Re: [PATCH] sched/fair: Restore env status before goto redo in load_balance()

[Xunlei Pang]

Ping Peter

> From: Xunlei Pang <pang.xunlei@xxxxxxxxxx>
>
> In load_balance(), some members of lb_env will be assigned with
> new values in LBF_DST_PINNED case. But lb_env::flags may still
> retain LBF_ALL_PINNED if no proper tasks were found afterwards
> due to another balance, task affinity changing, etc, which can
> really happen because busiest rq lock has already been released.
>
> This is wrong, for example with env.dst_cpu assigned new_dst_cpu
> when going back to "redo" label, it may cause should_we_balance()
> to return false which is unreasonable.
>
> This patch restores proper status of env before "goto redo", and
> improves "out_all_pinned" and "out_one_pinned" labels.
>
> Signed-off-by: Xunlei Pang <pang.xunlei@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 35 ++++++++++++++++++++---------------
>  1 file changed, 20 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ee595ef..45bbda1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6843,6 +6843,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                 .dst_cpu        = this_cpu,
>                 .dst_rq         = this_rq,
>                 .dst_grpmask    = sched_group_cpus(sd->groups),
> +               .new_dst_cpu    = -1,
>                 .idle           = idle,
>                 .loop_break     = sched_nr_migrate_break,
>                 .cpus           = cpus,
> @@ -6977,12 +6978,19 @@ more_balance:
>                 /* All tasks on this runqueue were pinned by CPU affinity */
>                 if (unlikely(env.flags & LBF_ALL_PINNED)) {
>                         cpumask_clear_cpu(cpu_of(busiest), cpus);
> -                       if (!cpumask_empty(cpus)) {
> -                               env.loop = 0;
> -                               env.loop_break = sched_nr_migrate_break;
> -                               goto redo;
> +                       if (env.new_dst_cpu != -1) {
> +                               env.new_dst_cpu = -1;
> +                               cpumask_or(cpus, cpus,
> +                                       sched_group_cpus(sd->groups));
> +                               cpumask_and(cpus, cpus, cpu_active_mask);
> +
> +                               env.dst_cpu = this_cpu;
> +                               env.dst_rq = this_rq;
>                         }
> -                       goto out_all_pinned;
> +                       env.flags &= ~LBF_SOME_PINNED;
> +                       env.loop = 0;
> +                       env.loop_break = sched_nr_migrate_break;
> +                       goto redo;
>                 }
>         }
>
> @@ -7009,7 +7017,7 @@ more_balance:
>                                 raw_spin_unlock_irqrestore(&busiest->lock,
>                                                             flags);
>                                 env.flags |= LBF_ALL_PINNED;
> -                               goto out_one_pinned;
> +                               goto out_active_balanced;
>                         }
>
>                         /*
> @@ -7058,26 +7066,23 @@ more_balance:
>  out_balanced:
>         /*
>          * We reach balance although we may have faced some affinity
> -        * constraints. Clear the imbalance flag if it was set.
> +        * constraints.
> +        *
> +        * When LBF_ALL_PINNED was not set, clear the imbalance flag
> +        * if it was set.
>          */
> -       if (sd_parent) {
> +       if (sd_parent && !(env.flags & LBF_ALL_PINNED)) {
>                 int *group_imbalance = &sd_parent->groups->sgc->imbalance;
>
>                 if (*group_imbalance)
>                         *group_imbalance = 0;
>         }
>
> -out_all_pinned:
> -       /*
> -        * We reach balance because all tasks are pinned at this level so
> -        * we can't migrate them. Let the imbalance flag set so parent level
> -        * can try to migrate them.
> -        */
>         schedstat_inc(sd, lb_balanced[idle]);
>
>         sd->nr_balance_failed = 0;
>
> -out_one_pinned:
> +out_active_balanced:
>         /* tune up the balancing interval */
>         if (((env.flags & LBF_ALL_PINNED) &&
>                         sd->balance_interval < MAX_PINNED_INTERVAL) ||
> --
> 1.9.1
>
>
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