[PATCH 4.9 056/153] dm round robin: revert "use percpu repeat_count and current_path"

Greg Kroah-Hartman
2017-03-10T09:19:53Z

From: Mike Snitzer <snitzer@redhat.com>

commit 37a098e9d10db6e2efc05fe61e3a6ff2e9802c53 upstream.

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).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/md/dm-round-robin.c | 67 +++++++++-----------------------------------
1 file changed, 14 insertions(+), 53 deletions(-)

--- 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_selec

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_selec
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_sel
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(st
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 = {