[PATCH 3/5] sched/fair: Call update_curr() before unthrottling the hierarchy

[K Prateek Nayak]

Subsequent commits will allow update_curr() to throttle the hierarchy
when the runtime accounting exceeds allocated quota. Call update_curr()
before the unthrottle event, and in tg_unthrottle_up() to catch up on
any remaining runtime and stabilize the "runtime_remaining" and
"throttle_count" for that cfs_rq.

Doing an update_curr() early ensures the cfs_rq is not throttled right
back up again when the unthrottle is in progress.

Since all callers of unthrottle_cfs_rq(), except one, already update the
rq_clock and call rq_clock_start_loop_update(), move the
update_rq_clock() in unthrottle_cfs_rq() to the lonely caller that
doesn't update the rq_clock.

Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
 kernel/sched/fair.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 23742d928b51..b3b3172702a9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6711,6 +6711,15 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
 	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
 	struct task_struct *p, *tmp;
 
+	/*
+	 * If cfs_rq->curr is set, the cfs_rq might not have caught up
+	 * since the last clock update. Do it now before we begin
+	 * queueing task onto it to save the need for unnecessarily
+	 * unthrottle the hierarchy for this cfs_rq to be throttled
+	 * right back again.
+	 */
+	update_curr(cfs_rq);
+
 	if (--cfs_rq->throttle_count)
 		return 0;
 
@@ -6853,14 +6862,16 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 	 * We can't unthrottle this cfs_rq without any runtime remaining because
 	 * any enqueue in tg_unthrottle_up() will immediately trigger a throttle,
 	 * which is not supposed to happen on unthrottle path.
+	 *
+	 * Catch up on the remaining runtime since last clock update before
+	 * checking runtime remaining.
 	 */
+	update_curr(cfs_rq);
 	if (cfs_rq->runtime_enabled && cfs_rq->runtime_remaining <= 0)
 		return;
 
 	cfs_rq->throttled = 0;
 
-	update_rq_clock(rq);
-
 	scoped_guard(raw_spinlock, &cfs_b->lock) {
 		list_del_rcu(&cfs_rq->throttled_list);
 
@@ -6935,6 +6946,7 @@ static inline void __unthrottle_cfs_rq_async(struct cfs_rq *cfs_rq)
 	bool first;
 
 	if (rq == this_rq()) {
+		update_rq_clock(rq);
 		unthrottle_cfs_rq(cfs_rq);
 		return;
 	}
-- 
2.43.0