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* Copyright (C) 2004-2005 IBM Corp. All Rights Reserved.
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* Copyright (C) 2006-2009 NEC Corporation.
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* Module Author: Stefan Bader, IBM
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* Modified by: Kiyoshi Ueda, NEC
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* This file is released under the GPL.
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* queue-length path selector - choose a path with the least number of
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#include "dm-path-selector.h"
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#include <linux/slab.h>
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#include <linux/ctype.h>
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#include <linux/errno.h>
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#include <linux/module.h>
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#include <linux/atomic.h>
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#define DM_MSG_PREFIX "multipath queue-length"
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#define QL_VERSION "0.1.0"
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struct list_head valid_paths;
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struct list_head failed_paths;
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struct list_head list;
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unsigned repeat_count;
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atomic_t qlen; /* the number of in-flight I/Os */
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static struct selector *alloc_selector(void)
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struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
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INIT_LIST_HEAD(&s->valid_paths);
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INIT_LIST_HEAD(&s->failed_paths);
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static int ql_create(struct path_selector *ps, unsigned argc, char **argv)
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struct selector *s = alloc_selector();
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static void ql_free_paths(struct list_head *paths)
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struct path_info *pi, *next;
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list_for_each_entry_safe(pi, next, paths, list) {
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static void ql_destroy(struct path_selector *ps)
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struct selector *s = ps->context;
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ql_free_paths(&s->valid_paths);
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ql_free_paths(&s->failed_paths);
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static int ql_status(struct path_selector *ps, struct dm_path *path,
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status_type_t type, char *result, unsigned maxlen)
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/* When called with NULL path, return selector status/args. */
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DMEMIT("%d ", atomic_read(&pi->qlen));
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case STATUSTYPE_TABLE:
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DMEMIT("%u ", pi->repeat_count);
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static int ql_add_path(struct path_selector *ps, struct dm_path *path,
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int argc, char **argv, char **error)
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struct selector *s = ps->context;
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struct path_info *pi;
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unsigned repeat_count = QL_MIN_IO;
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* Arguments: [<repeat_count>]
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* <repeat_count>: The number of I/Os before switching path.
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* If not given, default (QL_MIN_IO) is used.
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*error = "queue-length ps: incorrect number of arguments";
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if ((argc == 1) && (sscanf(argv[0], "%u", &repeat_count) != 1)) {
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*error = "queue-length ps: invalid repeat count";
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/* Allocate the path information structure */
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pi = kmalloc(sizeof(*pi), GFP_KERNEL);
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*error = "queue-length ps: Error allocating path information";
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pi->repeat_count = repeat_count;
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atomic_set(&pi->qlen, 0);
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path->pscontext = pi;
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list_add_tail(&pi->list, &s->valid_paths);
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static void ql_fail_path(struct path_selector *ps, struct dm_path *path)
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struct selector *s = ps->context;
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struct path_info *pi = path->pscontext;
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list_move(&pi->list, &s->failed_paths);
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static int ql_reinstate_path(struct path_selector *ps, struct dm_path *path)
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struct selector *s = ps->context;
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struct path_info *pi = path->pscontext;
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list_move_tail(&pi->list, &s->valid_paths);
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* Select a path having the minimum number of in-flight I/Os
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static struct dm_path *ql_select_path(struct path_selector *ps,
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unsigned *repeat_count, size_t nr_bytes)
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struct selector *s = ps->context;
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struct path_info *pi = NULL, *best = NULL;
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if (list_empty(&s->valid_paths))
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/* Change preferred (first in list) path to evenly balance. */
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list_move_tail(s->valid_paths.next, &s->valid_paths);
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list_for_each_entry(pi, &s->valid_paths, list) {
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(atomic_read(&pi->qlen) < atomic_read(&best->qlen)))
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if (!atomic_read(&best->qlen))
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*repeat_count = best->repeat_count;
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static int ql_start_io(struct path_selector *ps, struct dm_path *path,
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struct path_info *pi = path->pscontext;
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atomic_inc(&pi->qlen);
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static int ql_end_io(struct path_selector *ps, struct dm_path *path,
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struct path_info *pi = path->pscontext;
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atomic_dec(&pi->qlen);
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static struct path_selector_type ql_ps = {
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.name = "queue-length",
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.module = THIS_MODULE,
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.destroy = ql_destroy,
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.add_path = ql_add_path,
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.fail_path = ql_fail_path,
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.reinstate_path = ql_reinstate_path,
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.select_path = ql_select_path,
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.start_io = ql_start_io,
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static int __init dm_ql_init(void)
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int r = dm_register_path_selector(&ql_ps);
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DMERR("register failed %d", r);
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DMINFO("version " QL_VERSION " loaded");
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static void __exit dm_ql_exit(void)
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int r = dm_unregister_path_selector(&ql_ps);
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DMERR("unregister failed %d", r);
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module_init(dm_ql_init);
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module_exit(dm_ql_exit);
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MODULE_AUTHOR("Stefan Bader <Stefan.Bader at de.ibm.com>");
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"(C) Copyright IBM Corp. 2004,2005 All Rights Reserved.\n"
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DM_NAME " path selector to balance the number of in-flight I/Os"
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MODULE_LICENSE("GPL");