linux-kernel - Re: [PATCH RESEND v9 10/10] sched: move cfs task on a CPU with higher capacity

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Date:	Fri, 20 Feb 2015 14:54:09 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH RESEND v9 10/10] sched: move cfs task on a CPU with higher capacity

On 20 February 2015 at 12:27, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Jan 15, 2015 at 11:09:30AM +0100, Vincent Guittot wrote:
>> As a sidenote, this will note generate more spurious ilb because we already
>
> s/note/not/
>
>> trig an ilb if there is more than 1 busy cpu. If this cpu is the only one that
>> has a task, we will trig the ilb once for migrating the task.
>
>> +static inline bool nohz_kick_needed(struct rq *rq)
>>  {
>>       unsigned long now = jiffies;
>>       struct sched_domain *sd;
>>       struct sched_group_capacity *sgc;
>>       int nr_busy, cpu = rq->cpu;
>> +     bool kick = false;
>>
>>       if (unlikely(rq->idle_balance))
>> +             return false;
>>
>>         /*
>>       * We may be recently in ticked or tickless idle mode. At the first
>> @@ -7472,38 +7498,44 @@ static inline int nohz_kick_needed(struct rq *rq)
>>        * balancing.
>>        */
>>       if (likely(!atomic_read(&nohz.nr_cpus)))
>> +             return false;
>>
>>       if (time_before(now, nohz.next_balance))
>> +             return false;
>>
>>       if (rq->nr_running >= 2)
>> +             return true;
>
> So this,
>
>>       rcu_read_lock();
>>       sd = rcu_dereference(per_cpu(sd_busy, cpu));
>>       if (sd) {
>>               sgc = sd->groups->sgc;
>>               nr_busy = atomic_read(&sgc->nr_busy_cpus);
>>
>> +             if (nr_busy > 1) {
>> +                     kick = true;
>> +                     goto unlock;
>> +             }
>> +
>>       }
>>
>> +     sd = rcu_dereference(rq->sd);
>> +     if (sd) {
>> +             if ((rq->cfs.h_nr_running >= 1) &&
>> +                             check_cpu_capacity(rq, sd)) {
>> +                     kick = true;
>> +                     goto unlock;
>> +             }
>> +     }
>
> vs this: how would we ever get here?
>
> If h_nr_running > 1, must then not nr_running > 1 as well?

you're right,
but the test above can trig a kick with h_nr_running == 1 whereas the
other tests may not

>
>>
>> +     sd = rcu_dereference(per_cpu(sd_asym, cpu));
>>       if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
>>                                 sched_domain_span(sd)) < cpu))
>> +             kick = true;
>
> For consistencies sake I would've added a goto unlock here as well.

i can add the goto

>
>> +unlock:
>>       rcu_read_unlock();
>> +     return kick;
>>  }
--
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