UPSTREAM: sched/fair: Initialize throttle_count for new task-groups lazily

Cgroup created inside throttled group must inherit current throttle_count.
Broken throttle_count allows to nominate throttled entries as a next buddy,
later this leads to null pointer dereference in pick_next_task_fair().

This patch initialize cfs_rq->throttle_count at first enqueue: laziness
allows to skip locking all rq at group creation. Lazy approach also allows
to skip full sub-tree scan at throttling hierarchy (not in this patch).

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bsegall@google.com
Link: http://lkml.kernel.org/r/146608182119.21870.8439834428248129633.stgit@buzz
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrey Ulanov <andreyu@google.com>

(cherry-picked from commit 094f469172e00d6ab0a3130b0e01c83b3cf3a98d)

BUG=b:62255954
TEST=kernel compiles

Change-Id: I4b39f8ea3ff951fc59807b50c1bddde241c53efd
Reviewed-on: https://chromium-review.googlesource.com/521225
Commit-Ready: Andrey Ulanov <andreyu@google.com>
Tested-by: Andrey Ulanov <andreyu@google.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
(cherry picked from commit 43966ab2563e2df7cd0434350ee01d26ecaa615c)
Reviewed-on: https://chromium-review.googlesource.com/523026
Commit-Queue: Andrey Ulanov <andreyu@google.com>

kernel/sched/fair.c

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a9049a1..78dbcb4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3958,6 +3958,26 @@
 	if (!cfs_bandwidth_used())
 		return;
 
+	/* Synchronize hierarchical throttle counter: */
+	if (unlikely(!cfs_rq->throttle_uptodate)) {
+		struct rq *rq = rq_of(cfs_rq);
+		struct cfs_rq *pcfs_rq;
+		struct task_group *tg;
+
+		cfs_rq->throttle_uptodate = 1;
+
+		/* Get closest up-to-date node, because leaves go first: */
+		for (tg = cfs_rq->tg->parent; tg; tg = tg->parent) {
+			pcfs_rq = tg->cfs_rq[cpu_of(rq)];
+			if (pcfs_rq->throttle_uptodate)
+				break;
+		}
+		if (tg) {
+			cfs_rq->throttle_count = pcfs_rq->throttle_count;
+			cfs_rq->throttled_clock_task = rq_clock_task(rq);
+		}
+	}
+
 	/* an active group must be handled by the update_curr()->put() path */
 	if (!cfs_rq->runtime_enabled || cfs_rq->curr)
 		return;

kernel/sched/sched.h

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e22f476..e30a22a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -421,7 +421,7 @@
 
 	u64 throttled_clock, throttled_clock_task;
 	u64 throttled_clock_task_time;
-	int throttled, throttle_count;
+	int throttled, throttle_count, throttle_uptodate;
 	struct list_head throttled_list;
 #endif /* CONFIG_CFS_BANDWIDTH */
 #endif /* CONFIG_FAIR_GROUP_SCHED */