workqueue: don't butcher idle workers on an offline CPU

Currently, during CPU offlining, after all pending work items are
drained, the trustee butchers all workers.  Also, on CPU onlining
failure, workqueue_cpu_callback() ensures that the first idle worker
is destroyed.  Combined, these guarantee that an offline CPU doesn't
have any worker for it once all the lingering work items are finished.

This guarantee isn't really necessary and makes CPU on/offlining more
expensive than needs to be, especially for platforms which use CPU
hotplug for powersaving.

This patch lets offline CPUs removes idle worker butchering from the
trustee and let a CPU which failed onlining keep the created first
worker.  The first worker is created if the CPU doesn't have any
during CPU_DOWN_PREPARE and started right away.  If onlining succeeds,
the rebind_workers() call in CPU_ONLINE will rebind it like any other
workers.  If onlining fails, the worker is left alone till the next
try.

This makes CPU hotplugs cheaper by allowing global_cwqs to keep
workers across them and simplifies code.

Note that trustee doesn't re-arm idle timer when it's done and thus
the disassociated global_cwq will keep all workers until it comes back
online.  This will be improved by further patches.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: remove CPU offline trustee

With the previous changes, a disassociated global_cwq now can run as
an unbound one on its own - it can create workers as necessary to
drain remaining works after the CPU has been brought down and manage
the number of workers using the usual idle timer mechanism making
trustee completely redundant except for the actual unbinding
operation.

This patch removes the trustee and let a disassociated global_cwq
manage itself.  Unbinding is moved to a work item (for CPU affinity)
which is scheduled and flushed from CPU_DONW_PREPARE.

This patch moves nr_running clearing outside gcwq and manager locks to
simplify the code.  As nr_running is unused at the point, this is
safe.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: simplify CPU hotplug code

With trustee gone, CPU hotplug code can be simplified.

* gcwq_claim/release_management() now grab and release gcwq lock too
  respectively and gained _and_lock and _and_unlock postfixes.

* All CPU hotplug logic was implemented in workqueue_cpu_callback()
  which was called by workqueue_cpu_up/down_callback() for the correct
  priority.  This was because up and down paths shared a lot of logic,
  which is no longer true.  Remove workqueue_cpu_callback() and move
  all hotplug logic into the two actual callbacks.

This patch doesn't make any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueues: Introduce new flag WQ_POWER_EFFICIENT for power oriented workqueues

Workqueues can be performance or power-oriented. Currently, most workqueues are
bound to the CPU they were created on. This gives good performance (due to cache
effects) at the cost of potentially waking up otherwise idle cores (Idle from
scheduler's perspective. Which may or may not be physically idle) just to
process some work. To save power, we can allow the work to be rescheduled on a
core that is already awake.

Workqueues created with the WQ_UNBOUND flag will allow some power savings.
However, we don't change the default behaviour of the system.  To enable
power-saving behaviour, a new config option CONFIG_WQ_POWER_EFFICIENT needs to
be turned on. This option can also be overridden by the
workqueue.power_efficient boot parameter.

tj: Updated config description and comments.  Renamed
    CONFIG_WQ_POWER_EFFICIENT to CONFIG_WQ_POWER_EFFICIENT_DEFAULT.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

[zwliew: backported to linux 3.0]
Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>

Conflicts:
	kernel/workqueue.c

Change-Id: I35b444251b9df2f3d5b42d7462b170055fa02bd3
workqueue: Add system wide power_efficient workqueues

This patch adds system wide workqueues aligned towards power saving. This is
done by allocating them with WQ_UNBOUND flag if 'wq_power_efficient' is set to
'true'.

tj: updated comments a bit.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

Change-Id: I060e04bb7b216fbe19c977d1fa2203d42beaf508
[zwliew: adapt for linux-3.0]
Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>