizes the energy. */
                                           ^^^^^^^^^^
nit-pick: would say explicitly "best_energy" here, just to avoid
confusion about (non) possible overflows in the following if check ;)

> +	for_each_cpu_and(cpu, &p->cpus_allowed, sched_domain_span(sd)) {
> +		if (!task_fits(p, cpu) || cpu == prev_cpu)

nit-pick: to me it would read better as:

                if (cpu == prev_cpu)
                        continue;
                if (!task_fits(p, cpu))
                        continue;

but it's more matter of (personal) taste then efficiency.

> +			continue;
> +		cur_energy = compute_energy(p, cpu);
> +		if (cur_energy < best_energy) {
> +			best_energy = cur_energy;
> +			best_cpu = cpu;
> +		}
> +	}
> +
> +	/*
> +	 * We pick the best CPU only if it saves at least 1.5% of the
> +	 * energy used by prev_cpu.
> +	 */
> +	if ((prev_energy - best_energy) > (prev_energy >> 6))
> +		return best_cpu;
> +
> +	return prev_cpu;
> +}

[...]

> @@ -6555,6 +6613,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>  			break;
>  		}
> 
> +		/*
> +		 * Energy-aware task placement is performed on the highest
> +		 * non-overutilized domain spanning over cpu and prev_cpu.
> +		 */
> +		if (want_energy && !sd_overutilized(tmp) &&
> +		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
> +			energy_sd = tmp;
> +

Not entirely sure, but I was trying to understand if we can avoid to
modify the definition of want_affine (in the previous chunk) and move
this block before the previous "if (want_affine..." (in mainline but
not in this chunk), which will became an else, e.g.

        if (want_energy && !sd_overutilized(tmp) &&
                // ...
        else if (want_energy && !sd_overutilized(tmp) &&
                // ...

Isn't that the same?

Maybe there is a code path I'm missing... but otherwise it seems a
more self contained modification of select_task_rq_fair...

>  		if (tmp->flags & sd_flag)
>  			sd = tmp;
>  		else if (!want_affine)
> @@ -6586,6 +6652,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>  			if (want_affine)
>  				current->recent_used_cpu = cpu;
>  		}
> +	} else if (energy_sd) {
> +		new_cpu = find_energy_efficient_cpu(energy_sd, p, prev_cpu);
>  	} else {
>  		new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);
>  	}

-- 
#include <best/regards.h>

Patrick Bellasi