dm-queue-length.c

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/*
 * 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");