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

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Date:   Wed, 15 Apr 2020 11:39:35 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>,
        Alessio Balsini <balsini@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] sched/deadline: Make DL capacity-aware

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