sched/fair: Prevent throttling in early pick_next_task_fair()

The optimized task selection logic optimistically selects a new task
to run without first doing a full put_prev_task(). This is so that we
can avoid a put/set on the common ancestors of the old and new task.

Similarly, we should only call check_cfs_rq_runtime() to throttle
eligible groups if they're part of the common ancestry, otherwise it
is possible to end up with no eligible task in the simple task
selection.

Imagine:
		/root
	/prev		/next
	/A		/B

If our optimistic selection ends up throttling /next, we goto simple
and our put_prev_task() ends up throttling /prev, after which we're
going to bug out in set_next_entity() because there aren't any tasks
left.

Avoid this scenario by only throttling common ancestors.

Reported-by: Mohammed Naser <mnaser@vexxhost.com>
Reported-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Ben Segall <bsegall@google.com>
[ munged Changelog ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Roman Gushchin <klamm@yandex-team.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: pjt@google.com
Fixes: 678d5718d8d0 ("sched/fair: Optimize cgroup pick_next_task_fair()")
Link: http://lkml.kernel.org/r/xm26wq1oswoq.fsf@sword-of-the-dawn.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0d4632f..84ada05 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5322,18 +5322,21 @@ again:
                 * entity, update_curr() will update its vruntime, otherwise
                 * forget we've ever seen it.
                 */
-               if (curr && curr->on_rq)
-                       update_curr(cfs_rq);
-               else
-                       curr = NULL;
+               if (curr) {
+                       if (curr->on_rq)
+                               update_curr(cfs_rq);
+                       else
+                               curr = NULL;
 
-               /*
-                * This call to check_cfs_rq_runtime() will do the throttle and
-                * dequeue its entity in the parent(s). Therefore the 'simple'
-                * nr_running test will indeed be correct.
-                */
-               if (unlikely(check_cfs_rq_runtime(cfs_rq)))
-                       goto simple;
+                       /*
+                        * This call to check_cfs_rq_runtime() will do the
+                        * throttle and dequeue its entity in the parent(s).
+                        * Therefore the 'simple' nr_running test will indeed
+                        * be correct.
+                        */
+                       if (unlikely(check_cfs_rq_runtime(cfs_rq)))
+                               goto simple;
+               }
 
                se = pick_next_entity(cfs_rq, curr);
                cfs_rq = group_cfs_rq(se);