linux/drivers/md/dm-service-time.c
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   1/*
   2 * Copyright (C) 2007-2009 NEC Corporation.  All Rights Reserved.
   3 *
   4 * Module Author: Kiyoshi Ueda
   5 *
   6 * This file is released under the GPL.
   7 *
   8 * Throughput oriented path selector.
   9 */
  10
  11#include "dm.h"
  12#include "dm-path-selector.h"
  13
  14#include <linux/slab.h>
  15#include <linux/module.h>
  16
  17#define DM_MSG_PREFIX   "multipath service-time"
  18#define ST_MIN_IO       1
  19#define ST_MAX_RELATIVE_THROUGHPUT      100
  20#define ST_MAX_RELATIVE_THROUGHPUT_SHIFT        7
  21#define ST_MAX_INFLIGHT_SIZE    ((size_t)-1 >> ST_MAX_RELATIVE_THROUGHPUT_SHIFT)
  22#define ST_VERSION      "0.2.0"
  23
  24struct selector {
  25        struct list_head valid_paths;
  26        struct list_head failed_paths;
  27};
  28
  29struct path_info {
  30        struct list_head list;
  31        struct dm_path *path;
  32        unsigned repeat_count;
  33        unsigned relative_throughput;
  34        atomic_t in_flight_size;        /* Total size of in-flight I/Os */
  35};
  36
  37static struct selector *alloc_selector(void)
  38{
  39        struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
  40
  41        if (s) {
  42                INIT_LIST_HEAD(&s->valid_paths);
  43                INIT_LIST_HEAD(&s->failed_paths);
  44        }
  45
  46        return s;
  47}
  48
  49static int st_create(struct path_selector *ps, unsigned argc, char **argv)
  50{
  51        struct selector *s = alloc_selector();
  52
  53        if (!s)
  54                return -ENOMEM;
  55
  56        ps->context = s;
  57        return 0;
  58}
  59
  60static void free_paths(struct list_head *paths)
  61{
  62        struct path_info *pi, *next;
  63
  64        list_for_each_entry_safe(pi, next, paths, list) {
  65                list_del(&pi->list);
  66                kfree(pi);
  67        }
  68}
  69
  70static void st_destroy(struct path_selector *ps)
  71{
  72        struct selector *s = ps->context;
  73
  74        free_paths(&s->valid_paths);
  75        free_paths(&s->failed_paths);
  76        kfree(s);
  77        ps->context = NULL;
  78}
  79
  80static int st_status(struct path_selector *ps, struct dm_path *path,
  81                     status_type_t type, char *result, unsigned maxlen)
  82{
  83        unsigned sz = 0;
  84        struct path_info *pi;
  85
  86        if (!path)
  87                DMEMIT("0 ");
  88        else {
  89                pi = path->pscontext;
  90
  91                switch (type) {
  92                case STATUSTYPE_INFO:
  93                        DMEMIT("%d %u ", atomic_read(&pi->in_flight_size),
  94                               pi->relative_throughput);
  95                        break;
  96                case STATUSTYPE_TABLE:
  97                        DMEMIT("%u %u ", pi->repeat_count,
  98                               pi->relative_throughput);
  99                        break;
 100                }
 101        }
 102
 103        return sz;
 104}
 105
 106static int st_add_path(struct path_selector *ps, struct dm_path *path,
 107                       int argc, char **argv, char **error)
 108{
 109        struct selector *s = ps->context;
 110        struct path_info *pi;
 111        unsigned repeat_count = ST_MIN_IO;
 112        unsigned relative_throughput = 1;
 113
 114        /*
 115         * Arguments: [<repeat_count> [<relative_throughput>]]
 116         *      <repeat_count>: The number of I/Os before switching path.
 117         *                      If not given, default (ST_MIN_IO) is used.
 118         *      <relative_throughput>: The relative throughput value of
 119         *                      the path among all paths in the path-group.
 120         *                      The valid range: 0-<ST_MAX_RELATIVE_THROUGHPUT>
 121         *                      If not given, minimum value '1' is used.
