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

The sloppy nature of lockless access to percpu pointers
(s->current_path) in rr_select_path(), from multiple threads, is
causing some paths to used more than others -- which results in less
IO performance being observed.

Revert these upstream commits to restore truly symmetric round-robin
IO submission in DM multipath:

b0b477c dm round robin: use percpu 'repeat_count' and 'current_path'
802934b dm round robin: do not use this_cpu_ptr() without having preemption disabled

There is no benefit to all this complexity if repeat_count = 1 (which is
the recommended default).

Cc: stable@vger.kernel.org # 4.6+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 14 insertions, 53 deletions

diff --git a/drivers/md/dm-round-robin.c b/drivers/md/dm-round-robin.c
index 6c25213ab38c..bdbb7e6e8212 100644
--- a/drivers/md/dm-round-robin.c
+++ b/drivers/md/dm-round-robin.c
@@ -17,8 +17,8 @@
 #include <linux/module.h>
 
 #define DM_MSG_PREFIX "multipath round-robin"
-#define RR_MIN_IO     1000
-#define RR_VERSION    "1.1.0"
+#define RR_MIN_IO     1
+#define RR_VERSION    "1.2.0"
 
 /*-----------------------------------------------------------------
  * Path-handling code, paths are held in lists
@@ -47,44 +47,19 @@ 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)
-                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;
+        if (s) {
+                INIT_LIST_HEAD(&s->valid_paths);
+                INIT_LIST_HEAD(&s->invalid_paths);
+                spin_lock_init(&s->lock);
+        }
 
         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)
@@ -105,8 +80,6 @@ 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;
 }
@@ -157,6 +130,11 @@ 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) {
@@ -183,9 +161,6 @@ 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);
 }
@@ -208,29 +183,15 @@ 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;
-
-        local_irq_save(flags);
-        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) {
-                        local_irq_restore(flags);
-                        return current_path;
-                }
-        }
 
-        spin_lock(&s->lock);
+        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 current_path;
+        return pi ? pi->path : NULL;
 }
 
 static struct path_selector_type rr_ps = {