[PATCH] sched: update last_balance only when actually balanced

[Rik van Riel <riel@redhat.com>]

Currently rebalance_domains will happily update sd->last_balance
even if should_we_balance() decides that this CPU should not do
any load balancing for the sd.

This can prevent the CPU that should load balance for the sd from
doing load balancing, which can delay active balancing essentially
forever.

The solution is to only update sd->last_balance when load_balance()
actually did something.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
 kernel/sched/fair.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 45943b2..4f7fee2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7187,7 +7187,9 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 				 */
 				idle = idle_cpu(cpu) ? CPU_IDLE : CPU_NOT_IDLE;
 			}
-			sd->last_balance = jiffies;
+			/* Only update if we actually balanced. */
+			if (continue_balancing)
+				sd->last_balance = jiffies;
 			interval = get_sd_balance_interval(sd, idle != CPU_IDLE);
 		}
 		if (need_serialize)