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/*
* Copyright (C) 2007-2009 NEC Corporation. All Rights Reserved.
*
* Module Author: Kiyoshi Ueda
*
* This file is released under the GPL.
*
* Throughput oriented path selector.
*/
#include "dm.h"
#include "dm-path-selector.h"
#include <linux/slab.h>
#include <linux/module.h>
#define DM_MSG_PREFIX "multipath service-time"
#define ST_MIN_IO 1
#define ST_MAX_RELATIVE_THROUGHPUT 100
#define ST_MAX_RELATIVE_THROUGHPUT_SHIFT 7
#define ST_MAX_INFLIGHT_SIZE ((size_t)-1 >> ST_MAX_RELATIVE_THROUGHPUT_SHIFT)
#define ST_VERSION "0.2.0"
struct selector {
struct list_head valid_paths;
struct list_head failed_paths;
};
struct path_info {
struct list_head list;
struct dm_path *path;
unsigned repeat_count;
unsigned relative_throughput;
atomic_t in_flight_size; /* Total size of in-flight I/Os */
};
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->failed_paths);
}
return s;
}
static int st_create(struct path_selector *ps, unsigned argc, char **argv)
{
struct selector *s = alloc_selector();
if (!s)
return -ENOMEM;
ps->context = s;
return 0;
}
static void free_paths(struct list_head *paths)
{
struct path_info *pi, *next;
list_for_each_entry_safe(pi, next, paths, list) {
list_del(&pi->list);
kfree(pi);
}
}
static void st_destroy(struct path_selector *ps)
{
struct selector *s = ps->context;
free_paths(&s->valid_paths);
free_paths(&s->failed_paths);
kfree(s);
ps->context = NULL;
}
static int st_status(struct path_selector *ps, struct dm_path *path,
status_type_t type, char *result, unsigned maxlen)
{
unsigned sz = 0;
struct path_info *pi;
if (!path)
DMEMIT("0 ");
else {
pi = path->pscontext;
switch (type) {
case STATUSTYPE_INFO:
DMEMIT("%d %u ", atomic_read(&pi->in_flight_size),
pi->relative_throughput);
break;
case STATUSTYPE_TABLE:
DMEMIT("%u %u ", pi->repeat_count,
pi->relative_throughput);
break;
}
}
return sz;
}
static int st_add_path(struct path_selector *ps, struct dm_path *path,
int argc, char **argv, char **error)
{
struct selector *s = ps->context;
struct path_info *pi;
unsigned repeat_count = ST_MIN_IO;
unsigned relative_throughput = 1;
char dummy;
/*
* Arguments: [<repeat_count> [<relative_throughput>]]
* <repeat_count>: The number of I/Os before switching path.
* If not given, default (ST_MIN_IO) is used.
* <relative_throughput>: The relative throughput value of
* the path among all paths in the path-group.
* The valid range: 0-<ST_MAX_RELATIVE_THROUGHPUT>
* If not given, minimum value '1' is used.
* If '0' is given, the path isn't selected while
* other paths having a positive value are
* available.
*/
if (argc > 2) {
*error = "service-time ps: incorrect number of arguments";
return -EINVAL;
}
if (argc && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
*error = "service-time ps: invalid repeat count";
return -EINVAL;
}
if ((argc == 2) &&
(sscanf(argv[1], "%u%c", &relative_throughput, &dummy) != 1 ||
relative_throughput > ST_MAX_RELATIVE_THROUGHPUT)) {
*error = "service-time ps: invalid relative_throughput value";
return -EINVAL;
}
/* allocate the path */
pi = kmalloc(sizeof(*pi), GFP_KERNEL);
if (!pi) {
*error = "service-time ps: Error allocating path context";
return -ENOMEM;
}
pi->path = path;
pi->repeat_count = repeat_count;
pi->relative_throughput = relative_throughput;
atomic_set(&pi->in_flight_size, 0);
path->pscontext = pi;
list_add_tail(&pi->list, &s->valid_paths);
return 0;
}
static void st_fail_path(struct path_selector *ps, struct dm_path *path)
{
struct selector *s = ps->context;
struct path_info *pi = path->pscontext;
list_move(&pi->list, &s->failed_paths);
}
static int st_reinstate_path(struct path_selector *ps, struct dm_path *path)
{
struct selector *s = ps->context;
struct path_info *pi = path->pscontext;
list_move_tail(&pi->list, &s->valid_paths);
return 0;
}
/*
* Compare the estimated service time of 2 paths, pi1 and pi2,
* for the incoming I/O.
