/*
 * 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;

        /*
         * 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", &repeat_count) != 1)) {
                *error = "service-time ps: invalid repeat count";
                return -EINVAL;
        }

        if ((argc == 2) &&
            (sscanf(argv[1], "%u", &relative_throughput) != 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");