linux-kernel - [PATCH RT 4/8] sched: Optimize migration_cpu

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Date:   Fri, 09 Jul 2021 17:59:57 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Tom Zanussi <zanussi@...nel.org>,
        "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>, stable@...nel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: [PATCH RT 4/8] sched: Optimize migration_cpu_stop()

5.10.47-rt46-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@...radead.org>

commit 3f1bc119cd7fc987c8ed25ffb717f99403bb308c upstream.

When the purpose of migration_cpu_stop() is to migrate the task to
'any' valid CPU, don't migrate the task when it's already running on a
valid CPU.

Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Cc: stable@...nel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Valentin Schneider <valentin.schneider@....com>
Link: https://lkml.kernel.org/r/20210224131355.569238629@infradead.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 kernel/sched/core.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6880c300c624..9cbe12d8c5bd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1972,14 +1972,25 @@ static int migration_cpu_stop(void *data)
 			complete = true;
 		}
 
-		if (dest_cpu < 0)
+		if (dest_cpu < 0) {
+			if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask))
+				goto out;
+
 			dest_cpu = cpumask_any_distribute(&p->cpus_mask);
+		}
 
 		if (task_on_rq_queued(p))
 			rq = __migrate_task(rq, &rf, p, dest_cpu);
 		else
 			p->wake_cpu = dest_cpu;
 
+		/*
+		 * XXX __migrate_task() can fail, at which point we might end
+		 * up running on a dodgy CPU, AFAICT this can only happen
+		 * during CPU hotplug, at which point we'll get pushed out
+		 * anyway, so it's probably not a big deal.
+		 */
+
 	} else if (pending) {
 		/*
 		 * This happens when we get migrated between migrate_enable()'s
-- 
2.30.2