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

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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>
This commit is contained in:
Mike Snitzer 2016-02-17 15:04:05 -05:00
parent 90a4323ccf
commit b0b477c7e0
1 changed files with 48 additions and 12 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

60
drivers/md/dm-round-robin.c
View file

@ -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) {
INIT_LIST_HEAD(&s->valid_paths);
INIT_LIST_HEAD(&s->invalid_paths);
spin_lock_init(&s->lock);
}
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 = {