Re: [PATCH v4 2/3] sched/fair: Clean up active balance nr_balance_failed trickery

[Vincent Guittot]

On Thu, 1 Apr 2021 at 21:30, Valentin Schneider
<valentin.schneider@xxxxxxx> wrote:
>
> When triggering an active load balance, sd->nr_balance_failed is set to
> such a value that any further can_migrate_task() using said sd will ignore
> the output of task_hot().
>
> This behaviour makes sense, as active load balance intentionally preempts a
> rq's running task to migrate it right away, but this asynchronous write is
> a bit shoddy, as the stopper thread might run active_load_balance_cpu_stop
> before the sd->nr_balance_failed write either becomes visible to the
> stopper's CPU or even happens on the CPU that appended the stopper work.
>
> Add a struct lb_env flag to denote active balancing, and use it in
> can_migrate_task(). Remove the sd->nr_balance_failed write that served the
> same purpose. Cleanup the LBF_DST_PINNED active balance special case.
>
> Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
>  kernel/sched/fair.c | 31 +++++++++++++++----------------
>  1 file changed, 15 insertions(+), 16 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04d5e14fa261..d8077f82a380 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7422,6 +7422,7 @@ enum migration_type {
>  #define LBF_NEED_BREAK 0x02
>  #define LBF_DST_PINNED  0x04
>  #define LBF_SOME_PINNED        0x08
> +#define LBF_ACTIVE_LB  0x10
>
>  struct lb_env {
>         struct sched_domain     *sd;
> @@ -7583,10 +7584,13 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>                  * our sched_group. We may want to revisit it if we couldn't
>                  * meet load balance goals by pulling other tasks on src_cpu.
>                  *
> -                * Avoid computing new_dst_cpu for NEWLY_IDLE or if we have
> -                * already computed one in current iteration.
> +                * Avoid computing new_dst_cpu
> +                * - for NEWLY_IDLE
> +                * - if we have already computed one in current iteration
> +                * - if it's an active balance
>                  */
> -               if (env->idle == CPU_NEWLY_IDLE || (env->flags & LBF_DST_PINNED))
> +               if (env->idle == CPU_NEWLY_IDLE ||
> +                   env->flags & (LBF_DST_PINNED | LBF_ACTIVE_LB))
>                         return 0;
>
>                 /* Prevent to re-select dst_cpu via env's CPUs: */
> @@ -7611,10 +7615,14 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>
>         /*
>          * Aggressive migration if:
> -        * 1) destination numa is preferred
> -        * 2) task is cache cold, or
> -        * 3) too many balance attempts have failed.
> +        * 1) active balance
> +        * 2) destination numa is preferred
> +        * 3) task is cache cold, or
> +        * 4) too many balance attempts have failed.
>          */
> +       if (env->flags & LBF_ACTIVE_LB)
> +               return 1;
> +
>         tsk_cache_hot = migrate_degrades_locality(p, env);
>         if (tsk_cache_hot == -1)
>                 tsk_cache_hot = task_hot(p, env);
> @@ -9805,9 +9813,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                                         active_load_balance_cpu_stop, busiest,
>                                         &busiest->active_balance_work);
>                         }
> -
> -                       /* We've kicked active balancing, force task migration. */
> -                       sd->nr_balance_failed = sd->cache_nice_tries+1;
>                 }
>         } else {
>                 sd->nr_balance_failed = 0;
> @@ -9957,13 +9962,7 @@ static int active_load_balance_cpu_stop(void *data)
>                         .src_cpu        = busiest_rq->cpu,
>                         .src_rq         = busiest_rq,
>                         .idle           = CPU_IDLE,
> -                       /*
> -                        * can_migrate_task() doesn't need to compute new_dst_cpu
> -                        * for active balancing. Since we have CPU_IDLE, but no
> -                        * @dst_grpmask we need to make that test go away with lying
> -                        * about DST_PINNED.
> -                        */
> -                       .flags          = LBF_DST_PINNED,
> +                       .flags          = LBF_ACTIVE_LB,
>                 };
>
>                 schedstat_inc(sd->alb_count);
> --
> 2.25.1
>