sched: fix cpu hotplug

Commit 79c537998d143b127c8c662a403c3356cb885f1c upstream

the CPU hotplug problems (crashes under high-volume unplug+replug
tests) seem to be related to migrate_dead_tasks().

Firstly I added traces to see all tasks being migrated with
migrate_live_tasks() and migrate_dead_tasks(). On my setup the problem
pops up (the one with "se == NULL" in the loop of
pick_next_task_fair()) shortly after the traces indicate that some has
been migrated with migrate_dead_tasks()). btw., I can reproduce it
much faster now with just a plain cpu down/up loop.

[disclaimer] Well, unless I'm really missing something important in
this late hour [/desclaimer] pick_next_task() is not something
appropriate for migrate_dead_tasks() :-)

the following change seems to eliminate the problem on my setup
(although, I kept it running only for a few minutes to get a few
messages indicating migrate_dead_tasks() does move tasks and the
system is still ok)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 1e4596c68dd0..c54077ca1ed1 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5728,6 +5728,7 @@ static void migrate_dead_tasks(unsigned int dead_cpu)
 		next = pick_next_task(rq, rq->curr);
 		if (!next)
 			break;
+		next->sched_class->put_prev_task(rq, next);
 		migrate_dead(dead_cpu, next);
 
 	}