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

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Message-Id: <20180704142455.16035-1-matt@codeblueprint.co.uk>
Date:   Wed,  4 Jul 2018 15:24:55 +0100
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Ingo Molnar <mingo@...nel.org>,
        Mike Galbraith <umgwanakikbuti@...il.com>
Subject: [PATCH] sched/fair: Avoid divide by zero when rebalancing domains

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@...nel.org>
Cc: Mike Galbraith <umgwanakikbuti@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Matt Fleming <matt@...eblueprint.co.uk>
---
 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