linux/drivers/md/dm-ps-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      "0.3.0"
  23
  24struct selector {
  25        struct list_head valid_paths;
  26        struct list_head failed_paths;
  27        spinlock_t lock;
  28};
  29
  30struct path_info {
  31        struct list_head list;
  32        struct dm_path *path;
  33        unsigned repeat_count;
  34        unsigned relative_throughput;
  35        atomic_t in_flight_size;        /* Total size of in-flight I/Os */
  36};
  37
  38static struct selector *alloc_selector(void)
  39{
  40        struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
  41
  42        if (s) {
  43                INIT_LIST_HEAD(&s->valid_paths);
  44                INIT_LIST_HEAD(&s->failed_paths);
  45                spin_lock_init(&s->lock);
  46        }
  47
  48        return s;
  49}
  50
  51static int st_create(struct path_selector *ps, unsigned argc, char **argv)
  52{
  53        struct selector *s = alloc_selector();
  54
  55        if (!s)
  56                return -ENOMEM;
  57
  58        ps->context = s;
  59        return 0;
  60}
  61
  62static void free_paths(struct list_head *paths)
  63{
  64        struct path_info *pi, *next;
  65
  66        list_for_each_entry_safe(pi, next, paths, list) {
  67                list_del(&pi->list);
  68                kfree(pi);
  69        }
  70}
  71
  72static void st_destroy(struct path_selector *ps)
  73{
  74        struct selector *s = ps->context;
  75
  76        free_paths(&s->valid_paths);
  77        free_paths(&s->failed_paths);
  78        kfree(s);
  79        ps->context = NULL;
  80}
  81
  82static int st_status(struct path_selector *ps, struct dm_path *path,
  83                     status_type_t type, char *result, unsigned maxlen)
  84{
  85        unsigned sz = 0;
  86        struct path_info *pi;
  87
  88        if (!path)
  89                DMEMIT("0 ");
  90        else {
  91                pi = path->pscontext;
  92
  93                switch (type) {
  94                case STATUSTYPE_INFO:
  95                        DMEMIT("%d %u ", atomic_read(&pi->in_flight_size),
  96                               pi->relative_throughput);
  97                        break;
  98                case STATUSTYPE_TABLE:
  99                        DMEMIT("%u %u ", pi->repeat_count,
 100                               pi->relative_throughput);
 101                        break;
 102                }
 103        }
 104
 105        return sz;
 106}
 107
 108static int st_add_path(struct path_selector *ps, struct dm_path *path,
 109                       int argc, char **argv, char **error)
 110{
 111        struct selector *s = ps->context;
 112        struct path_info *pi;
 113        unsigned repeat_count = ST_MIN_IO;
 114        unsigned relative_throughput = 1;
 115        char dummy;
 116        unsigned long flags;
 117
 118        /*
 119         * Arguments: [<repeat_count> [<relative_throughput>]]
 120         *      <repeat_count>: The number of I/Os before switching path.
 121         *                      If not given, default (ST_MIN_IO) is used.
 122         *      <relative_throughput>: The relative throughput value of
 123         *                      the path among all paths in the path-group.
 124         *                      The valid range: 0-<ST_MAX_RELATIVE_THROUGHPUT>
 125         *                      If not given, minimum value '1' is used.
 126         *                      If '0' is given, the path isn't selected while
 127         *                      other paths having a positive value are
 128         *                      available.
