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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>
(cherry picked from commit 0668106ca3865ba945e155097fb042bf66d364d3)
Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Stefan Guendhoer <stefan@guendhoer.com>
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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>
(cherry picked from commit cee22a15052faa817e3ec8985a28154d3fabc7aa)
Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Stefan Guendhoer <stefan@guendhoer.com>
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When there is serious memory pressure, all workers in a pool could be
blocked, and a new thread cannot be created because it requires memory
allocation.
In this situation a WQ_MEM_RECLAIM workqueue will wake up the
rescuer thread to do some work.
The rescuer will only handle requests that are already on ->worklist.
If max_requests is 1, that means it will handle a single request.
The rescuer will be woken again in 100ms to handle another max_requests
requests.
I've seen a machine (running a 3.0 based "enterprise" kernel) with
thousands of requests queued for xfslogd, which has a max_requests of
1, and is needed for retiring all 'xfs' write requests. When one of
the worker pools gets into this state, it progresses extremely slowly
and possibly never recovers (only waited an hour or two).
With this patch we leave a pool_workqueue on mayday list
until it is clearly no longer in need of assistance. This allows
all requests to be handled in a timely fashion.
We keep each pool_workqueue on the mayday list until
need_to_create_worker() is false, and no work for this workqueue is
found in the pool.
I have tested this in combination with a (hackish) patch which forces
all work items to be handled by the rescuer thread. In that context
it significantly improves performance. A similar patch for a 3.0
kernel significantly improved performance on a heavy work load.
Thanks to Jan Kara for some design ideas, and to Dongsu Park for
some comments and testing.
tj: Inverted the lock order between wq_mayday_lock and pool->lock with
a preceding patch and simplified this patch. Added comment and
updated changelog accordingly. Dongsu spotted missing get_pwq()
in the simplified code.
Cc: Dongsu Park <dongsu.park@profitbricks.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Stefan Guendhoer <stefan@guendhoer.com>
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