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>

1 files changed, 27 insertions, 0 deletions

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 623488fdc1f5..fc0136b604f2 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -303,6 +303,33 @@ enum {
 	WQ_CPU_INTENSIVE	= 1 << 5, /* cpu instensive workqueue */
 	WQ_SYSFS		= 1 << 6, /* visible in sysfs, see wq_sysfs_register() */
 
+	/*
+	 * Per-cpu workqueues are generally preferred because they tend to
+	 * show better performance thanks to cache locality.  Per-cpu
+	 * workqueues exclude the scheduler from choosing the CPU to
+	 * execute the worker threads, which has an unfortunate side effect
+	 * of increasing power consumption.
+	 *
+	 * The scheduler considers a CPU idle if it doesn't have any task
+	 * to execute and tries to keep idle cores idle to conserve power;
+	 * however, for example, a per-cpu work item scheduled from an
+	 * interrupt handler on an idle CPU will force the scheduler to
+	 * excute the work item on that CPU breaking the idleness, which in
+	 * turn may lead to more scheduling choices which are sub-optimal
+	 * in terms of power consumption.
+	 *
+	 * Workqueues marked with WQ_POWER_EFFICIENT are per-cpu by default
+	 * but become unbound if workqueue.power_efficient kernel param is
+	 * specified.  Per-cpu workqueues which are identified to
+	 * contribute significantly to power-consumption are identified and
+	 * marked with this flag and enabling the power_efficient mode
+	 * leads to noticeable power saving at the cost of small
+	 * performance disadvantage.
+	 *
+	 * http://thread.gmane.org/gmane.linux.kernel/1480396
+	 */
+	WQ_POWER_EFFICIENT	= 1 << 7,
+
 	__WQ_DRAINING		= 1 << 16, /* internal: workqueue is draining */
 	__WQ_ORDERED		= 1 << 17, /* internal: workqueue is ordered */