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