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SubjectRe: [PATCH] sched/cfs: fix spurious active migration
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On 29/11/2019 15:04, Vincent Guittot wrote:
> The load balance can fail to find a suitable task during the periodic check
> because the imbalance is smaller than half of the load of the waiting
> tasks. This results in the increase of the number of failed load balance,
> which can end up to start an active migration. This active migration is
> useless because the current running task is not a better choice than the
> waiting ones. In fact, the current task was probably not running but
> waiting for the CPU during one of the previous attempts and it had already
> not been selected.
>
> When load balance fails too many times to migrate a task, we should relax
> the contraint on the maximum load of the tasks that can be migrated
> similarly to what is done with cache hotness.
>
> Before the rework, load balance used to set the imbalance to the average
> load_per_task in order to mitigate such situation. This increased the
> likelihood of migrating a task but also of selecting a larger task than
> needed while more appropriate ones were in the list.

Why not use '&& !env->sd->nr_balance_failed' then? Too aggressive? But
the average load_per_task was calculated at each load balance attempt,
so it would have led to a migration at the first load balance.

This would be in sync with the LB_MIN check in the same switch case
(migrate_load). Although LB_MIN is false by default.

> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>
> I haven't seen any noticable performance changes on the benchmarks that I
> usually run but the problem can be easily highlight with a simple test
> with 9 always running tasks on 8 cores.
>
> kernel/sched/fair.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e0d662a..d1b4fa7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7433,7 +7433,14 @@ static int detach_tasks(struct lb_env *env)
> load < 16 && !env->sd->nr_balance_failed)
> goto next;
>
> - if (load/2 > env->imbalance)
> + /*
> + * Make sure that we don't migrate too much load.
> + * Nevertheless, let relax the constraint if
> + * scheduler fails to find a good waiting task to
> + * migrate.
> + */
> + if (load/2 > env->imbalance &&
> + env->sd->nr_balance_failed <= env->sd->cache_nice_tries)
> goto next;
>
> env->imbalance -= load;
>

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