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

Dietmar Eggemann
2015-09-18T10:34:32Z

On 02/09/15 18:11, Leo Yan wrote:
> 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);
> }
>

It's true that on your SMP system the target sched_group 'sg_target'
depends only on 'task_cpu(p)' because this determines sched_domain 'sd'
(and so the order of sched_groups for the iteration).

So the current do-while loop to select 'sg_target' for an SMP system
makes little sense.

But why should we favour the first sched_group (cluster) (the one w/ the
lower max_cap_cpu number) in this situation? …