dm round robin: use percpu 'repeat_count' and 'current_path'

Now that dm-mpath core is lockless in the per-IO fast path it is
critical, for performance, to have the .select_path hook
(rr_select_path) also be as lockless as possible.

The new percpu members of 'struct selector' allow for lockless support
of 'repeat_count' governed repeat use of a previously selected path.  If
a path fails while it is 'current_path' the worst case is concurrent IO
might be mapped to the failed path until the .fail_path hook
(rr_fail_path) is called.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-round-robin.c

@@ -17,7 +17,7 @@
 #include <linux/module.h>
 
 #define DM_MSG_PREFIX "multipath round-robin"
-#define RR_MIN_IO     1
+#define RR_MIN_IO     1000
 #define RR_VERSION    "1.1.0"
 
 /*-----------------------------------------------------------------
@@ -47,19 +47,44 @@ struct selector {
 	struct list_head valid_paths;
 	struct list_head invalid_paths;
 	spinlock_t lock;
+	struct dm_path * __percpu *current_path;
+	struct percpu_counter repeat_count;
 };
 
+static void set_percpu_current_path(struct selector *s, struct dm_path *path)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		*per_cpu_ptr(s->current_path, cpu) = path;
+}
+
 static struct selector *alloc_selector(void)
 {
 	struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
 
-	if (s) {
+	if (!s)
+		return NULL;
+
 	INIT_LIST_HEAD(&s->valid_paths);
 	INIT_LIST_HEAD(&s->invalid_paths);
 	spin_lock_init(&s->lock);
-	}
+
+	s->current_path = alloc_percpu(struct dm_path *);
+	if (!s->current_path)
+		goto out_current_path;
+	set_percpu_current_path(s, NULL);
+
+	if (percpu_counter_init(&s->repeat_count, 0, GFP_KERNEL))
+		goto out_repeat_count;
 
 	return s;
+
+out_repeat_count:
+	free_percpu(s->current_path);
+out_current_path:
+	kfree(s);
+	return NULL;;
 }
 
 static int rr_create(struct path_selector *ps, unsigned argc, char **argv)
@@ -80,6 +105,8 @@ static void rr_destroy(struct path_selector *ps)
 
 	free_paths(&s->valid_paths);
 	free_paths(&s->invalid_paths);
+	free_percpu(s->current_path);
+	percpu_counter_destroy(&s->repeat_count);
 	kfree(s);
 	ps->context = NULL;
 }
@@ -130,11 +157,6 @@ static int rr_add_path(struct path_selector *ps, struct dm_path *path,
 		return -EINVAL;
 	}
 
-	if (repeat_count > 1) {
-		DMWARN_LIMIT("repeat_count > 1 is deprecated, using 1 instead");
-		repeat_count = 1;
-	}
-
 	/* allocate the path */
 	pi = kmalloc(sizeof(*pi), GFP_KERNEL);
 	if (!pi) {
@@ -161,6 +183,9 @@ static void rr_fail_path(struct path_selector *ps, struct dm_path *p)
 	struct path_info *pi = p->pscontext;
 
 	spin_lock_irqsave(&s->lock, flags);
+	if (p == *this_cpu_ptr(s->current_path))
+		set_percpu_current_path(s, NULL);
+
 	list_move(&pi->list, &s->invalid_paths);
 	spin_unlock_irqrestore(&s->lock, flags);
 }
@@ -183,15 +208,26 @@ static struct dm_path *rr_select_path(struct path_selector *ps, size_t nr_bytes)
 	unsigned long flags;
 	struct selector *s = ps->context;
 	struct path_info *pi = NULL;
+	struct dm_path *current_path = NULL;
+
+	current_path = *this_cpu_ptr(s->current_path);
+	if (current_path) {
+		percpu_counter_dec(&s->repeat_count);
+		if (percpu_counter_read_positive(&s->repeat_count) > 0)
+			return current_path;
+	}
 
 	spin_lock_irqsave(&s->lock, flags);
 	if (!list_empty(&s->valid_paths)) {
 		pi = list_entry(s->valid_paths.next, struct path_info, list);
 		list_move_tail(&pi->list, &s->valid_paths);
+		percpu_counter_set(&s->repeat_count, pi->repeat_count);
+		set_percpu_current_path(s, pi->path);
+		current_path = pi->path;
 	}
 	spin_unlock_irqrestore(&s->lock, flags);
 
-	return pi ? pi->path : NULL;
+	return current_path;
 }
 
 static struct path_selector_type rr_ps = {