[RFC PATCH 12/22] sched/fair: Prepare bandwidth distribution to unthrottle partial throttles right away

[K Prateek Nayak]

Unthrottling a partially throttled hierarchy is cheap since it only
involves updating the cfs_rq's throttle status and does not involve
modifing the hierarchical indicators via tg_tree walks or enqueuing the
cfs_rq back.

Unthrottle a partially throttled cfs_rq right away. This also helps
avoiding a subtle race with async throttling, specifically:

  distribute_cfs_runtime()
    rq_lock_irqsave(rq)
    # Replenish bandwidth		schedule()
    unthrottle_cfs_rq_async()		  local_irq_disable()
      smp_call_function_single_async() -> # IPI pending
    rq_unlock_irqrestore()		  rq_lock()
					  pick_task_fair()
					    check_cfs_rq_runtime()
					    # runtime_remaining > 0 but
					    # cfs_rq->throttled != 0

The above can lead pick_eevdf() failing to find a sched_entity which is
now avoided.

Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
 kernel/sched/fair.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 091493bc8506..8824e89a3ede 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6320,6 +6320,17 @@ static bool distribute_cfs_runtime(struct cfs_bandwidth *cfs_b)
 
 		/* we check whether we're throttled above */
 		if (cfs_rq->runtime_remaining > 0) {
+			/*
+			 * Partially throttled cfs_rq is cheap to unthrottle
+			 * since they only require updating the throttled
+			 * status of cfs_rq. Save the need for reacquiring the
+			 * rq_lock and a possible IPI by unthrottling it here.
+			 */
+			if (!cfs_rq_h_throttled(cfs_rq)) {
+				unthrottle_cfs_rq(cfs_rq, false);
+				goto next;
+			}
+
 			if (cpu_of(rq) != this_cpu) {
 				unthrottle_cfs_rq_async(cfs_rq);
 			} else {
-- 
2.43.0