drivers/md/dm-service-time.c

   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
  24 struct selector {
  25         struct list_head valid_paths;
  26         struct list_head failed_paths;
  27 };
  28
  29 struct 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
  37 static 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
  49 static 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
  60 static 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
  70 static 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
  80 static 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
 106 static 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         char dummy;
 114
 115         /*
 116          * Arguments: [<repeat_count> [<relative_throughput>]]
 117          *      <repeat_count>: The number of I/Os before switching path.
 118          *                      If not given, default (ST_MIN_IO) is used.
 119          *      <relative_throughput>: The relative throughput value of
 120          *                      the path among all paths in the path-group.
 121          *                      The valid range: 0-<ST_MAX_RELATIVE_THROUGHPUT>
 122          *                      If not given, minimum value '1' is used.
 123          *                      If '0' is given, the path isn't selected while
 124          *                      other paths having a positive value are
 125          *                      available.
 126          */
 127         if (argc > 2) {
 128                 *error = "service-time ps: incorrect number of arguments";
 129                 return -EINVAL;
 130         }
 131
 132         if (argc && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
 133                 *error = "service-time ps: invalid repeat count";
 134                 return -EINVAL;
 135         }
 136
 137         if ((argc == 2) &&
 138             (sscanf(argv[1], "%u%c", &relative_throughput, &dummy) != 1 ||
 139              relative_throughput > ST_MAX_RELATIVE_THROUGHPUT)) {
 140                 *error = "service-time ps: invalid relative_throughput value";
 141                 return -EINVAL;
 142         }
 143
 144         /* allocate the path */
 145         pi = kmalloc(sizeof(*pi), GFP_KERNEL);
 146         if (!pi) {
 147                 *error = "service-time ps: Error allocating path context";
 148                 return -ENOMEM;
 149         }
 150
 151         pi->path = path;
 152         pi->repeat_count = repeat_count;
 153         pi->relative_throughput = relative_throughput;
 154         atomic_set(&pi->in_flight_size, 0);
 155
 156         path->pscontext = pi;
 157
 158         list_add_tail(&pi->list, &s->valid_paths);
 159
 160         return 0;
 161 }
 162
 163 static void st_fail_path(struct path_selector *ps, struct dm_path *path)
 164 {
 165         struct selector *s = ps->context;
 166         struct path_info *pi = path->pscontext;
 167
 168         list_move(&pi->list, &s->failed_paths);
 169 }
 170
 171 static int st_reinstate_path(struct path_selector *ps, struct dm_path *path)
 172 {
 173         struct selector *s = ps->context;
 174         struct path_info *pi = path->pscontext;
 175
 176         list_move_tail(&pi->list, &s->valid_paths);
 177
 178         return 0;
 179 }
 180
 181 /*
 182  * Compare the estimated service time of 2 paths, pi1 and pi2,
 183  * for the incoming I/O.
 184  *
 185  * Returns:
 186  * < 0 : pi1 is better
 187  * 0   : no difference between pi1 and pi2
 188  * > 0 : pi2 is better
 189  *
 190  * Description:
 191  * Basically, the service time is estimated by:
 192  *     ('pi->in-flight-size' + 'incoming') / 'pi->relative_throughput'
 193  * To reduce the calculation, some optimizations are made.
 194  * (See comments inline)
 195  */
 196 static int st_compare_load(struct path_info *pi1, struct path_info *pi2,
 197                            size_t incoming)
 198 {
 199         size_t sz1, sz2, st1, st2;
 200
 201         sz1 = atomic_read(&pi1->in_flight_size);
 202         sz2 = atomic_read(&pi2->in_flight_size);
 203
 204         /*
 205          * Case 1: Both have same throughput value. Choose less loaded path.
 206          */
 207         if (pi1->relative_throughput == pi2->relative_throughput)
 208                 return sz1 - sz2;
 209
 210         /*
 211          * Case 2a: Both have same load. Choose higher throughput path.
 212          * Case 2b: One path has no throughput value. Choose the other one.
 213          */
 214         if (sz1 == sz2 ||
 215             !pi1->relative_throughput || !pi2->relative_throughput)
 216                 return pi2->relative_throughput - pi1->relative_throughput;
 217
 218         /*
 219          * Case 3: Calculate service time. Choose faster path.
