[3.8.y.z extended stable] Patch "sched: Avoid throttle_cfs_rq() racing with period_timer stopping" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    sched: Avoid throttle_cfs_rq() racing with period_timer stopping

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.15.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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>From db19c96cf65df6b5faf70fc58f8d12cce5d1e8a1 Mon Sep 17 00:00:00 2001
From: Ben Segall <bsegall at google.com>
Date: Wed, 16 Oct 2013 11:16:32 -0700
Subject: sched: Avoid throttle_cfs_rq() racing with period_timer stopping

commit f9f9ffc237dd924f048204e8799da74f9ecf40cf upstream.

throttle_cfs_rq() doesn't check to make sure that period_timer is running,
and while update_curr/assign_cfs_runtime does, a concurrently running
period_timer on another cpu could cancel itself between this cpu's
update_curr and throttle_cfs_rq(). If there are no other cfs_rqs running
in the tg to restart the timer, this causes the cfs_rq to be stranded
forever.

Fix this by calling __start_cfs_bandwidth() in throttle if the timer is
inactive.

(Also add some sched_debug lines for cfs_bandwidth.)

Tested: make a run/sleep task in a cgroup, loop switching the cgroup
between 1ms/100ms quota and unlimited, checking for timer_active=0 and
throttled=1 as a failure. With the throttle_cfs_rq() change commented out
this fails, with the full patch it passes.

Signed-off-by: Ben Segall <bsegall at google.com>
Signed-off-by: Peter Zijlstra <peterz at infradead.org>
Cc: pjt at google.com
Link: http://lkml.kernel.org/r/20131016181632.22647.84174.stgit@sword-of-the-dawn.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Chris J Arges <chris.j.arges at ubuntu.com>
[ Chris J Arges: backport to 3.8 ]
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/sched/debug.c | 10 ++++++++++
 kernel/sched/fair.c  |  2 ++
 2 files changed, 12 insertions(+)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 7ae4c4c..3539029 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -231,7 +231,17 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 	SEQ_printf(m, "  .%-30s: %d\n", "tg->runnable_avg",
 			atomic_read(&cfs_rq->tg->runnable_avg));
 #endif
+#endif
+#ifdef CONFIG_CFS_BANDWIDTH
+	SEQ_printf(m, "  .%-30s: %d\n", "tg->cfs_bandwidth.timer_active",
+			cfs_rq->tg->cfs_bandwidth.timer_active);
+	SEQ_printf(m, "  .%-30s: %d\n", "throttled",
+			cfs_rq->throttled);
+	SEQ_printf(m, "  .%-30s: %d\n", "throttle_count",
+			cfs_rq->throttle_count);
+#endif

+#ifdef CONFIG_FAIR_GROUP_SCHED
 	print_cfs_group_stats(m, cpu, cfs_rq->tg);
 #endif
 }
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e9ab160..6c8930e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2268,6 +2268,8 @@ static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
 	cfs_rq->throttled_clock = rq->clock;
 	raw_spin_lock(&cfs_b->lock);
 	list_add_tail_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq);
+	if (!cfs_b->timer_active)
+		__start_cfs_bandwidth(cfs_b);
 	raw_spin_unlock(&cfs_b->lock);
 }

--
1.8.3.2