[tip: sched/urgent] sched: Optimize migration_cpu_stop()

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     9eca0f53b1c2f5acb85e84673e263bf996817a24
Gitweb:        https://git.kernel.org/tip/9eca0f53b1c2f5acb85e84673e263bf996817a24
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Wed, 24 Feb 2021 11:21:35 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 01 Mar 2021 11:02:14 +01:00

sched: Optimize migration_cpu_stop()

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@xxxxxxxxxx
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Link: https://lkml.kernel.org/r/20210224131355.569238629@xxxxxxxxxxxxx
---
 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 84b657f..ac05afb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1936,14 +1936,25 @@ 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