*
* Returns:
* < 0 : pi1 is better
* 0 : no difference between pi1 and pi2
* > 0 : pi2 is better
*
* Description:
* Basically, the service time is estimated by:
* ('pi->in-flight-size' + 'incoming') / 'pi->relative_throughput'
* To reduce the calculation, some optimizations are made.
* (See comments inline)
*/
static int st_compare_load(struct path_info *pi1, struct path_info *pi2,
size_t incoming)
{
size_t sz1, sz2, st1, st2;
sz1 = atomic_read(&pi1->in_flight_size);
sz2 = atomic_read(&pi2->in_flight_size);
/*
* Case 1: Both have same throughput value. Choose less loaded path.
*/
if (pi1->relative_throughput == pi2->relative_throughput)
return sz1 - sz2;
/*
* Case 2a: Both have same load. Choose higher throughput path.
* Case 2b: One path has no throughput value. Choose the other one.
*/
if (sz1 == sz2 ||
!pi1->relative_throughput || !pi2->relative_throughput)
return pi2->relative_throughput - pi1->relative_throughput;
/*
* Case 3: Calculate service time. Choose faster path.
* Service time using pi1:
* st1 = (sz1 + incoming) / pi1->relative_throughput
* Service time using pi2:
* st2 = (sz2 + incoming) / pi2->relative_throughput
*
* To avoid the division, transform the expression to use
* multiplication.
* Because ->relative_throughput > 0 here, if st1 < st2,
* the expressions below are the same meaning:
* (sz1 + incoming) / pi1->relative_throughput <
* (sz2 + incoming) / pi2->relative_throughput
* (sz1 + incoming) * pi2->relative_throughput <
* (sz2 + incoming) * pi1->relative_throughput
* So use the later one.
*/
sz1 += incoming;
sz2 += incoming;
if (unlikely(sz1 >= ST_MAX_INFLIGHT_SIZE ||
sz2 >= ST_MAX_INFLIGHT_SIZE)) {
/*
* Size may be too big for multiplying pi->relative_throughput
* and overflow.
* To avoid the overflow and mis-selection, shift down both.
*/
sz1 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
sz2 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
}
st1 = sz1 * pi2->relative_throughput;
st2 = sz2 * pi1->relative_throughput;
if (st1 != st2)
return st1 - st2;
/*
* Case 4: Service time is equal. Choose higher throughput path.
*/
return pi2->relative_throughput - pi1->relative_throughput;
}
static struct dm_path *st_select_path(struct path_selector *ps,
unsigned *repeat_count, size_t nr_bytes)
{
struct selector *s = ps->context;
struct path_info *pi = NULL, *best = NULL;
if (list_empty(&s->valid_paths))
return NULL;
/* Change preferred (first in list) path to evenly balance. */
list_move_tail(s->valid_paths.next, &s->valid_paths);
list_for_each_entry(pi, &s->valid_paths, list)
if (!best || (st_compare_load(pi, best, nr_bytes) < 0))
best = pi;
if (!best)
return NULL;
*repeat_count = best->repeat_count;
return best->path;
}
static int st_start_io(struct path_selector *ps, struct dm_path *path,
size_t nr_bytes)
{
struct path_info *pi = path->pscontext;
atomic_add(nr_bytes, &pi->in_flight_size);
return 0;
}
static int st_end_io(struct path_selector *ps, struct dm_path *path,
size_t nr_bytes)
{
struct path_info *pi = path->pscontext;
atomic_sub(nr_bytes, &pi->in_flight_size);
return 0;
}
static struct path_selector_type st_ps = {
.name = "service-time",
.module = THIS_MODULE,
.table_args = 2,
.info_args = 2,
.create = st_create,
.destroy = st_destroy,
.status = st_status,
.add_path = st_add_path,
.fail_path = st_fail_path,
.reinstate_path = st_reinstate_path,
.select_path = st_select_path,
.start_io = st_start_io,
.end_io = st_end_io,
};
static int __init dm_st_init(void)
{
int r = dm_register_path_selector(&st_ps);
if (r < 0)
DMERR("register failed %d", r);
DMINFO("version " ST_VERSION " loaded");
return r;
}
static void __exit dm_st_exit(void)
{
int r = dm_unregister_path_selector(&st_ps);
if (r < 0)
DMERR("unregister failed %d", r);
}
module_init(dm_st_init);
module_exit(dm_st_exit);
MODULE_DESCRIPTION(DM_NAME " throughput oriented path selector");
MODULE_AUTHOR("Kiyoshi Ueda <k-ueda@ct.jp.nec.com>");
MODULE_LICENSE("GPL");
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