[PATCH] PM: runtime: Allocate pm_wq with WQ_HIGHPRI

["Chung-Kai (Michael) Mei" <chungkai@google.com>]

Asynchronous runtime PM work (pm_runtime_work()) and genpd power-off
work (genpd_power_off_work_fn()) run on pm_wq. While such a work item
executes a ->runtime_suspend() or ->runtime_resume() callback, the
device's runtime_status is RPM_SUSPENDING or RPM_RESUMING with
dev->power.lock dropped, and genpd_power_off_work_fn() holds the genpd
lock.

If another task calls pm_runtime_get_sync() on the device in that
window, rpm_resume() sleeps on dev->power.wait_queue in
TASK_UNINTERRUPTIBLE until the work item completes. A wait queue has
no owner, so the waiter cannot lend its priority to the kworker, and
its latency is bounded by how quickly the pm_wq worker gets CPU time.
Because asynchronous runtime PM work is queued onto pm_wq by the PM
core, individual drivers cannot select a different workqueue.

pm_wq workers run at nice 0 and can be kept off the CPU for a long
time by higher-weight CFS tasks. On an arm64 Android device whose GPU
driver uses runtime PM autosuspend, render threads were observed
blocked in rpm_resume() for 5 to 19 ms while the runtime PM callback
itself needed less than 60 us of CPU time. For more than 99% of that
time the pm_wq worker was runnable, preempted by nice -20 tasks.

Allocate pm_wq with WQ_HIGHPRI so that its work items are served by the
unbound high-priority worker pools (nice -20), like other
latency-sensitive kernel workqueues such as kblockd. This does not
change the ordering or concurrency semantics of pm_wq. It only reduces
the time a runtime PM transition spends waiting for CPU time, and with
it the latency seen by synchronous runtime PM callers.

Signed-off-by: Chung-Kai (Michael) Mei <chungkai@google.com>
---
 kernel/power/main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/power/main.c b/kernel/power/main.c
index 5429e9f19b655..1e8ae76356b23 100644
--- a/kernel/power/main.c
+++ b/kernel/power/main.c
@@ -1125,7 +1125,7 @@ EXPORT_SYMBOL_GPL(pm_wq);
 
 static int __init pm_start_workqueues(void)
 {
-	pm_wq = alloc_workqueue("pm", WQ_UNBOUND, 0);
+	pm_wq = alloc_workqueue("pm", WQ_UNBOUND | WQ_HIGHPRI, 0);
 	if (!pm_wq)
 		return -ENOMEM;
 

base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222
-- 
2.56.0.307.g5fe7bd680c-goog