dm-service-time.c

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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 <[email protected]>");
MODULE_LICENSE("GPL");