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SubjectRe: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE_WAKE
On 06/11/2015 10:35 PM, Mike Galbraith wrote:
> On Thu, 2015-05-28 at 13:05 +0200, Peter Zijlstra wrote:
>
>> @@ -5022,22 +5026,28 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>> * If both cpu and prev_cpu are part of this domain,
>> * cpu is a valid SD_WAKE_AFFINE target.
>> */
>> - if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
>> - cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
>> + if (want_affine && !affine_sd &&
>> + (tmp->flags & SD_WAKE_AFFINE) &&
>> + cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
>> affine_sd = tmp;
>> - break;
>> - }
>>
>> if (tmp->flags & sd_flag)
>> sd = tmp;
>> + else if (!want_affine || (want_affine && affine_sd))
>> + break;
>> }
>
> Hm, new_cpu == cpu.
>
>> - if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync))
>> + if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync)) {
>> prev_cpu = cpu;
>> + sd = NULL; /* WAKE_AFFINE trumps BALANCE_WAKE */
>> + }
>
> If branch above is not taken, new_cpu remains cpu.
>
>> if (sd_flag & SD_BALANCE_WAKE) {
>> - new_cpu = select_idle_sibling(p, prev_cpu);
>> - goto unlock;
>> + int tmp = select_idle_sibling(p, prev_cpu);
>> + if (tmp >= 0) {
>> + new_cpu = tmp;
>> + goto unlock;
>> + }
>> }
>
> If select_idle_sibling() returns -1, new_cpu remains cpu.
>
>>
>> while (sd) {
>
> If sd == NULL, we fall through and try to pull wakee despite nacked-by
> tsk_cpus_allowed() or wake_affine().
>

So maybe add a check in the if (sd_flag & SD_BALANCE_WAKE) for something
like this

if (tmp >= 0) {
new_cpu = tmp;
goto unlock;
} else if (!want_affine) {
new_cpu = prev_cpu;
}

so we can make sure we're not being pushed onto a cpu that we aren't
allowed on? Thanks,

Josef



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