linux-kernel - [PATCH 3/8] sched: Dont run cpu-online with balance

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Date:   Sat, 16 Jan 2021 12:30:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org, tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
        valentin.schneider@....com, cai@...hat.com,
        vincent.donnefort@....com, decui@...rosoft.com, paulmck@...nel.org,
        vincent.guittot@...aro.org, rostedt@...dmis.org, tj@...nel.org,
        peterz@...radead.org
Subject: [PATCH 3/8] sched: Dont run cpu-online with balance_push() enabled

We don't need to push away tasks when we come online, mark the push
complete right before the CPU dies.

XXX hotplug state machine has trouble with rollback here.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/core.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7320,10 +7320,12 @@ static void balance_push_set(int cpu, bo
 	struct rq_flags rf;
 
 	rq_lock_irqsave(rq, &rf);
-	if (on)
+	if (on) {
+		WARN_ON_ONCE(rq->balance_callback);
 		rq->balance_callback = &balance_push_callback;
-	else
+	} else if (rq->balance_callback == &balance_push_callback) {
 		rq->balance_callback = NULL;
+	}
 	rq_unlock_irqrestore(rq, &rf);
 }
 
@@ -7441,6 +7443,10 @@ int sched_cpu_activate(unsigned int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	struct rq_flags rf;
 
+	/*
+	 * Make sure that when the hotplug state machine does a roll-back
+	 * we clear balance_push. Ideally that would happen earlier...
+	 */
 	balance_push_set(cpu, false);
 
 #ifdef CONFIG_SCHED_SMT
@@ -7608,6 +7614,12 @@ int sched_cpu_dying(unsigned int cpu)
 	}
 	rq_unlock_irqrestore(rq, &rf);
 
+	/*
+	 * Should really be after we clear cpu_online(), but we're in
+	 * stop_machine(), so it all works.
+	 */
+	balance_push_set(cpu, false);
+
 	calc_load_migrate(rq);
 	update_max_interval();
 	nohz_balance_exit_idle(rq);