linux-kernel - Re: [PATCH v3 04/10] sched/fair: rework load

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Date:   Wed, 2 Oct 2019 11:24:51 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v3 04/10] sched/fair: rework load_balance

On 02/10/2019 10:23, Vincent Guittot wrote:
> On Tue, 1 Oct 2019 at 18:53, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>
>> On 01/10/2019 10:14, Vincent Guittot wrote:
>>> On Mon, 30 Sep 2019 at 18:24, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>
>>>> Hi Vincent,
>>>>
>>>> On 19/09/2019 09:33, Vincent Guittot wrote:
>>
> [...]
> 
>>
>>>>> +             if (busiest->group_weight == 1 || sds->prefer_sibling) {
>>>>> +                     /*
>>>>> +                      * When prefer sibling, evenly spread running tasks on
>>>>> +                      * groups.
>>>>> +                      */
>>>>> +                     env->balance_type = migrate_task;
>>>>> +                     env->imbalance = (busiest->sum_h_nr_running - local->sum_h_nr_running) >> 1;
>>>>> +                     return;
>>>>> +             }
>>>>> +
>>>>> +             /*
>>>>> +              * If there is no overload, we just want to even the number of
>>>>> +              * idle cpus.
>>>>> +              */
>>>>> +             env->balance_type = migrate_task;
>>>>> +             env->imbalance = max_t(long, 0, (local->idle_cpus - busiest->idle_cpus) >> 1);
>>>>
>>>> Why do we need a max_t(long, 0, ...) here and not for the 'if
>>>> (busiest->group_weight == 1 || sds->prefer_sibling)' case?
>>>
>>> For env->imbalance = (busiest->sum_h_nr_running - local->sum_h_nr_running) >> 1;
>>>
>>> either we have sds->prefer_sibling && busiest->sum_nr_running >
>>> local->sum_nr_running + 1
>>
>> I see, this corresponds to
>>
>> /* Try to move all excess tasks to child's sibling domain */
>>        if (sds.prefer_sibling && local->group_type == group_has_spare &&
>>            busiest->sum_h_nr_running > local->sum_h_nr_running + 1)
>>                goto force_balance;
>>
>> in find_busiest_group, I assume.
> 
> yes. But it seems that I missed a case:
> 
> prefer_sibling is set
> busiest->sum_h_nr_running <= local->sum_h_nr_running + 1 so we skip
> goto force_balance above
> But env->idle != CPU_NOT_IDLE  and local->idle_cpus >
> (busiest->idle_cpus + 1) so we also skip goto out_balance and finally
> call calculate_imbalance()
> 
> in calculate_imbalance with prefer_sibling set, imbalance =
> (busiest->sum_h_nr_running - local->sum_h_nr_running) >> 1;
> 
> so we probably want something similar to max_t(long, 0,
> (busiest->sum_h_nr_running - local->sum_h_nr_running) >> 1)

Makes sense.

Caught a couple of

[  369.310464] 0-3->4-7 2->5 env->imbalance = 2147483646
[  369.310796] 0-3->4-7 2->4 env->imbalance = 2147483647

in this if condition on h620 running hackbench.