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, 15 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index c3bfacb92910..37dfd720de79 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3320,6 +3320,21 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         that this also can be controlled per-workqueue for
                         workqueues visible under /sys/bus/workqueue/.
 
+        workqueue.power_efficient
+                        Per-cpu workqueues are generally preferred because
+                        they show better performance thanks to cache
+                        locality; unfortunately, per-cpu workqueues tend to
+                        be more power hungry than unbound workqueues.
+
+                        Enabling this makes the per-cpu workqueues which
+                        were observed to contribute significantly to power
+                        consumption unbound, leading to measurably lower
+                        power usage at the cost of small performance
+                        overhead.
+
+                        The default value of this parameter is determined by
+                        the config option CONFIG_WQ_POWER_EFFICIENT_DEFAULT.
+
         x2apic_phys     [X86-64,APIC] Use x2apic physical mode instead of
                         default x2apic cluster mode on platforms
                         supporting x2apic.