[PATCH] sched/fair: Avoid divide by zero when rebalancing domains

[Matt Fleming]

It's possible that the CPU doing nohz idle balance hasn't had its own
load updated for many seconds. This can lead to huge deltas between
rq->avg_stamp and rq->clock when rebalancing, and has been seen to
cause the following crash:

 divide error: 0000 [#1] SMP
 Call Trace:
  [<ffffffff810bcba8>] update_sd_lb_stats+0xe8/0x560
  [<ffffffff810bd04d>] find_busiest_group+0x2d/0x4b0
  [<ffffffff810bd640>] load_balance+0x170/0x950
  [<ffffffff810be3ff>] rebalance_domains+0x13f/0x290
  [<ffffffff810852bc>] __do_softirq+0xec/0x300
  [<ffffffff8108578a>] irq_exit+0xfa/0x110
  [<ffffffff816167d9>] reschedule_interrupt+0xc9/0xd0

Make sure we update the rq clock and load before balancing.

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2f0a0be4d344..2c81662c858a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9597,6 +9597,16 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
 	 */
 	smp_mb();
 
+	/*
+	 * Ensure this_rq's clock and load are up-to-date before we
+	 * rebalance since it's possible that they haven't been
+	 * updated for multiple schedule periods, i.e. many seconds.
+	 */
+	raw_spin_lock_irq(&this_rq->lock);
+	update_rq_clock(this_rq);
+	cpu_load_update_idle(this_rq);
+	raw_spin_unlock_irq(&this_rq->lock);
+
 	for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
 		if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
 			continue;
-- 
2.13.6