 122         *                      If '0' is given, the path isn't selected while
 123         *                      other paths having a positive value are
 124         *                      available.
 125         */
 126        if (argc > 2) {
 127                *error = "service-time ps: incorrect number of arguments";
 128                return -EINVAL;
 129        }
 130
 131        if (argc && (sscanf(argv[0], "%u", &repeat_count) != 1)) {
 132                *error = "service-time ps: invalid repeat count";
 133                return -EINVAL;
 134        }
 135
 136        if ((argc == 2) &&
 137            (sscanf(argv[1], "%u", &relative_throughput) != 1 ||
 138             relative_throughput > ST_MAX_RELATIVE_THROUGHPUT)) {
 139                *error = "service-time ps: invalid relative_throughput value";
 140                return -EINVAL;
 141        }
 142
 143        /* allocate the path */
 144        pi = kmalloc(sizeof(*pi), GFP_KERNEL);
 145        if (!pi) {
 146                *error = "service-time ps: Error allocating path context";
 147                return -ENOMEM;
 148        }
 149
 150        pi->path = path;
 151        pi->repeat_count = repeat_count;
 152        pi->relative_throughput = relative_throughput;
 153        atomic_set(&pi->in_flight_size, 0);
 154
 155        path->pscontext = pi;
 156
 157        list_add_tail(&pi->list, &s->valid_paths);
 158
 159        return 0;
 160}
 161
 162static void st_fail_path(struct path_selector *ps, struct dm_path *path)
 163{
 164        struct selector *s = ps->context;
 165        struct path_info *pi = path->pscontext;
 166
 167        list_move(&pi->list, &s->failed_paths);
 168}
 169
 170static int st_reinstate_path(struct path_selector *ps, struct dm_path *path)
 171{
 172        struct selector *s = ps->context;
 173        struct path_info *pi = path->pscontext;
 174
 175        list_move_tail(&pi->list, &s->valid_paths);
 176
 177        return 0;
 178}
 179
 180/*
 181 * Compare the estimated service time of 2 paths, pi1 and pi2,
 182 * for the incoming I/O.
 183 *
 184 * Returns:
 185 * < 0 : pi1 is better
 186 * 0   : no difference between pi1 and pi2
 187 * > 0 : pi2 is better
 188 *
 189 * Description:
 190 * Basically, the service time is estimated by:
 191 *     ('pi->in-flight-size' + 'incoming') / 'pi->relative_throughput'
 192 * To reduce the calculation, some optimizations are made.
 193 * (See comments inline)
 194 */
 195static int st_compare_load(struct path_info *pi1, struct path_info *pi2,
 196                           size_t incoming)
 197{
 198        size_t sz1, sz2, st1, st2;
 199
 200        sz1 = atomic_read(&pi1->in_flight_size);
 201        sz2 = atomic_read(&pi2->in_flight_size);
 202
 203        /*
 204         * Case 1: Both have same throughput value. Choose less loaded path.
 205         */
 206        if (pi1->relative_throughput == pi2->relative_throughput)
 207                return sz1 - sz2;
 208
 209        /*
 210         * Case 2a: Both have same load. Choose higher throughput path.
 211         * Case 2b: One path has no throughput value. Choose the other one.
 212         */
 213        if (sz1 == sz2 ||
 214            !pi1->relative_throughput || !pi2->relative_throughput)
 215                return pi2->relative_throughput - pi1->relative_throughput;
 216
 217        /*
 218         * Case 3: Calculate service time. Choose faster path.
 219         *         Service time using pi1:
 220         *             st1 = (sz1 + incoming) / pi1->relative_throughput
 221         *         Service time using pi2:
 222         *             st2 = (sz2 + incoming) / pi2->relative_throughput
 223         *
 224         *         To avoid the division, transform the expression to use
 225         *         multiplication.
