[PATCH v9 10/10] sched: move cfs task on a CPU with higher capacity

[Vincent Guittot]

On 21 November 2014 at 13:37, Morten Rasmussen <morten.rasmussen at arm.com> wrote:
> On Mon, Nov 03, 2014 at 04:54:47PM +0000, Vincent Guittot wrote:

>>
>> +     /*
>> +      * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
>> +      * It's worth migrating the task if the src_cpu's capacity is reduced
>> +      * because of other sched_class or IRQs whereas capacity stays
>> +      * available on dst_cpu.
>> +      */
>> +     if ((env->idle != CPU_NOT_IDLE) &&
>> +                     (env->src_rq->cfs.h_nr_running == 1)) {
>> +             unsigned long src_eff_capacity, dst_eff_capacity;
>> +
>> +             dst_eff_capacity = 100;
>> +             dst_eff_capacity *= capacity_of(env->dst_cpu);
>> +             dst_eff_capacity *= capacity_orig_of(env->src_cpu);
>> +
>> +             src_eff_capacity = sd->imbalance_pct;
>> +             src_eff_capacity *= capacity_of(env->src_cpu);
>> +             src_eff_capacity *= capacity_orig_of(env->dst_cpu);
>
> Do we need to scale by capacity_orig? Shouldn't the absolute capacity be
> better?
>
> if (capacity_of(env->src) * sd->imbalance_pct < capacity_of(env->dst) *
> 100) ?

we don't want to compare absolute capacity between CPUs but to compare
the reduction of their capacity because we want to choose the CPU
which is less used  by RT tasks or irq

Regards,
Vincent
>
> Isn't it the absolute available capacity that matters? For SMP
> capacity_orig is the same and cancels out and doesn't change anything.
> For big.LITTLE we would rather have the task run on a big where rt/irq
> eats 30% than a little cpu where rq/irq eats 5%, assuming big capacity
> is much bigger than little capacity so the absolute available capacity
> (~cycles/time) is larger on the big cpu.
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