 129         */
 130        if (argc > 2) {
 131                *error = "service-time ps: incorrect number of arguments";
 132                return -EINVAL;
 133        }
 134
 135        if (argc && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
 136                *error = "service-time ps: invalid repeat count";
 137                return -EINVAL;
 138        }
 139
 140        if (repeat_count > 1) {
 141                DMWARN_LIMIT("repeat_count > 1 is deprecated, using 1 instead");
 142                repeat_count = 1;
 143        }
 144
 145        if ((argc == 2) &&
 146            (sscanf(argv[1], "%u%c", &relative_throughput, &dummy) != 1 ||
 147             relative_throughput > ST_MAX_RELATIVE_THROUGHPUT)) {
 148                *error = "service-time ps: invalid relative_throughput value";
 149                return -EINVAL;
 150        }
 151
 152        /* allocate the path */
 153        pi = kmalloc(sizeof(*pi), GFP_KERNEL);
 154        if (!pi) {
 155                *error = "service-time ps: Error allocating path context";
 156                return -ENOMEM;
 157        }
 158
 159        pi->path = path;
 160        pi->repeat_count = repeat_count;
 161        pi->relative_throughput = relative_throughput;
 162        atomic_set(&pi->in_flight_size, 0);
 163
 164        path->pscontext = pi;
 165
 166        spin_lock_irqsave(&s->lock, flags);
 167        list_add_tail(&pi->list, &s->valid_paths);
 168        spin_unlock_irqrestore(&s->lock, flags);
 169
 170        return 0;
 171}
 172
 173static void st_fail_path(struct path_selector *ps, struct dm_path *path)
 174{
 175        struct selector *s = ps->context;
 176        struct path_info *pi = path->pscontext;
 177        unsigned long flags;
 178
 179        spin_lock_irqsave(&s->lock, flags);
 180        list_move(&pi->list, &s->failed_paths);
 181        spin_unlock_irqrestore(&s->lock, flags);
 182}
 183
 184static int st_reinstate_path(struct path_selector *ps, struct dm_path *path)
 185{
 186        struct selector *s = ps->context;
 187        struct path_info *pi = path->pscontext;
 188        unsigned long flags;
 189
 190        spin_lock_irqsave(&s->lock, flags);
 191        list_move_tail(&pi->list, &s->valid_paths);
 192        spin_unlock_irqrestore(&s->lock, flags);
 193
 194        return 0;
 195}
 196
 197/*
 198 * Compare the estimated service time of 2 paths, pi1 and pi2,
 199 * for the incoming I/O.
 200 *
 201 * Returns:
 202 * < 0 : pi1 is better
 203 * 0   : no difference between pi1 and pi2
 204 * > 0 : pi2 is better
 205 *
 206 * Description:
 207 * Basically, the service time is estimated by:
 208 *     ('pi->in-flight-size' + 'incoming') / 'pi->relative_throughput'
 209 * To reduce the calculation, some optimizations are made.
 210 * (See comments inline)
 211 */
 212static int st_compare_load(struct path_info *pi1, struct path_info *pi2,
 213                           size_t incoming)
 214{
 215        size_t sz1, sz2, st1, st2;
 216
 217        sz1 = atomic_read(&pi1->in_flight_size);
 218        sz2 = atomic_read(&pi2->in_flight_size);
 219
 220        /*
 221         * Case 1: Both have same throughput value. Choose less loaded path.
 222         */
 223        if (pi1->relative_throughput == pi2->relative_throughput)
 224                return sz1 - sz2;
 225
 226        /*
 227         * Case 2a: Both have same load. Choose higher throughput path.
 228         * Case 2b: One path has no throughput value. Choose the other one.
 229         */
 230        if (sz1 == sz2 ||
 231            !pi1->relative_throughput || !pi2->relative_throughput)
 232                return pi2->relative_throughput - pi1->relative_throughput;
 233
 234        /*
 235         * Case 3: Calculate service time. Choose faster path.
 236         *         Service time using pi1:
 237         *             st1 = (sz1 + incoming) / pi1->relative_throughput
 238         *         Service time using pi2:
 239         *             st2 = (sz2 + incoming) / pi2->relative_throughput
 240         *
 241         *         To avoid the division, transform the expression to use
 242         *         multiplication.
 243         *         Because ->relative_throughput > 0 here, if st1 < st2,
 244         *         the expressions below are the same meaning:
 245         *             (sz1 + incoming) / pi1->relative_throughput <
 246         *                 (sz2 + incoming) / pi2->relative_throughput
 247         *             (sz1 + incoming) * pi2->relative_throughput <
 248         *                 (sz2 + incoming) * pi1->relative_throughput
 249         *         So use the later one.
