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

Vincent Guittot
2015-02-20T13:54:33Z

On 20 February 2015 at 12:27, Peter Zijlstra <peterz@infradead.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;
>> }