sched: Fix race on toggling cfs_bandwidth_used

When we transition cfs_bandwidth_used to false, any currently
throttled groups will incorrectly return false from cfs_rq_throttled.
While tg_set_cfs_bandwidth will unthrottle them eventually, currently
running code (including at least dequeue_task_fair and
distribute_cfs_runtime) will cause errors.

Fix this by turning off cfs_bandwidth_used only after unthrottling all
cfs_rqs.

Tested: toggle bandwidth back and forth on a loaded cgroup. Caused
crashes in minutes without the patch, hasn't crashed with it.

Signed-off-by: Ben Segall <bsegall@google.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: pjt@google.com
Link: http://lkml.kernel.org/r/20131016181611.22647.80365.stgit@sword-of-the-dawn.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

3 files changed, 19 insertions, 9 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7c61f313521d..450a34b2a637 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7436,7 +7436,12 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
 
         runtime_enabled = quota != RUNTIME_INF;
         runtime_was_enabled = cfs_b->quota != RUNTIME_INF;
-        account_cfs_bandwidth_used(runtime_enabled, runtime_was_enabled);
+        /*
+         * If we need to toggle cfs_bandwidth_used, off->on must occur
+         * before making related changes, and on->off must occur afterwards
+         */
+        if (runtime_enabled && !runtime_was_enabled)
+                cfs_bandwidth_usage_inc();
         raw_spin_lock_irq(&cfs_b->lock);
         cfs_b->period = ns_to_ktime(period);
         cfs_b->quota = quota;
@@ -7462,6 +7467,8 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
                         unthrottle_cfs_rq(cfs_rq);
                 raw_spin_unlock_irq(&rq->lock);
         }
+        if (runtime_was_enabled && !runtime_enabled)
+                cfs_bandwidth_usage_dec();
 out_unlock:
         mutex_unlock(&cfs_constraints_mutex);
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 813dd61a9b43..ebd187f50339 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2845,13 +2845,14 @@ static inline bool cfs_bandwidth_used(void)
         return static_key_false(&__cfs_bandwidth_used);
 }
 
-void account_cfs_bandwidth_used(int enabled, int was_enabled)
+void cfs_bandwidth_usage_inc(void)
 {
-        /* only need to count groups transitioning between enabled/!enabled */
-        if (enabled && !was_enabled)
-                static_key_slow_inc(&__cfs_bandwidth_used);
-        else if (!enabled && was_enabled)
-                static_key_slow_dec(&__cfs_bandwidth_used);
+        static_key_slow_inc(&__cfs_bandwidth_used);
+}
+
+void cfs_bandwidth_usage_dec(void)
+{
+        static_key_slow_dec(&__cfs_bandwidth_used);
 }
 #else /* HAVE_JUMP_LABEL */
 static bool cfs_bandwidth_used(void)
@@ -2859,7 +2860,8 @@ static bool cfs_bandwidth_used(void)
         return true;
 }
 
-void account_cfs_bandwidth_used(int enabled, int was_enabled) {}
+void cfs_bandwidth_usage_inc(void) {}
+void cfs_bandwidth_usage_dec(void) {}
 #endif /* HAVE_JUMP_LABEL */
 
 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ffc708717b70..4e650acffed7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1352,7 +1352,8 @@ extern void print_rt_stats(struct seq_file *m, int cpu);
 extern void init_cfs_rq(struct cfs_rq *cfs_rq);
 extern void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq);
 
-extern void account_cfs_bandwidth_used(int enabled, int was_enabled);
+extern void cfs_bandwidth_usage_inc(void);
+extern void cfs_bandwidth_usage_dec(void);
 
 #ifdef CONFIG_NO_HZ_COMMON
 enum rq_nohz_flag_bits {