[PATCH V2] driver core: Use unbound workqueue for deferred probes

[Yogesh Lal]

Deferred probe usually runs only on pinned kworkers, which might take
longer time if a device contains multiple sub-devices. One such case
is of sound card on mobile devices, where we have good number of
mixers and controls per mixer.

We observed boot up improvement - deferred probes take ~600ms when bound
to little core kworker and ~200ms when deferred probe is queued on
unbound wq. This is due to scheduler moving the worker running deferred
probe work to big CPUs. Without this change, we see the worker is running
on LITTLE CPU due to affinity.

Since kworker runs deferred probe of several devices, the locality may
not be important. Also, init thread executing driver initcalls, can
potentially migrate as it has cpu affinity set to all cpus.In addition
to this, async probes use unbounded workqueue. So, using unbounded wq for
deferred probes looks to be similar to these w.r.t. scheduling behavior.

Signed-off-by: Yogesh Lal <ylal@xxxxxxxxxxxxxx>
---
Changes in v2:
Updating the Changelog with issue description.

 drivers/base/dd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 9179825f..c9c174a 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -182,7 +182,7 @@ static void driver_deferred_probe_trigger(void)
 	 * Kick the re-probe thread.  It may already be scheduled, but it is
 	 * safe to kick it again.
 	 */
-	schedule_work(&deferred_probe_work);
+	queue_work(system_unbound_wq, &deferred_probe_work);
 }
 
 /**
-- 
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