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Date:   Thu, 5 Jul 2018 15:43:54 +0100
From:   Matt Fleming <matt@codeblueprint.co.uk>
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc:     kernel test robot <xiaolong.ye@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Mike Galbraith <umgwanakikbuti@gmail.com>, lkp@01.org
Subject: Re: [lkp-robot] [sched/fair] fbd5188493:
 WARNING:inconsistent_lock_state
Message-ID: <20180705144354.GC3864@codeblueprint.co.uk>
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On Thu, 05 Jul, at 02:24:58PM, Matt Fleming wrote:
> 
> Hmm.. it still looks to me like we should be saving and restoring IRQs
> since this can be called from IRQ context, no?
> 
> The patch was a forward-port from one of our SLE kernels, and I messed
> up the IRQ flag balancing for the v4.18-rc3 code :-(
 
Something like this?

---->8----

From 9b152d8dadec04ac631300d86a92552e57e81db5 Mon Sep 17 00:00:00 2001
From: Matt Fleming <matt@codeblueprint.co.uk>
Date: Wed, 4 Jul 2018 14:22:51 +0100
Subject: [PATCH v2] 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@kernel.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
---
 kernel/sched/fair.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

Changes in v2: Balance IRQ flags properly.

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2f0a0be4d344..150b92c7c9d1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9676,6 +9676,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 {
 	int this_cpu = this_rq->cpu;
 	unsigned int flags;
+	struct rq_flags rf;
 
 	if (!(atomic_read(nohz_flags(this_cpu)) & NOHZ_KICK_MASK))
 		return false;
@@ -9692,6 +9693,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 	if (!(flags & NOHZ_KICK_MASK))
 		return false;
 
+	/*
+	 * 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.
+	 */
+	rq_lock_irqsave(this_rq, &rf);
+	update_rq_clock(this_rq);
+	cpu_load_update_idle(this_rq);
+	rq_unlock_irqrestore(this_rq, &rf);
+
 	_nohz_idle_balance(this_rq, flags, idle);
 
 	return true;
-- 
2.13.6