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@google.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
CC: Konstantin Khorenko <khorenko@parallels.com>
CC: Paul Turner <pjt@google.com>
CC: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1403684382.3462.42.camel@tkhai
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 29 insertions, 1 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e50234ba0b27..2dbc63d1a847 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 a140c6a8c947..923fe32db6b3 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)