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SubjectRe: [PATCH v2 5/6] sched/deadline: Make DL capacity-aware
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On 30/04/2020 15:10, Pavan Kondeti wrote:
> On Mon, Apr 27, 2020 at 10:37:08AM +0200, Dietmar Eggemann wrote:
>> From: Luca Abeni <luca.abeni@santannapisa.it>

[...]

>> @@ -1653,10 +1654,19 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
>> * other hand, if it has a shorter deadline, we
>> * try to make it stay here, it might be important.
>> */
>> - if (unlikely(dl_task(curr)) &&
>> - (curr->nr_cpus_allowed < 2 ||
>> - !dl_entity_preempt(&p->dl, &curr->dl)) &&
>> - (p->nr_cpus_allowed > 1)) {
>> + select_rq = unlikely(dl_task(curr)) &&
>> + (curr->nr_cpus_allowed < 2 ||
>> + !dl_entity_preempt(&p->dl, &curr->dl)) &&
>> + p->nr_cpus_allowed > 1;
>> +
>> + /*
>> + * Take the capacity of the CPU into account to
>> + * ensure it fits the requirement of the task.
>> + */
>> + if (static_branch_unlikely(&sched_asym_cpucapacity))
>> + select_rq |= !dl_task_fits_capacity(p, cpu);
>> +
>> + if (select_rq) {
>> int target = find_later_rq(p);
>
> I see that find_later_rq() checks if the previous CPU is part of
> later_mask and returns it immediately. So we don't migrate the
> task in the case where there previous CPU can't fit the task and
> there are no idle CPUs on which the task can fit. LGTM.

Hope I understand you here. I don't think that [patch 6/6] provides this
already.

In case 'later_mask' has no fitting CPUs, 'max_cpu' is set in the
otherwise empty 'later_mask'. But 'max_cpu' is not necessary task_cpu(p).

Example on Juno [L b b L L L] with thread0-0 (big task)

cpudl_find [thread0-0 2117] orig later_mask=0,3-4 later_mask=0
find_later_rq [thread0-0 2117] task_cpu=2 later_mask=0

A tweak could be added favor task_cpu(p) in case it is amongst the CPUs
with the maximum capacity in cpudl_find() for the !fit case.

[...]

>> +/*
>> + * Verify the fitness of task @p to run on @cpu taking into account the
>> + * CPU original capacity and the runtime/deadline ratio of the task.
>> + *
>> + * The function will return true if the CPU original capacity of the
>> + * @cpu scaled by SCHED_CAPACITY_SCALE >= runtime/deadline ratio of the
>> + * task and false otherwise.
>> + */
>> +static inline bool dl_task_fits_capacity(struct task_struct *p, int cpu)
>> +{
>> + unsigned long cap = arch_scale_cpu_capacity(cpu);
>> +
>> + return cap_scale(p->dl.dl_deadline, cap) >= p->dl.dl_runtime;
>> +}
>> +
>
> This is same as
>
> return p->dl.dl_bw >> (BW_SHIFT - SCHED_CAPACITY_SHIFT) <= cap
>
> Correct? If yes, would it be better to use this?

We could use sched_dl_entity::dl_density (dl_runtime / dl_deadline) but
then I would have to export BW_SHIFT.

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