sched/fair: Refill bandwidth before scaling

In order to prevent possible hardlockup of sched_cfs_period_timer()
loop, loop count is introduced to denote whether to scale quota and
period or not. However, scale is done between forwarding period timer
and refilling cfs bandwidth runtime, which means that period timer is
forwarded with old "period" while runtime is refilled with scaled
"quota".

Move do_sched_cfs_period_timer() before scaling to solve this.

Fixes: 2e8e192 ("sched/fair: Limit sched_cfs_period_timer() loop to avoid hard lockup")
Signed-off-by: Huaixin Chang <changhuaixin@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ben Segall <bsegall@google.com>
Reviewed-by: Phil Auld <pauld@redhat.com>
Link: https://lkml.kernel.org/r/20200420024421.22442-3-changhuaixin@linux.alibaba.com

kernel/sched/fair.c

@@ -5159,6 +5159,8 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 		if (!overrun)
 			break;
 
+		idle = do_sched_cfs_period_timer(cfs_b, overrun, flags);
+
 		if (++count > 3) {
 			u64 new, old = ktime_to_ns(cfs_b->period);
 
@@ -5188,8 +5190,6 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 			/* reset count so we don't come right back in here */
 			count = 0;
 		}
-
-		idle = do_sched_cfs_period_timer(cfs_b, overrun, flags);
 	}
 	if (idle)
 		cfs_b->period_active = 0;