linux-kernel - Re: [PATCH v3] sched/fair: use utilization to select misfit task

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Date:   Fri, 2 Aug 2019 15:27:18 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org
Cc:     pauld@...hat.com, srikar@...ux.vnet.ibm.com,
        quentin.perret@....com, dietmar.eggemann@....com,
        Morten.Rasmussen@....com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3] sched/fair: use utilization to select misfit task

On 02/08/2019 13:56, Vincent Guittot wrote:
> utilization is used to detect a misfit task but the load is then used to
> select the task on the CPU which can lead to select a small task with
> high weight instead of the task that triggered the misfit migration.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
> Keep tracking load instead of utilization but check that 
> task doesn't fit CPU's capacity when selecting the task to detach as
> suggested by Valentin 
> 

I find that one much better :) The very same fix could be applied
regardless of the rework, so FWIW (providing the changelog gets a bit
clearer - maybe squash the comment in it):
Acked-by: Valentin Schneider <valentin.schneider@....com>


One more thing though: we actually face the same problem in active balance,
i.e. the misfit task could sleep/get preempted before the CPU stopper
actually runs, and that latter would migrate $random.

I was thinking of passing the lb_env to stop_one_cpu_nowait() - we'd have
to rebuild it in active_load_balance_cpu_stop() anyway, but we could copy
things like env.migration_type so that we remember that we should be
picking a misfit task and have a similar detach guard.

Sadly, the lb_env struct is on load_balance()'s stack, and that's a nowait
call :/

Peter/Thomas, would there be much hate in adding some sort of flags field
to struct cpu_stop_work? Or if you see a better alternative, I'm all ears.


>  kernel/sched/fair.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 53e64a7..8496118 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7487,15 +7487,9 @@ static int detach_tasks(struct lb_env *env)
>  			break;
>  
>  		case migrate_misfit:
> -			load = task_h_load(p);
> -
> -			/*
> -			 * utilization of misfit task might decrease a bit
> -			 * since it has been recorded. Be conservative in the
> -			 * condition.
> -			 */
> -			if (load < env->imbalance)
> -				goto next;
> +			/* This is not a misfit task */
> +                       if (task_fits_capacity(p, capacity_of(env->src_cpu)))
> +			       goto next;
>  
>  			env->imbalance = 0;
>  			break;
>