Re: [PATCH] sched/fair: Do not scan non-movable tasks several times

[Vincent Guittot]

On Thu, 14 Dec 2023 at 15:42, Konstantin Khorenko
<khorenko@xxxxxxxxxxxxx> wrote:
>
> If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK and all
> tasks are not movable, detach_tasks() should not iterate more than tasks
> available in the busiest rq.
>
> Previously the (env->loop > env->loop_max) condition prevented us from

It's usually better to give the commit directly when we know it :
Before commit : b0defa7ae03e ("sched/fair: Make sure to try to detach
at least one movable task"),
the (env->loop > env->loop_max) condition prevented us from ...

> scanning non-movable tasks more than rq size times, but after we start
> checking the LBF_ALL_PINNED flag, the "all tasks are not movable" case
> is under threat.
>

Fixes: b0defa7ae03e ("sched/fair: Make sure to try to detach at least
one movable task")

> Signed-off-by: Konstantin Khorenko <khorenko@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d7a3c63a2171..faa2a765e899 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11219,7 +11219,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                 .dst_rq         = this_rq,
>                 .dst_grpmask    = group_balance_mask(sd->groups),
>                 .idle           = idle,
> -               .loop_break     = SCHED_NR_MIGRATE_BREAK,
>                 .cpus           = cpus,
>                 .fbq_type       = all,
>                 .tasks          = LIST_HEAD_INIT(env.tasks),
> @@ -11265,6 +11264,12 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                  * correctly treated as an imbalance.
>                  */
>                 env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
> +               /*
> +                * If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK
> +                * and all tasks are not movable, detach_tasks() should not
> +                * iterate more than tasks available in rq.
> +                */
> +               env.loop_break = min(SCHED_NR_MIGRATE_BREAK, busiest->nr_running);

Should it be after more_balance: ?
In case we do "more_balance:" on a new_dst_cpu and it ends up that
finally there is no more movable task as we released the lock of
busiest rq in the meantime ?

Also you can remove one more superfluous init of loop_break:

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11361,7 +11361,6 @@ static int load_balance(int this_cpu, struct
rq *this_rq,
                         */
                        if (!cpumask_subset(cpus, env.dst_grpmask)) {
                                env.loop = 0;
-                               env.loop_break = SCHED_NR_MIGRATE_BREAK;
                                goto redo;
                        }
                        goto out_all_pinned;

>
>  more_balance:
>                 rq_lock_irqsave(busiest, &rf);
> --
> 2.39.3
>