Re: [RFCv5 PATCH 32/46] sched: Energy-aware wake-up task placement

Leo Yan
2015-09-02T17:11:44Z

On Tue, Jul 07, 2015 at 07:24:15PM +0100, Morten Rasmussen wrote:
> Let available compute capacity and estimated energy impact select
> wake-up target cpu when energy-aware scheduling is enabled and the
> system in not over-utilized (above the tipping point).
>
> energy_aware_wake_cpu() attempts to find group of cpus with sufficient
> compute capacity to accommodate the task and find a cpu with enough spare
> capacity to handle the task within that group. Preference is given to
> cpus with enough spare capacity at the current OPP. Finally, the energy
> impact of the new target and the previous task cpu is compared to select
> the wake-up target cpu.
>
> cc: Ingo Molnar <mingo@redhat.com>
> cc: Peter Zijlstra <peterz@infradead.org>
>
> Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
> ---
> kernel/sched/fair.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 84 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0f7dbda4..01f7337 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5427,6 +5427,86 @@ static int select_idle_sibling(struct task_struct *p, int target)
> return target;
> }
>
> +static int energy_aware_wake_cpu(struct task_struct *p, int target)
> +{
> + struct sched_domain *sd;
> + struct sched_group *sg, *sg_target;
> + int target_max_cap = INT_MAX;
> + int target_cpu = task_cpu(p);
> + int i;
> +
> + sd = rcu_dereference(per_cpu(sd_ea, task_cpu(p)));
> +
> + if (!sd)
> + return target;
> +
> + sg = sd->groups;
> + sg_target = sg;
> +
> + /*
> + * Find group with sufficient capacity. We only get here if no cpu is
> + * overutilized. We may end up overutilizing a cpu by adding the task,
> + * but that should not be any worse than select_idle_sibling().
> + * load_balance() should sort it out later as we get above the tipping
> + * point.
> + */
> + do {
> + /* Assuming all cpus are the same in group */
> + int max_cap_cpu = group_first_cpu(sg);
> +
> + /*
> + * Assume smaller max capacity means more energy-efficient.
> + * Ideally we should query the energy model for the right
> + * answer but it easily ends up in an exhaustive search.
> + */
> + if (capacity_of(max_cap_cpu) < target_max_cap &&
> + task_fits_capacity(p, max_cap_cpu)) {
> + sg_target = sg;
> + target_max_cap = capacity_of(max_cap_cpu);
> + }

Here should consider scenario for two groups have same capacity?
This will benefit for the case LITTLE.LITTLE. So the code will be
looks like below:

int target_sg_cpu = INT_MAX;

if (capacity_of(max_cap_cpu) <= target_max_cap &&
task_fits_capacity(p, max_cap_cpu)) {

if ((capacity_of(max_cap_cpu) == target_max_cap) &&
(target_sg_cpu < max_cap_cpu))
continue;

target_sg_cpu = max_cap_cpu;
sg_target = sg;
target_max_cap = capacity_of(max_cap_cpu);
}

> + } while (sg = sg->next, sg != sd->groups);
> +
> + /* Find cpu with sufficient capacity */
> + for_each_cpu_and(i, tsk_cpus_allowed(p), sched_group_cpus(sg_target)) {
> + /*
> + * p's blocked utilization is still accounted for on prev_cpu
> + * so prev_cpu will receive a negative bias due the double
> + * accouting. However, the blocked utilization may be zero.
> + */
> + int new_usage = get_cpu_usage(i) + task_utilization(p);
> +
> + if (new_usage > capacity_orig_of(i))
> + continue;
> +
> + if (new_usage < capacity_curr_of(i)) {
> + target_cpu = i;
> + if (cpu_rq(i)->nr_running)
> + break;
> + }
> +
> + /* cpu has capacity at higher OPP, keep it as fallback */
> + if (target_cpu == task_cpu(p))
> + target_cpu = i;
> + }
> +
> + if (target_cpu != task_cpu(p)) {
> + struct energy_env eenv = {
> + .usage_delta = task_utilization(p),
> + .src_cpu = task_cpu(p),
> + .dst_cpu = target_cpu,
> + };
> +
> + /* Not enough spare capacity on previous cpu */
> + if (cpu_overutilized(task_cpu(p)))
> + return target_cpu;
> +
> + if (energy_diff(&eenv) >= 0)
> + return task_cpu(p);
> + }
> +
> + return target_cpu;
> +}
> +
> /*
> * select_task_rq_fair: Select target runqueue for the waking task in domains
> * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> @@ -5479,7 +5559,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> prev_cpu = cpu;
>
> if (sd_flag & SD_BALANCE_WAKE && want_sibling) {
> - new_cpu = select_idle_sibling(p, prev_cpu);
> + if (energy_aware() && !cpu_rq(cpu)->rd->overutilized)
> + new_cpu = energy_aware_wake_cpu(p, prev_cpu);
> + else
> + new_cpu = select_idle_sibling(p, prev_cpu);
> goto unlock;
> }
>
> --
> 1.9.1
>
> --
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