linux-kernel - Re: [PATCH] sched/fair: vruntime should normalize when switching from fair

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Date:   Wed, 29 Aug 2018 16:33:32 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steve Muckle <smuckle@...gle.com>,
        Miguel de Dios <migueldedios@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, Todd Kjos <tkjos@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Patrick Bellasi <Patrick.Bellasi@....com>,
        Chris Redpath <Chris.Redpath@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        John Dias <joaodias@...gle.com>
Subject: Re: [PATCH] sched/fair: vruntime should normalize when switching from
 fair

On 08/29/2018 12:59 PM, Peter Zijlstra wrote:
> On Wed, Aug 29, 2018 at 11:54:58AM +0100, Dietmar Eggemann wrote:
>> I forgot to mention that since fair_task's cpu affinity is restricted to
>> CPU4, there is no call to set_task_cpu()->migrate_task_rq_fair() since if
>> (task_cpu(p) != cpu) fails.
>>
>> I think the combination of cpu affinity of the fair_task to CPU4 and the
>> fact that the scheduler runs on CPU1 when waking fair_task (with the two
>> cpus not sharing LLC) while TTWU_QUEUE is enabled is the situation in which
>> this vruntime issue can happen.
> 
> Ohhh, D'0h. A remote wakeup that doesn't migrate.

Ah, there is this WF_MIGRATED flag, perfect for the distinction whether 
a task migrated or not.

> That would suggest something like so:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b39fb596f6c1..b3b62cf37fb6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9638,7 +9638,8 @@ static inline bool vruntime_normalized(struct task_struct *p)
>   	 * - A task which has been woken up by try_to_wake_up() and
>   	 *   waiting for actually being woken up by sched_ttwu_pending().
>   	 */
> -	if (!se->sum_exec_runtime || p->state == TASK_WAKING)
> +	if (!se->sum_exec_runtime ||
> +	    (p->state == TASK_WAKING && p->sched_remote_wakeup))
>   		return true;
>   
>   	return false;
Yes, this solves the issue for the case I described. Using 
'p->sched_remote_wakeup' (WF_MIGRATED) looks more elegant than using 
'p->sched_class == &fair_sched_class'.