[PATCH 4/8] md/raid5: raise NR_STRIPE_HASH_LOCKS from 8 to 32

[Hiroshi Nishida <nishidafmly@gmail.com>]

The stripe cache hash is striped across NR_STRIPE_HASH_LOCKS spinlocks
(see stripe_hash_locks_hash()).  The value has been 8 since the per-hash
locking was introduced, which is small for modern many-core servers:
with only 8 buckets, stripe cache lookup and allocation contend on the
same few locks once the CPU count greatly exceeds the bucket count.

Raise it to 32.  Two constraints bound the choice:

  - STRIPE_HASH_LOCKS_MASK is (NR_STRIPE_HASH_LOCKS - 1) and is used as
    a bitmask in stripe_hash_locks_hash(), so the value must be a power
    of two.

  - raid5_quiesce() acquires every hash lock plus device_lock at once
    via lock_all_device_hash_locks_irq().  That holds
    NR_STRIPE_HASH_LOCKS + 1 locks simultaneously, which must stay below
    MAX_LOCK_DEPTH (48) so the held-lock array does not overflow when
    lockdep is enabled.

32 is the largest power of two that satisfies both: 32 + 1 leaves
headroom under 48, whereas 64 would exceed it.  (The pre-existing
"must remain below 64" comment understates this; MAX_LOCK_DEPTH is the
real ceiling.)  STRIPE_HASH_LOCKS_MASK and all NR_STRIPE_HASH_LOCKS-
sized arrays scale automatically.

This is purely a lock-striping change; it does not affect stripe cache
correctness.  The benefit appears on many-core systems, while on small
systems the extra buckets are harmless.

Tested: loop-device RAID-5 create, write/verify, fail a disk, rebuild
onto a spare and scrub all complete cleanly.

Assisted-by: Claude:claude-opus-4-8 [Claude Code]
Signed-off-by: Hiroshi Nishida <nishidafmly@gmail.com>
---
 drivers/md/raid5.h | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 57349737d393..3efab71ebef7 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -498,12 +498,14 @@ struct disk_info {
 #define HASH_MASK		(NR_HASH - 1)
 #define MAX_STRIPE_BATCH	8
 
-/* NOTE NR_STRIPE_HASH_LOCKS must remain below 64.
- * This is because we sometimes take all the spinlocks
- * and creating that much locking depth can cause
- * problems.
+/* NR_STRIPE_HASH_LOCKS must be a power of two, since
+ * STRIPE_HASH_LOCKS_MASK masks with (NR_STRIPE_HASH_LOCKS - 1).
+ * It must also be small enough that taking all of them at once in
+ * lock_all_device_hash_locks_irq(), plus device_lock, keeps the held
+ * lock count below MAX_LOCK_DEPTH (48) with lockdep enabled.  32 is the
+ * largest power of two that satisfies both constraints.
  */
-#define NR_STRIPE_HASH_LOCKS 8
+#define NR_STRIPE_HASH_LOCKS 32
 #define STRIPE_HASH_LOCKS_MASK (NR_STRIPE_HASH_LOCKS - 1)
 
 struct r5worker {
-- 
2.43.0