linux-kernel - [PATCH 4.1 117/202] sched/fair: Prevent throttling in early pick_next_task

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Message-Id: <20151018014907.245719036@linuxfoundation.org>
Date:	Sat, 17 Oct 2015 18:58:20 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Mohammed Naser <mnaser@...xhost.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Ben Segall <bsegall@...gle.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roman Gushchin <klamm@...dex-team.ru>,
	Thomas Gleixner <tglx@...utronix.de>, pjt@...gle.com,
	Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 4.1 117/202] sched/fair: Prevent throttling in early pick_next_task_fair()

4.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ben Segall <bsegall@...gle.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 <mnaser@...xhost.com>
Reported-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Signed-off-by: Ben Segall <bsegall@...gle.com>
[ munged Changelog ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Roman Gushchin <klamm@...dex-team.ru>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: pjt@...gle.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@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/sched/fair.c |   25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5126,18 +5126,21 @@ 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);

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