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

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

This reverts commit 120732ac4d7ba94f5a120aa06fb2b1c025e038e6.

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 b2e001acf3b0 ("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, 120732ac4d7b ("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 935c38c7e6f3 ("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
120732ac4d7b 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,
120732ac4d7b 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@kernel.org>
Fixes: 120732ac4d7b ("workqueue: make sure delayed work run in local cpu")
Link: http://lkml.kernel.org/g/20160120211926.GJ10810@quack.suse.cz
Cc: stable@vger.kernel.org
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Daniel Bilik <daniel.bilik@neosystem.cz>
Cc: Jan Kara <jack@suse.cz>
Cc: Shaohua Li <shli@fb.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Bilik <daniel.bilik@neosystem.cz>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>