linux-kernel - [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers

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Date:	Tue, 17 Jul 2012 10:12:20 -0700
From:	Tejun Heo <tj@...nel.org>
To:	linux-kernel@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, peterz@...radead.org,
	tglx@...utronix.de, linux-pm@...r.kernel.org
Subject: [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers

Currently, workqueue destroys all workers for offline CPUs unless
there are lingering work items.  On some platforms, CPU hotplugs are
used as part of runtime powersaving and thus can be very frequent.
The overhead of destroying and re-creating workers on each cycle can
be noticeable.

In addition, being different from the usual mode of opration,
kill-all-idle involves separate driving logic, which currently is
implemented across trustee_thread() and CPU hotplug notifier.  This is
rather complex and ugly.

This patchset updates workqueue CPU hotplug path such that a
disassociated global_cwq (the per-cpu worker pool) is run as if it's
running as an unbound one (WQ_UNBOUND) - executing work items ASAP
without concurrency management while following the usual worker
management based on idle timer.

While this makes rebinding somewhat more complicated, as it has to be
able to rebind idle workers too, it allows overall hotplug path to be
much simpler.  A disassociating global_cwq simply becomes an unbound
one at the end of CPU_DOWN_PREPARE and makes use of the same logic
already existing for WQ_UNBOUND.  While re-associating, it switches
back to a bound one.  Except during the transitions themselves, the
usual worker management is always in effect making trustee
unnecessary.

 0001-workqueue-perform-cpu-down-operations-from-low-prior.patch
 0002-workqueue-drop-CPU_DYING-notifier-operation.patch
 0003-workqueue-ROGUE-workers-are-UNBOUND-workers.patch
 0004-workqueue-use-mutex-for-global_cwq-manager-exclusion.patch
 0005-workqueue-drop-bind-from-create_worker.patch
 0006-workqueue-reimplement-CPU-online-rebinding-to-handle.patch
 0007-workqueue-don-t-butcher-idle-workers-on-an-offline-C.patch
 0008-workqueue-remove-CPU-offline-trustee.patch
 0009-workqueue-simplify-CPU-hotplug-code.patch

0001 is fix for an unlikely race condition.  0002-0005 prepare for CPU
hotplug reimplementation.  0006 reimplements re-binding such that it
can handle idle workers too.  0007 drops killing idle workers of
offline CPUs.  0008 replaces trustee with the usual management logic.
0009 refactors hotplug path a bit to clean it up.

This patchset is on top of "workqueue: reimplement high priority using
a separate worker pool" patchset[1] and available on the following git
branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git review-wq-hotplug

 include/linux/cpu.h |    5 
 kernel/workqueue.c  |  738 ++++++++++++++++++++--------------------------------
 2 files changed, 296 insertions(+), 447 deletions(-)

Thanks.

--
tejun

[1] http://lwn.net/Articles/506029/
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