Re: WARNING: CPU: 1 PID: 15 at kernel/sched/sched.h:804 assert_clock_updated.isra.62.part.63+0x25/0x27

[Matt Fleming <matt@codeblueprint.co.uk>]

On Sat, 28 Jan, at 08:21:05AM, Mike Galbraith wrote:
> Running Steven's hotplug stress script in tip.today.  Config is
> NOPREEMPT, tune for maximum build time (enterprise default-ish).
> 
> [   75.268049] x86: Booting SMP configuration:
> [   75.268052] smpboot: Booting Node 0 Processor 1 APIC 0x2
> [   75.279994] smpboot: Booting Node 0 Processor 2 APIC 0x4
> [   75.294617] smpboot: Booting Node 0 Processor 4 APIC 0x1
> [   75.310698] smpboot: Booting Node 0 Processor 5 APIC 0x3
> [   75.359056] smpboot: CPU 3 is now offline
> [   75.415505] smpboot: CPU 4 is now offline
> [   75.479985] smpboot: CPU 5 is now offline
> [   75.550674] ------------[ cut here ]------------
> [   75.550678] WARNING: CPU: 1 PID: 15 at kernel/sched/sched.h:804 assert_clock_updated.isra.62.part.63+0x25/0x27
> [   75.550679] rq->clock_update_flags < RQCF_ACT_SKIP

The following patch queued in tip/sched/core should fix this issue:

---->8----

>From 4d25b35ea3729affd37d69c78191ce6f92766e1a Mon Sep 17 00:00:00 2001
From: Matt Fleming <matt@codeblueprint.co.uk>
Date: Wed, 26 Oct 2016 16:15:44 +0100
Subject: [PATCH] sched/fair: Restore previous rq_flags when migrating tasks in
 hotplug

__migrate_task() can return with a different runqueue locked than the
one we passed as an argument. So that we can repin the lock in
migrate_tasks() (and keep the update_rq_clock() bit) we need to
restore the old rq_flags before repinning.

Note that it wouldn't be correct to change move_queued_task() to repin
because of the change of runqueue and the fact that having an
up-to-date clock on the initial rq doesn't mean the new rq has one
too.

Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7f983e83a353..3b248b03ad8f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5608,7 +5608,7 @@ static void migrate_tasks(struct rq *dead_rq)
 {
 	struct rq *rq = dead_rq;
 	struct task_struct *next, *stop = rq->stop;
-	struct rq_flags rf;
+	struct rq_flags rf, old_rf;
 	int dest_cpu;
 
 	/*
@@ -5669,6 +5669,13 @@ static void migrate_tasks(struct rq *dead_rq)
 			continue;
 		}
 
+		/*
+		 * __migrate_task() may return with a different
+		 * rq->lock held and a new cookie in 'rf', but we need
+		 * to preserve rf::clock_update_flags for 'dead_rq'.
+		 */
+		old_rf = rf;
+
 		/* Find suitable destination for @next, with force if needed. */
 		dest_cpu = select_fallback_rq(dead_rq->cpu, next);
 
@@ -5677,6 +5684,7 @@ static void migrate_tasks(struct rq *dead_rq)
 			raw_spin_unlock(&rq->lock);
 			rq = dead_rq;
 			raw_spin_lock(&rq->lock);
+			rf = old_rf;
 		}
 		raw_spin_unlock(&next->pi_lock);
 	}
-- 
2.10.0