/*
 * Copyright (C) 2004-2005 IBM Corp.  All Rights Reserved.
 * Copyright (C) 2006-2009 NEC Corporation.
 *
 * dm-queue-length.c
 *
 * Module Author: Stefan Bader, IBM
 * Modified by: Kiyoshi Ueda, NEC
 *
 * This file is released under the GPL.
 *
 * queue-length path selector - choose a path with the least number of
 * in-flight I/Os.
 */

#include "dm.h"
#include "dm-path-selector.h"

#include <linux/slab.h>
#include <linux/ctype.h>
#include <linux/errno.h>
#include <linux/module.h>
#include <linux/atomic.h>

#define DM_MSG_PREFIX   "multipath queue-length"
#define QL_MIN_IO       128
#define QL_VERSION      "0.1.0"

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;
        atomic_t                qlen;   /* the number 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 ql_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 ql_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 ql_destroy(struct path_selector *ps)
{
        struct selector *s = ps->context;

        ql_free_paths(&s->valid_paths);
        ql_free_paths(&s->failed_paths);
        kfree(s);
        ps->context = NULL;
}

static int ql_status(struct path_selector *ps, struct dm_path *path,
                     status_type_t type, char *result, unsigned maxlen)
{
        unsigned sz = 0;
        struct path_info *pi;

        /* When called with NULL path, return selector status/args. */
        if (!path)
                DMEMIT("0 ");
        else {
                pi = path->pscontext;

                switch (type) {
                case STATUSTYPE_INFO:
                        DMEMIT("%d ", atomic_read(&pi->qlen));
                        break;
                case STATUSTYPE_TABLE:
                        DMEMIT("%u ", pi->repeat_count);
                        break;
                }
        }

        return sz;
}

static int ql_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 = QL_MIN_IO;
        char dummy;

        /*
         * Arguments: [<repeat_count>]
         *      <repeat_count>: The number of I/Os before switching path.
         *                      If not given, default (QL_MIN_IO) is used.
         */
        if (argc > 1) {
                *error = "queue-length ps: incorrect number of arguments";
                return -EINVAL;
        }

        if ((argc == 1) && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
                *error = "queue-length ps: invalid repeat count";
                return -EINVAL;
        }

        /* Allocate the path information structure */
        pi = kmalloc(sizeof(*pi), GFP_KERNEL);
        if (!pi) {
                *error = "queue-length ps: Error allocating path information";
                return -ENOMEM;
        }

        pi->path = path;
        pi->repeat_count = repeat_count;
        atomic_set(&pi->qlen, 0);

        path->pscontext = pi;

        list_add_tail(&pi->list, &s->valid_paths);

        return 0;
}

static void ql_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 ql_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;
}

/*
 * Select a path having the minimum number of in-flight I/Os
 */
static struct dm_path *ql_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 ||
                    (atomic_read(&pi->qlen) < atomic_read(&best->qlen)))
                        best = pi;

                if (!atomic_read(&best->qlen))
                        break;
        }

        if (!best)
                return NULL;

        *repeat_count = best->repeat_count;

        return best->path;
}

static int ql_start_io(struct path_selector *ps, struct dm_path *path,
                       size_t nr_bytes)
{
        struct path_info *pi = path->pscontext;

        atomic_inc(&pi->qlen);

        return 0;
}

static int ql_end_io(struct path_selector *ps, struct dm_path *path,
                     size_t nr_bytes)
{
        struct path_info *pi = path->pscontext;

        atomic_dec(&pi->qlen);

        return 0;
}

static struct path_selector_type ql_ps = {
        .name           = "queue-length",
        .module         = THIS_MODULE,
        .table_args     = 1,
        .info_args      = 1,
        .create         = ql_create,
        .destroy        = ql_destroy,
        .status         = ql_status,
        .add_path       = ql_add_path,
        .fail_path      = ql_fail_path,
        .reinstate_path = ql_reinstate_path,
        .select_path    = ql_select_path,
        .start_io       = ql_start_io,
        .end_io         = ql_end_io,
};

static int __init dm_ql_init(void)
{
        int r = dm_register_path_selector(&ql_ps);

        if (r < 0)
                DMERR("register failed %d", r);

        DMINFO("version " QL_VERSION " loaded");

        return r;
}

static void __exit dm_ql_exit(void)
{
        int r = dm_unregister_path_selector(&ql_ps);

        if (r < 0)
                DMERR("unregister failed %d", r);
}

module_init(dm_ql_init);
module_exit(dm_ql_exit);

MODULE_AUTHOR("Stefan Bader <Stefan.Bader at de.ibm.com>");
MODULE_DESCRIPTION(
        "(C) Copyright IBM Corp. 2004,2005   All Rights Reserved.\n"
        DM_NAME " path selector to balance the number of in-flight I/Os"
);
MODULE_LICENSE("GPL");