[PATCH 4/6] md/raid5: make the stripe batch size a module parameter

[Hiroshi Nishida <nishidafmly@gmail.com>]

handle_active_stripes() dequeues up to MAX_STRIPE_BATCH (8) stripes from a
worker group under a single device_lock acquisition, and
raid5_wakeup_stripe_thread() divides group->stripes_cnt by the same value
to decide how many additional workers to wake.  The two uses must move
together: the dequeue cap bounds how many stripes one worker drains, while
the spawn divisor sizes the worker fan-out to match.

Make the batch size a per-array value chosen by a new max_stripe_batch
module parameter (1-32, default 8, so behaviour is unchanged out of the
box).  Both the dequeue cap and the spawn divisor read the single
conf->max_stripe_batch, so they cannot drift apart.  The on-stack batch[]
array in handle_active_stripes() is sized to the STRIPE_BATCH_MAX (32)
upper bound; only conf->max_stripe_batch entries are ever used.

The value is resolved when an array is created.  On busy multi-threaded
arrays a larger batch amortises the device_lock over more stripes at some
latency cost; unlike the hash-lock count and the cache-size limit there is
no general hardware signal for the best batch size, so the default is left
at the historical 8 and the value is simply exposed for tuning.

Signed-off-by: Hiroshi Nishida <nishidafmly@gmail.com>
---
 drivers/md/raid5.c | 15 ++++++++++++---
 drivers/md/raid5.h | 12 +++++++++++-
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index e41d3fc92dd0..5f0825c5effe 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -78,6 +78,11 @@ module_param(stripe_cache_size_max, uint, 0644);
 MODULE_PARM_DESC(stripe_cache_size_max,
 		 "Maximum the per-array stripe_cache_size may be raised to.  0 (the default) derives the limit from system memory (never below the historical 32768), so large-memory hosts can grow the stripe cache without a recompile while small ones are not offered a limit above what RAM can back.  A non-zero value sets a fixed limit");
 
+static unsigned int max_stripe_batch = STRIPE_BATCH_DEFAULT;
+module_param(max_stripe_batch, uint, 0644);
+MODULE_PARM_DESC(max_stripe_batch,
+		 "Number of stripes a worker thread handles per device_lock acquisition, 1-32 (default 8).  Larger values amortise the lock over more stripes on busy multi-threaded arrays at some latency cost.  Read when an array is created");
+
 static bool devices_handle_discard_safely = false;
 module_param(devices_handle_discard_safely, bool, 0644);
 MODULE_PARM_DESC(devices_handle_discard_safely,
@@ -221,7 +226,7 @@ static void raid5_wakeup_stripe_thread(struct stripe_head *sh)
 	/* at least one worker should run to avoid race */
 	queue_work_on(sh->cpu, raid5_wq, &group->workers[0].work);
 
-	thread_cnt = group->stripes_cnt / MAX_STRIPE_BATCH - 1;
+	thread_cnt = group->stripes_cnt / conf->max_stripe_batch - 1;
 	/* wakeup more workers */
 	for (i = 1; i < conf->worker_cnt_per_group && thread_cnt > 0; i++) {
 		if (group->workers[i].working == false) {
@@ -6734,11 +6739,11 @@ static int handle_active_stripes(struct r5conf *conf, int group,
 				 struct list_head *temp_inactive_list)
 		__must_hold(&conf->device_lock)
 {
-	struct stripe_head *batch[MAX_STRIPE_BATCH], *sh;
+	struct stripe_head *batch[STRIPE_BATCH_MAX], *sh;
 	int i, batch_size = 0, hash;
 	bool release_inactive = false;
 
-	while (batch_size < MAX_STRIPE_BATCH &&
+	while (batch_size < conf->max_stripe_batch &&
 			(sh = __get_priority_stripe(conf, group)) != NULL)
 		batch[batch_size++] = sh;
 
@@ -7640,6 +7645,10 @@ static struct r5conf *setup_conf(struct mddev *mddev)
 	    !conf->temp_inactive_list)
 		goto abort;
 
+	/* Resolve the stripe batch size (see STRIPE_BATCH_* in raid5.h). */
+	conf->max_stripe_batch = clamp_t(int, max_stripe_batch,
+					 1, STRIPE_BATCH_MAX);
+
 #if PAGE_SIZE != DEFAULT_STRIPE_SIZE
 	conf->stripe_size = DEFAULT_STRIPE_SIZE;
 	conf->stripe_shift = ilog2(DEFAULT_STRIPE_SIZE) - 9;
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 10c45fa22955..69a41c1310af 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -490,7 +490,16 @@ struct disk_info {
 #define BYPASS_THRESHOLD	1
 #define NR_HASH			(PAGE_SIZE / sizeof(struct hlist_head))
 #define HASH_MASK		(NR_HASH - 1)
-#define MAX_STRIPE_BATCH	8
+/*
+ * Stripe batch size: how many stripes a worker dequeues from its group per
+ * device_lock acquisition in handle_active_stripes(); the same value divides
+ * group->stripes_cnt in raid5_wakeup_stripe_thread() to choose how many extra
+ * workers to spawn, so the two stay coupled.  Selected per array by the
+ * max_stripe_batch module parameter: STRIPE_BATCH_DEFAULT is the historical
+ * value and STRIPE_BATCH_MAX bounds the on-stack batch[] array.
+ */
+#define STRIPE_BATCH_DEFAULT	8
+#define STRIPE_BATCH_MAX	32
 
 /*
  * The stripe cache hash is striped across a power-of-two number of spinlocks,
@@ -690,6 +699,7 @@ struct r5conf {
 	struct r5worker_group	*worker_groups;
 	int			group_cnt;
 	int			worker_cnt_per_group;
+	int			max_stripe_batch; /* stripes/dequeue, 1..STRIPE_BATCH_MAX */
 	struct r5l_log		*log;
 	void			*log_private;
 
-- 
2.43.0