[PATCH 2/2] sched: Warn if we fail to migrate a task

[Paul Burton <paul.burton@mips.com>]

__set_cpus_allowed_ptr() makes use of stop_one_cpu() to call
migration_cpu_stop() in order to perform migration of a task away from
the CPU it's currently running on. If all is as expected then this
shouldn't fail, but as the preceding patch shows it's possible for this
assumption to be broken fairly subtly.

Add a warning to ensure that if stop_one_cpu() returns an error
(-ENOENT is the only one it can currently return) then we warn about it
in the kernel log, since this isn't expected to happen.

I considered propogating the error upwards, but this would require a
change to the return values allowed from the sched_setaffinity() syscall
and would require that user programs handle errors other than those
caused by their own bad input to the syscall. So for now this patch
simply warns, which is an improvement over the silent error & incorrect
scheduling we had before.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org

---

 kernel/sched/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2380bc228dd0..cda3affd45b7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1127,7 +1127,8 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
 		struct migration_arg arg = { p, dest_cpu };
 		/* Need help from migration thread: drop lock and wait. */
 		task_rq_unlock(rq, p, &rf);
-		stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
+		ret = stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
+		WARN_ON(ret);
 		tlb_migrate_finish(p->mm);
 		return 0;
 	} else if (task_on_rq_queued(p)) {
-- 
2.17.0