linux-kernel - Re: [PATCH 2/2 v2] sched: use load

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Date:   Fri, 2 Dec 2016 16:20:54 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Morten Rasmussen <morten.rasmussen@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Wanpeng Li <kernellwp@...il.com>,
        Yuyang Du <yuyang.du@...el.com>,
        Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH 2/2 v2] sched: use load_avg for selecting idlest group

On 30 November 2016 at 15:24, Morten Rasmussen <morten.rasmussen@....com> wrote:
> On Wed, Nov 30, 2016 at 02:54:00PM +0100, Vincent Guittot wrote:
>> On 30 November 2016 at 14:49, Vincent Guittot
>> <vincent.guittot@...aro.org> wrote:
>> > On 30 November 2016 at 13:49, Morten Rasmussen <morten.rasmussen@....com> wrote:
>> >> On Fri, Nov 25, 2016 at 04:34:33PM +0100, Vincent Guittot wrote:
>> >>> find_idlest_group() only compares the runnable_load_avg when looking for
>> >>> the least loaded group. But on fork intensive use case like hackbench
>> >
>> > [snip]
>> >
>> >>> +                             min_avg_load = avg_load;
>> >>> +                             idlest = group;
>> >>> +                     } else if ((runnable_load < (min_runnable_load + imbalance)) &&
>> >>> +                                     (100*min_avg_load > imbalance_scale*avg_load)) {
>> >>> +                             /*
>> >>> +                              * The runnable loads are close so we take
>> >>> +                              * into account blocked load through avg_load
>> >>> +                              *  which is blocked + runnable load
>> >>> +                              */
>> >>> +                             min_avg_load = avg_load;
>> >>>                               idlest = group;
>> >>>                       }
>> >>>
>> >>> @@ -5470,13 +5495,16 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>> >>>               goto no_spare;
>> >>>
>> >>>       if (this_spare > task_util(p) / 2 &&
>> >>> -         imbalance*this_spare > 100*most_spare)
>> >>> +         imbalance_scale*this_spare > 100*most_spare)
>> >>>               return NULL;
>> >>>       else if (most_spare > task_util(p) / 2)
>> >>>               return most_spare_sg;
>> >>>
>> >>>  no_spare:
>> >>> -     if (!idlest || 100*this_load < imbalance*min_load)
>> >>> +     if (!idlest ||
>> >>> +         (min_runnable_load > (this_runnable_load + imbalance)) ||
>> >>> +         ((this_runnable_load < (min_runnable_load + imbalance)) &&
>> >>> +                     (100*min_avg_load > imbalance_scale*this_avg_load)))
>> >>
>> >> I don't get why you have imbalance_scale applied to this_avg_load and
>> >> not min_avg_load. IIUC, you end up preferring non-local groups?
>> >
>> > In fact, I have keep the same condition that is used when looping the group.
>> > You're right that we should prefer local rq if avg_load are close and
>> > test the condition
>> > (100*this_avg_load > imbalance_scale*min_avg_load) instead
>>
>> Of course the correct condition is
>>  (100*this_avg_load < imbalance_scale*min_avg_load)
>
> Agreed, I should have read the entire thread before replying :-)

Interestingly, the original condition (100*min_avg_load >
imbalance_scale*this_avg_load) gives better performance result for the
hackbench test than the new one : (100*this_avg_load <
imbalance_scale*min_avg_load)

Matt,

Have you been able to get some results for the patchset ?

Vincent