3.19.8-ckt4 -stable review patch. If anyone has any objections, please let me
know.
------------------
From: Ben Segall <bseg...@google.com>
commit 54d27365cae88fbcc853b391dcd561e71acb81fa upstream.
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 <mna...@vexxhost.com>
Reported-by: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
Signed-off-by: Ben Segall <bseg...@google.com>
[ munged Changelog ]
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: H. Peter Anvin <h...@zytor.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Roman Gushchin <kl...@yandex-team.ru>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: p...@google.com
Fixes: 678d5718d8d0 ("sched/fair: Optimize cgroup pick_next_task_fair()")
Link:
http://lkml.kernel.org/r/xm26wq1oswoq....@sword-of-the-dawn.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mi...@kernel.org>
Signed-off-by: Kamal Mostafa <ka...@canonical.com>
---
kernel/sched/fair.c | 25 ++++++++++++++-----------
1 file changed, 14 insertions(+), 11 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 44cfeb3..9e78f9f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5028,18 +5028,21 @@ 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);
--
1.9.1
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