linux-kernel - Re: [PATCH v3 2/7] sched/fair: Clean up active balance nr_balance

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Date:   Wed, 17 Mar 2021 11:52:06 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>,
        Lingutla Chandrasekhar <clingutla@...eaurora.org>
Subject: Re: [PATCH v3 2/7] sched/fair: Clean up active balance
 nr_balance_failed trickery

On 11/03/2021 13:05, Valentin Schneider wrote:

[...]

> @@ -9952,7 +9954,8 @@ static int active_load_balance_cpu_stop(void *data)
>  			 * @dst_grpmask we need to make that test go away with lying
>  			 * about DST_PINNED.
>  			 */
> -			.flags		= LBF_DST_PINNED,
> +			.flags		= LBF_DST_PINNED |
> +					  LBF_ACTIVE_LB,

Since you now have a dedicated LBF flag for active balancing you could remove the
LBF_DST_PINNED here and adapt the if condition in can_migrate_task():

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f50a902bdf24..9f7feb512016 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7583,10 +7583,13 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
                 * 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.
+                * already computed one in current iteration or if it is 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: */
                for_each_cpu_and(cpu, env->dst_grpmask, env->cpus) {
@@ -9948,14 +9951,7 @@ 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 |
-                                         LBF_ACTIVE_LB,
+                       .flags          = LBF_ACTIVE_LB,
                };