[tip:sched/core] sched/fair: Disable runtime_enabled on dying rq

[tip-bot for Kirill Tkhai]

Commit-ID:  0e59bdaea75f12a7d7c03672f4ac22c0119a1bc0
Gitweb:     http://git.kernel.org/tip/0e59bdaea75f12a7d7c03672f4ac22c0119a1bc0
Author:     Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
AuthorDate: Wed, 25 Jun 2014 12:19:42 +0400
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sat, 5 Jul 2014 11:17:42 +0200

sched/fair: Disable runtime_enabled on dying rq

We kill rq->rd on the CPU_DOWN_PREPARE stage:

	cpuset_cpu_inactive -> cpuset_update_active_cpus -> partition_sched_domains ->
	-> cpu_attach_domain -> rq_attach_root -> set_rq_offline

This unthrottles all throttled cfs_rqs.

But the cpu is still able to call schedule() till

	take_cpu_down->__cpu_disable()

is called from stop_machine.

This case the tasks from just unthrottled cfs_rqs are pickable
in a standard scheduler way, and they are picked by dying cpu.
The cfs_rqs becomes throttled again, and migrate_tasks()
in migration_call skips their tasks (one more unthrottle
in migrate_tasks()->CPU_DYING does not happen, because rq->rd
is already NULL).

Patch sets runtime_enabled to zero. This guarantees, the runtime
is not accounted, and the cfs_rqs won't exceed given
cfs_rq->runtime_remaining = 1, and tasks will be pickable
in migrate_tasks(). runtime_enabled is recalculated again
when rq becomes online again.

Ben Segall also noticed, we always enable runtime in
tg_set_cfs_bandwidth(). Actually, we should do that for online
cpus only. To prevent races with unthrottle_offline_cfs_rqs()
we take get_online_cpus() lock.

Reviewed-by: Ben Segall <bsegall@xxxxxxxxxx>
Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
CC: Konstantin Khorenko <khorenko@xxxxxxxxxxxxx>
CC: Paul Turner <pjt@xxxxxxxxxx>
CC: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1403684382.3462.42.camel@tkhai
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/core.c |  8 +++++++-
 kernel/sched/fair.c | 22 ++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e50234b..2dbc63d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7817,6 +7817,11 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
 	if (period > max_cfs_quota_period)
 		return -EINVAL;
 
+	/*
+	 * Prevent race between setting of cfs_rq->runtime_enabled and
+	 * unthrottle_offline_cfs_rqs().
+	 */
+	get_online_cpus();
 	mutex_lock(&cfs_constraints_mutex);
 	ret = __cfs_schedulable(tg, period, quota);
 	if (ret)
@@ -7842,7 +7847,7 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
 	}
 	raw_spin_unlock_irq(&cfs_b->lock);
 
-	for_each_possible_cpu(i) {
+	for_each_online_cpu(i) {
 		struct cfs_rq *cfs_rq = tg->cfs_rq[i];
 		struct rq *rq = cfs_rq->rq;
 
@@ -7858,6 +7863,7 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
 		cfs_bandwidth_usage_dec();
 out_unlock:
 	mutex_unlock(&cfs_constraints_mutex);
+	put_online_cpus();
 
 	return ret;
 }
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a140c6a..923fe32 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3798,6 +3798,19 @@ static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
 	hrtimer_cancel(&cfs_b->slack_timer);
 }
 
+static void __maybe_unused update_runtime_enabled(struct rq *rq)
+{
+	struct cfs_rq *cfs_rq;
+
+	for_each_leaf_cfs_rq(rq, cfs_rq) {
+		struct cfs_bandwidth *cfs_b = &cfs_rq->tg->cfs_bandwidth;
+
+		raw_spin_lock(&cfs_b->lock);
+		cfs_rq->runtime_enabled = cfs_b->quota != RUNTIME_INF;
+		raw_spin_unlock(&cfs_b->lock);
+	}
+}
+
 static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
 {
 	struct cfs_rq *cfs_rq;
@@ -3811,6 +3824,12 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
 		 * there's some valid quota amount
 		 */
 		cfs_rq->runtime_remaining = 1;
+		/*
+		 * Offline rq is schedulable till cpu is completely disabled
+		 * in take_cpu_down(), so we prevent new cfs throttling here.
+		 */
+		cfs_rq->runtime_enabled = 0;
+
 		if (cfs_rq_throttled(cfs_rq))
 			unthrottle_cfs_rq(cfs_rq);
 	}
@@ -3854,6 +3873,7 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
 	return NULL;
 }
 static inline void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {}
+static inline void update_runtime_enabled(struct rq *rq) {}
 static inline void unthrottle_offline_cfs_rqs(struct rq *rq) {}
 
 #endif /* CONFIG_CFS_BANDWIDTH */
@@ -7362,6 +7382,8 @@ void trigger_load_balance(struct rq *rq)
 static void rq_online_fair(struct rq *rq)
 {
 	update_sysctl();
+
+	update_runtime_enabled(rq);
 }
 
 static void rq_offline_fair(struct rq *rq)
--
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