Re: [PATCH] sched: keep quiescent cpu out of idle balance loop

[Lei Wen <adrian.wenl@gmail.com>]

On Thu, Feb 20, 2014 at 4:50 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Feb 20, 2014 at 10:42:51AM +0800, Lei Wen wrote:
>> >> -     int ilb = cpumask_first(nohz.idle_cpus_mask);
>> >> +     int ilb;
>> >> +     int cpu = smp_processor_id();
>> >> +     struct sched_domain *tmp;
>> >>
>> >> -     if (ilb < nr_cpu_ids && idle_cpu(ilb))
>> >> -             return ilb;
>> >> +     for_each_domain(cpu, tmp) {
>> >> +             ilb = cpumask_first_and(nohz.idle_cpus_mask,
>> >> +                             sched_domain_span(tmp));
>> >> +             if (ilb < nr_cpu_ids && idle_cpu(ilb))
>> >> +                     return ilb;
>> >> +     }
>> >
>> > The ILB code is bad; but you just made it horrible. Don't add pointless
>> > for_each_domain() iterations.
>> >
>> > I'm thinking something like:
>> >
>> >   ilb = cpumask_first_and(nohz.idle_cpus_mask, this_rq()->rd.span);
>> >
>> > Should work just fine, no?
>>
>> Yes, it has the same result as my previous patch did.
>>
>> >
>> > Better still would be to maybe not participate in the ILB in the first
>> > place and leave this selection loop alone.
>>
>> Not quitely get your point here...
>> Do you mean that you want idle cpu selection be put in earlier place
>> than current find_new_ilb is?
>
> I meant that if you stop an idle CPU setting its bit in
> nohz.idle_cpus_mask, you don't have to mask it out either.

Understand it.
I would reformat my previous patch according to your suggestion.

Thanks,
Lei