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

tip-bot2 for Peter Zijlstra
2021-03-01T10:19:38Z

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@infradead.org>
AuthorDate: Wed, 24 Feb 2021 11:21:35 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
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@kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210224131355.569238629@infradead.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 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