 220          *         Service time using pi1:
 221          *             st1 = (sz1 + incoming) / pi1->relative_throughput
 222          *         Service time using pi2:
 223          *             st2 = (sz2 + incoming) / pi2->relative_throughput
 224          *
 225          *         To avoid the division, transform the expression to use
 226          *         multiplication.
 227          *         Because ->relative_throughput > 0 here, if st1 < st2,
 228          *         the expressions below are the same meaning:
 229          *             (sz1 + incoming) / pi1->relative_throughput <
 230          *                 (sz2 + incoming) / pi2->relative_throughput
 231          *             (sz1 + incoming) * pi2->relative_throughput <
 232          *                 (sz2 + incoming) * pi1->relative_throughput
 233          *         So use the later one.
 234          */
 235         sz1 += incoming;
 236         sz2 += incoming;
 237         if (unlikely(sz1 >= ST_MAX_INFLIGHT_SIZE ||
 238                      sz2 >= ST_MAX_INFLIGHT_SIZE)) {
 239                 /*
 240                  * Size may be too big for multiplying pi->relative_throughput
 241                  * and overflow.
 242                  * To avoid the overflow and mis-selection, shift down both.
 243                  */
 244                 sz1 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
 245                 sz2 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
 246         }
 247         st1 = sz1 * pi2->relative_throughput;
 248         st2 = sz2 * pi1->relative_throughput;
 249         if (st1 != st2)
 250                 return st1 - st2;
 251
 252         /*
 253          * Case 4: Service time is equal. Choose higher throughput path.
 254          */
 255         return pi2->relative_throughput - pi1->relative_throughput;
 256 }
 257
 258 static struct dm_path *st_select_path(struct path_selector *ps,
 259                                       unsigned *repeat_count, size_t nr_bytes)
 260 {
 261         struct selector *s = ps->context;
 262         struct path_info *pi = NULL, *best = NULL;
 263
 264         if (list_empty(&s->valid_paths))
 265                 return NULL;
 266
 267         /* Change preferred (first in list) path to evenly balance. */
 268         list_move_tail(s->valid_paths.next, &s->valid_paths);
 269
 270         list_for_each_entry(pi, &s->valid_paths, list)
 271                 if (!best || (st_compare_load(pi, best, nr_bytes) < 0))
 272                         best = pi;
 273
 274         if (!best)
 275                 return NULL;
 276
 277         *repeat_count = best->repeat_count;
 278
 279         return best->path;
 280 }
 281
 282 static int st_start_io(struct path_selector *ps, struct dm_path *path,
 283                        size_t nr_bytes)
 284 {
 285         struct path_info *pi = path->pscontext;
 286
 287         atomic_add(nr_bytes, &pi->in_flight_size);
 288
 289         return 0;
 290 }
 291
 292 static int st_end_io(struct path_selector *ps, struct dm_path *path,
 293                      size_t nr_bytes)
 294 {
 295         struct path_info *pi = path->pscontext;
 296
 297         atomic_sub(nr_bytes, &pi->in_flight_size);
 298
 299         return 0;
 300 }
 301
 302 static struct path_selector_type st_ps = {
 303         .name           = "service-time",
 304         .module         = THIS_MODULE,
 305         .table_args     = 2,
 306         .info_args      = 2,
 307         .create         = st_create,
 308         .destroy        = st_destroy,
 309         .status         = st_status,
 310         .add_path       = st_add_path,
 311         .fail_path      = st_fail_path,
 312         .reinstate_path = st_reinstate_path,
 313         .select_path    = st_select_path,
 314         .start_io       = st_start_io,
 315         .end_io         = st_end_io,
 316 };
 317
 318 static int __init dm_st_init(void)
 319 {
 320         int r = dm_register_path_selector(&st_ps);
 321
 322         if (r < 0)
 323                 DMERR("register failed %d", r);
 324
 325         DMINFO("version " ST_VERSION " loaded");
 326
 327         return r;
 328 }
 329
 330 static void __exit dm_st_exit(void)
 331 {
 332         int r = dm_unregister_path_selector(&st_ps);
 333
 334         if (r < 0)
 335                 DMERR("unregister failed %d", r);
 336 }
 337
 338 module_init(dm_st_init);
 339 module_exit(dm_st_exit);
 340
 341 MODULE_DESCRIPTION(DM_NAME " throughput oriented path selector");
 342 MODULE_AUTHOR("Kiyoshi Ueda <k-ueda@ct.jp.nec.com>");
 343 MODULE_LICENSE("GPL");