[PATCH RFC] sched/fair: fix sudden expiration of cfq quota in put_prev_task()

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

Pick_next_task_fair() must be sure that here is at least one runnable
task before calling put_prev_task(), but put_prev_task() can expire
last remains of cfs quota and throttle all currently runnable tasks.
As a result pick_next_task_fair() cannot find next task and crashes.

This patch leaves 1 in ->runtime_remaining when current assignation
expires and tries to refill it right after that. In the worst case
task will be scheduled once and throttled at the end of slice.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 kernel/sched/fair.c |   19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7ce18f3c097a..91785d077db4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3447,11 +3447,12 @@ static void expire_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 {
 	struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
 
-	/* if the deadline is ahead of our clock, nothing to do */
-	if (likely((s64)(rq_clock(rq_of(cfs_rq)) - cfs_rq->runtime_expires) < 0))
+	/* nothing to expire */
+	if (cfs_rq->runtime_remaining <= 0)
 		return;
 
-	if (cfs_rq->runtime_remaining < 0)
+	/* if the deadline is ahead of our clock, nothing to do */
+	if (likely((s64)(rq_clock(rq_of(cfs_rq)) - cfs_rq->runtime_expires) < 0))
 		return;
 
 	/*
@@ -3469,8 +3470,14 @@ static void expire_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 		/* extend local deadline, drift is bounded above by 2 ticks */
 		cfs_rq->runtime_expires += TICK_NSEC;
 	} else {
-		/* global deadline is ahead, expiration has passed */
-		cfs_rq->runtime_remaining = 0;
+		/*
+		 * Global deadline is ahead, expiration has passed.
+		 *
+		 * Do not expire runtime completely. Otherwise put_prev_task()
+		 * can throttle all tasks when we already checked nr_running or
+		 * put_prev_entity() can throttle already chosen next entity.
+		 */
+		cfs_rq->runtime_remaining = 1;
 	}
 }
 
@@ -3480,7 +3487,7 @@ static void __account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec)
 	cfs_rq->runtime_remaining -= delta_exec;
 	expire_cfs_rq_runtime(cfs_rq);
 
-	if (likely(cfs_rq->runtime_remaining > 0))
+	if (likely(cfs_rq->runtime_remaining > 1))
 		return;
 
 	/*