linux-kernel - Re: [PATCH v2 2/3] sched/fair: trigger asym

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Date:   Tue, 18 Dec 2018 12:00:16 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2 2/3] sched/fair: trigger asym_packing during idle load
 balance

On 18/12/2018 08:17, Vincent Guittot wrote:
[...]
>> That change looks fine. However, you're mentioning newidle load_balance()
>> not being triggered - you'd want to set root_domain->overload for any
>> newidle pull to happen, probably with something like this:
> 
> It's not needed in this case because the dst cpu is already the target
> cpu and the migration will happen during this idle load balance.
> Setting root_domain->overload is useful only if you want another cpu
> to pull the task during another coming load_balance (newly or normal
> idle ones) which is not the case here.
> 

Right, I got the check wrong. The thing is, if you want to go through a
newidle balance, you need to have that overload flag raised beforehand.

I was about to draw a diagram but I kinda already did in the log of
757ffdd705ee ("sched/fair: Set rq->rd->overload when misfit").

So you would first need to raise the flag, e.g. when updating the lb stats
on a low-priority CPU, and when a higher-priority CPU goes newly idle,
the flag is raised, gates to idle_balance() are opened, and a newidle pull
from low-priority to high-priority can happen.

>>
>> -----8<-----
>> @@ -8398,6 +8408,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>>                 sg = sg->next;
>>         } while (sg != env->sd->groups);
>>
>> +       if (check_asym_packing(env, sds))
>> +               sg_status |= SG_OVERLOAD;
>> +
>>  #ifdef CONFIG_NO_HZ_COMMON
>>         if ((env->flags & LBF_NOHZ_AGAIN) &&
>>             cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) {
>> ----->8-----
>>
>> It's similar to what is done for misfit, although that's yet another
>> 'twisted' use of that flag which we might want to rename (I suggested
>> something like 'need_idle_balance' a while back but it wasn't really
>> popular).