 226         *         Because ->relative_throughput > 0 here, if st1 < st2,
 227         *         the expressions below are the same meaning:
 228         *             (sz1 + incoming) / pi1->relative_throughput <
 229         *                 (sz2 + incoming) / pi2->relative_throughput
 230         *             (sz1 + incoming) * pi2->relative_throughput <
 231         *                 (sz2 + incoming) * pi1->relative_throughput
 232         *         So use the later one.
 233         */
 234        sz1 += incoming;
 235        sz2 += incoming;
 236        if (unlikely(sz1 >= ST_MAX_INFLIGHT_SIZE ||
 237                     sz2 >= ST_MAX_INFLIGHT_SIZE)) {
 238                /*
 239                 * Size may be too big for multiplying pi->relative_throughput
 240                 * and overflow.
 241                 * To avoid the overflow and mis-selection, shift down both.
 242                 */
 243                sz1 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
 244                sz2 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
 245        }
 246        st1 = sz1 * pi2->relative_throughput;
 247        st2 = sz2 * pi1->relative_throughput;
 248        if (st1 != st2)
 249                return st1 - st2;
 250
 251        /*
 252         * Case 4: Service time is equal. Choose higher throughput path.
 253         */
 254        return pi2->relative_throughput - pi1->relative_throughput;
 255}
 256
 257static struct dm_path *st_select_path(struct path_selector *ps,
 258                                      unsigned *repeat_count, size_t nr_bytes)
 259{
 260        struct selector *s = ps->context;
 261        struct path_info *pi = NULL, *best = NULL;
 262
 263        if (list_empty(&s->valid_paths))
 264                return NULL;
 265
 266        /* Change preferred (first in list) path to evenly balance. */
 267        list_move_tail(s->valid_paths.next, &s->valid_paths);
 268
 269        list_for_each_entry(pi, &s->valid_paths, list)
 270                if (!best || (st_compare_load(pi, best, nr_bytes) < 0))
 271                        best = pi;
 272
 273        if (!best)
 274                return NULL;
 275
 276        *repeat_count = best->repeat_count;
 277
 278        return best->path;
 279}
 280
 281static int st_start_io(struct path_selector *ps, struct dm_path *path,
 282                       size_t nr_bytes)
 283{
 284        struct path_info *pi = path->pscontext;
 285
 286        atomic_add(nr_bytes, &pi->in_flight_size);
 287
 288        return 0;
 289}
 290
 291static int st_end_io(struct path_selector *ps, struct dm_path *path,
 292                     size_t nr_bytes)
 293{
 294        struct path_info *pi = path->pscontext;
 295
 296        atomic_sub(nr_bytes, &pi->in_flight_size);
 297
 298        return 0;
 299}
 300
 301static struct path_selector_type st_ps = {
 302        .name           = "service-time",
 303        .module         = THIS_MODULE,
 304        .table_args     = 2,
 305        .info_args      = 2,
 306        .create         = st_create,
 307        .destroy        = st_destroy,
 308        .status         = st_status,
 309        .add_path       = st_add_path,
 310        .fail_path      = st_fail_path,
 311        .reinstate_path = st_reinstate_path,
 312        .select_path    = st_select_path,
 313        .start_io       = st_start_io,
 314        .end_io         = st_end_io,
 315};
 316
 317static int __init dm_st_init(void)
 318{
 319        int r = dm_register_path_selector(&st_ps);
 320
 321        if (r < 0)
 322                DMERR("register failed %d", r);
 323
 324        DMINFO("version " ST_VERSION " loaded");
 325
 326        return r;
 327}
 328
 329static void __exit dm_st_exit(void)
 330{
 331        int r = dm_unregister_path_selector(&st_ps);
 332
 333        if (r < 0)
 334                DMERR("unregister failed %d", r);
 335}
 336
 337module_init(dm_st_init);
 338module_exit(dm_st_exit);
 339
 340MODULE_DESCRIPTION(DM_NAME " throughput oriented path selector");
 341MODULE_AUTHOR("Kiyoshi Ueda <k-ueda@ct.jp.nec.com>");
 342MODULE_LICENSE("GPL");
 343