 250         */
 251        sz1 += incoming;
 252        sz2 += incoming;
 253        if (unlikely(sz1 >= ST_MAX_INFLIGHT_SIZE ||
 254                     sz2 >= ST_MAX_INFLIGHT_SIZE)) {
 255                /*
 256                 * Size may be too big for multiplying pi->relative_throughput
 257                 * and overflow.
 258                 * To avoid the overflow and mis-selection, shift down both.
 259                 */
 260                sz1 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
 261                sz2 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
 262        }
 263        st1 = sz1 * pi2->relative_throughput;
 264        st2 = sz2 * pi1->relative_throughput;
 265        if (st1 != st2)
 266                return st1 - st2;
 267
 268        /*
 269         * Case 4: Service time is equal. Choose higher throughput path.
 270         */
 271        return pi2->relative_throughput - pi1->relative_throughput;
 272}
 273
 274static struct dm_path *st_select_path(struct path_selector *ps, size_t nr_bytes)
 275{
 276        struct selector *s = ps->context;
 277        struct path_info *pi = NULL, *best = NULL;
 278        struct dm_path *ret = NULL;
 279        unsigned long flags;
 280
 281        spin_lock_irqsave(&s->lock, flags);
 282        if (list_empty(&s->valid_paths))
 283                goto out;
 284
 285        list_for_each_entry(pi, &s->valid_paths, list)
 286                if (!best || (st_compare_load(pi, best, nr_bytes) < 0))
 287                        best = pi;
 288
 289        if (!best)
 290                goto out;
 291
 292        /* Move most recently used to least preferred to evenly balance. */
 293        list_move_tail(&best->list, &s->valid_paths);
 294
 295        ret = best->path;
 296out:
 297        spin_unlock_irqrestore(&s->lock, flags);
 298        return ret;
 299}
 300
 301static int st_start_io(struct path_selector *ps, struct dm_path *path,
 302                       size_t nr_bytes)
 303{
 304        struct path_info *pi = path->pscontext;
 305
 306        atomic_add(nr_bytes, &pi->in_flight_size);
 307
 308        return 0;
 309}
 310
 311static int st_end_io(struct path_selector *ps, struct dm_path *path,
 312                     size_t nr_bytes, u64 start_time)
 313{
 314        struct path_info *pi = path->pscontext;
 315
 316        atomic_sub(nr_bytes, &pi->in_flight_size);
 317
 318        return 0;
 319}
 320
 321static struct path_selector_type st_ps = {
 322        .name           = "service-time",
 323        .module         = THIS_MODULE,
 324        .table_args     = 2,
 325        .info_args      = 2,
 326        .create         = st_create,
 327        .destroy        = st_destroy,
 328        .status         = st_status,
 329        .add_path       = st_add_path,
 330        .fail_path      = st_fail_path,
 331        .reinstate_path = st_reinstate_path,
 332        .select_path    = st_select_path,
 333        .start_io       = st_start_io,
 334        .end_io         = st_end_io,
 335};
 336
 337static int __init dm_st_init(void)
 338{
 339        int r = dm_register_path_selector(&st_ps);
 340
 341        if (r < 0)
 342                DMERR("register failed %d", r);
 343
 344        DMINFO("version " ST_VERSION " loaded");
 345
 346        return r;
 347}
 348
 349static void __exit dm_st_exit(void)
 350{
 351        int r = dm_unregister_path_selector(&st_ps);
 352
 353        if (r < 0)
 354                DMERR("unregister failed %d", r);
 355}
 356
 357module_init(dm_st_init);
 358module_exit(dm_st_exit);
 359
 360MODULE_DESCRIPTION(DM_NAME " throughput oriented path selector");
 361MODULE_AUTHOR("Kiyoshi Ueda <k-ueda@ct.jp.nec.com>");
 362MODULE_LICENSE("GPL");
 363