[PATCH] sched: sched: Fix rq->next_balance time updated to earlier than current time

[Tim Chen]

In traces on newidle_balance(), this_rq->next_balance
time goes backward and earlier than current time jiffies, e.g.

11.602 (         ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb739
11.624 (         ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
13.856 (         ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73b
13.910 (         ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
14.637 (         ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73c
14.666 (         ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c

It doesn't make sense to have a next_balance in the past.
Fix newidle_balance() and update_next_balance() so the next
balance time is at least jiffies+1.
---
 kernel/sched/fair.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1d75af1ecfb4..740a0572cbf1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9901,7 +9901,10 @@ update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
 
 	/* used by idle balance, so cpu_busy = 0 */
 	interval = get_sd_balance_interval(sd, 0);
-	next = sd->last_balance + interval;
+	if (time_after(jiffies+1, sd->last_balance + interval))
+		next = jiffies+1;
+	else
+		next = sd->last_balance + interval;
 
 	if (time_after(*next_balance, next))
 		*next_balance = next;
@@ -10681,6 +10684,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 
 out:
 	/* Move the next balance forward */
+	if (time_after(jiffies+1, this_rq->next_balance))
+		this_rq->next_balance = jiffies+1;
 	if (time_after(this_rq->next_balance, next_balance))
 		this_rq->next_balance = next_balance;
 
-- 
2.20.1