Re: [PATCH 3/4] sched/deadline: Make DL capacity-aware

Dietmar Eggemann
2020-04-15T09:39:52Z

On 10.04.20 14:52, Juri Lelli wrote:
> Hi,
>
> On 08/04/20 11:50, Dietmar Eggemann wrote:
>> From: Luca Abeni <luca.abeni@santannapisa.it>

[...]

>> @@ -1623,10 +1624,19 @@ 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;
>> +
>> + /*
>> + * We take into account the capacity of the CPU to
>> + * ensure it fits the requirement of the task.
>> + */
>> + if (static_branch_unlikely(&sched_asym_cpucapacity))
>> + select_rq |= !dl_task_fits_capacity(p, cpu);
>
> I'm thinking that, while dl_task_fits_capacity() works well when
> selecting idle cpus, in this case we should consider the fact that curr
> might be deadline as well and already consuming some of the rq capacity.
>
> Do you think we should try to take that into account, maybe using
> dl_rq->this_bw ?

So you're saying that cpudl_find(..., later_mask) could return 1 (w/
best_cpu (cp->elements[0].cpu) in later_mask).

And that this best_cpu could be a non-fitting CPU for p.

This could happen if cp->free_cpus is empty (no idle CPUs) so we take
cpudl_find()'s else path and in case p's deadline < cp->elements[0]
deadline.

We could condition the 'return 1' on best_cpu fitting p.

But should we do this for cpudl_find(..., NULL) calls from
check_preempt_equal_dl() as well or will this break GEDF?