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SubjectRe: [PATCH v3] sched/fair: use utilization to select misfit task
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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@linaro.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@arm.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;
>

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