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