[3.13.y-ckt stable] Patch "Revert "workqueue: make sure delayed work run in local cpu"" has been added to the 3.13.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    Revert "workqueue: make sure delayed work run in local cpu"

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt36.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 5e3652f751867d4904cea7a76d6bef49da6e1ab6 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj at kernel.org>
Date: Tue, 9 Feb 2016 16:11:26 -0500
Subject: Revert "workqueue: make sure delayed work run in local cpu"

commit 041bd12e272c53a35c54c13875839bcb98c999ce upstream.

This reverts commit 874bbfe600a660cba9c776b3957b1ce393151b76.

Workqueue used to implicity guarantee that work items queued without
explicit CPU specified are put on the local CPU.  Recent changes in
timer broke the guarantee and led to vmstat breakage which was fixed
by 176bed1de5bf ("vmstat: explicitly schedule per-cpu work on the CPU
we need it to run on").

vmstat is the most likely to expose the issue and it's quite possible
that there are other similar problems which are a lot more difficult
to trigger.  As a preventive measure, 874bbfe600a6 ("workqueue: make
sure delayed work run in local cpu") was applied to restore the local
CPU guarnatee.  Unfortunately, the change exposed a bug in timer code
which got fixed by 22b886dd1018 ("timers: Use proper base migration in
add_timer_on()").  Due to code restructuring, the commit couldn't be
backported beyond certain point and stable kernels which only had
874bbfe600a6 started crashing.

The local CPU guarantee was accidental more than anything else and we
want to get rid of it anyway.  As, with the vmstat case fixed,
874bbfe600a6 is causing more problems than it's fixing, it has been
decided to take the chance and officially break the guarantee by
reverting the commit.  A debug feature will be added to force foreign
CPU assignment to expose cases relying on the guarantee and fixes for
the individual cases will be backported to stable as necessary.

Signed-off-by: Tejun Heo <tj at kernel.org>
Fixes: 874bbfe600a6 ("workqueue: make sure delayed work run in local cpu")
Link: http://lkml.kernel.org/g/20160120211926.GJ10810@quack.suse.cz
Cc: Mike Galbraith <umgwanakikbuti at gmail.com>
Cc: Henrique de Moraes Holschuh <hmh at hmh.eng.br>
Cc: Daniel Bilik <daniel.bilik at neosystem.cz>
Cc: Jan Kara <jack at suse.cz>
Cc: Shaohua Li <shli at fb.com>
Cc: Sasha Levin <sasha.levin at oracle.com>
Cc: Ben Hutchings <ben at decadent.org.uk>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Daniel Bilik <daniel.bilik at neosystem.cz>
Cc: Jiri Slaby <jslaby at suse.cz>
Cc: Michal Hocko <mhocko at kernel.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/workqueue.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index dcae810..eb1cc22 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1475,13 +1475,13 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
 	timer_stats_timer_set_start_info(&dwork->timer);

 	dwork->wq = wq;
-	/* timer isn't guaranteed to run in this cpu, record earlier */
-	if (cpu == WORK_CPU_UNBOUND)
-		cpu = raw_smp_processor_id();
 	dwork->cpu = cpu;
 	timer->expires = jiffies + delay;

-	add_timer_on(timer, cpu);
+	if (unlikely(cpu != WORK_CPU_UNBOUND))
+		add_timer_on(timer, cpu);
+	else
+		add_timer(timer);
 }

 /**
--
2.7.0