[PATCH v2] fsverity: Remove WQ_UNBOUND from fsverity read workqueue

[Nathan Huckleberry <nhuck@google.com>]

WQ_UNBOUND causes significant scheduler latency on ARM64/Android.  This
is problematic for latency sensitive workloads, like I/O
post-processing.

Removing WQ_UNBOUND gives a 96% reduction in fsverity workqueue related
scheduler latency and improves app cold startup times by ~30ms.
WQ_UNBOUND was also removed from the dm-verity workqueue for the same
reason [1].

This code was tested by running Android app startup benchmarks and
measuring how long the fsverity workqueue spent in the runnable state.

Before
Total workqueue scheduler latency: 553800us
After
Total workqueue scheduler latency: 18962us

[1]: https://lore.kernel.org/all/20230202012348.885402-1-nhuck@google.com/

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
---
Changelog:
v1 -> v2:
- Added comment about WQ_UNBOUND
- Added info about related dm-verity patches in commit message

 fs/verity/verify.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/verity/verify.c b/fs/verity/verify.c
index f50e3b5b52c9..782b8b4a24c1 100644
--- a/fs/verity/verify.c
+++ b/fs/verity/verify.c
@@ -387,15 +387,15 @@ EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);
 int __init fsverity_init_workqueue(void)
 {
 	/*
-	 * Use an unbound workqueue to allow bios to be verified in parallel
-	 * even when they happen to complete on the same CPU.  This sacrifices
-	 * locality, but it's worthwhile since hashing is CPU-intensive.
-	 *
 	 * Also use a high-priority workqueue to prioritize verification work,
 	 * which blocks reads from completing, over regular application tasks.
+	 *
+	 * This workqueue is not marked as unbound for performance reasons.
+	 * Using an unbound workqueue for crypto operations causes excessive
+	 * scheduler latency on ARM64.
 	 */
 	fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
-						  WQ_UNBOUND | WQ_HIGHPRI,
+						  WQ_HIGHPRI,
 						  num_online_cpus());
 	if (!fsverity_read_workqueue)
 		return -ENOMEM;
-- 
2.40.0.rc1.284.g88254d51c5-goog