linux-kernel - Re: [PATCH 6/6] sched: Simplify set_affinity

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Date:   Wed, 24 Feb 2021 16:34:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Mel Gorman <mgorman@...e.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 6/6] sched: Simplify set_affinity_pending refcounts

On Wed, Feb 24, 2021 at 01:24:45PM +0100, Peter Zijlstra wrote:
> @@ -2199,12 +2199,16 @@ static int affine_move_task(struct rq *r
>  			push_task = get_task_struct(p);
>  		}
>  
> +		/*
> +		 * If there are pending waiters, but no pending stop_work,
> +		 * then complete now.
> +		 */
>  		pending = p->migration_pending;
> +		if (pending && !pending->stop_pending) {
>  			p->migration_pending = NULL;
>  			complete = true;
>  		}

> @@ -2282,12 +2286,13 @@ static int affine_move_task(struct rq *r
>  			if (task_on_rq_queued(p))
>  				rq = move_queued_task(rq, rf, p, dest_cpu);
>  
> +			if (!pending->stop_pending) {
> +				p->migration_pending = NULL;
> +				complete = true;
> +			}
>  		}
>  		task_rq_unlock(rq, p, rf);

Elsewhere Valentin argued something like the below ought to be possible.
I've not drawn diagrams yet, but if I understood his argument right it
should be possible.

---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1c56ac4df2c9..3ffbd1b76f3e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2204,9 +2204,10 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
 		 * then complete now.
 		 */
 		pending = p->migration_pending;
-		if (pending && !pending->stop_pending) {
+		if (pending) {
 			p->migration_pending = NULL;
-			complete = true;
+			if (!pending->stop_pending)
+				complete = true;
 		}
 
 		task_rq_unlock(rq, p, rf);
@@ -2286,10 +2287,9 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
 			if (task_on_rq_queued(p))
 				rq = move_queued_task(rq, rf, p, dest_cpu);
 
-			if (!pending->stop_pending) {
-				p->migration_pending = NULL;
+			p->migration_pending = NULL;
+			if (!pending->stop_pending)
 				complete = true;
-			}
 		}
 		task_rq_unlock(rq, p, rf);