fsverity: Remove WQ_UNBOUND from fsverity read workqueue

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/[email protected]/

Signed-off-by: Nathan Huckleberry <[email protected]>
Fixes: 8a1d0f9 ("fs-verity: add data verification hooks for ->readpages()")
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Eric Biggers <[email protected]>

Author: nhukc

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.
+	 * Use a high-priority workqueue to prioritize verification work, which
+	 * blocks reads from completing, over regular application tasks.
 	 *
-	 * Also use a high-priority workqueue to prioritize verification work,
-	 * which blocks reads from completing, over regular application tasks.
+	 * For performance reasons, don't use an unbound workqueue.  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;