root/drivers/md/dm-service-time.c

DEFINITIONS

1. alloc_selector
2. st_create
3. free_paths
4. st_destroy
5. st_status
6. st_add_path
7. st_fail_path
8. st_reinstate_path
9. st_compare_load
10. st_select_path
11. st_start_io
12. st_end_io
13. dm_st_init
14. dm_st_exit

   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 
  24 struct selector {
  25         struct list_head valid_paths;
  26         struct list_head failed_paths;
  27         spinlock_t lock;
  28 };
  29 
  30 struct 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 
  38 static 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 
  51 static 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 
  62 static 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 
  72 static 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 
  82 static 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 
 108 static 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 
 173 static 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 
 184 static 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  */
 212 static 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 
 274 static 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;
 296 out:
 297         spin_unlock_irqrestore(&s->lock, flags);
 298         return ret;
 299 }
 300 
 301 static 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 
 311 static int st_end_io(struct path_selector *ps, struct dm_path *path,
 312                      size_t nr_bytes)
 313 {
 314         struct path_info *pi = path->pscontext;
 315 
 316         atomic_sub(nr_bytes, &pi->in_flight_size);
 317 
 318         return 0;
 319 }
 320 
 321 static 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 
 337 static 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 
 349 static 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 
 357 module_init(dm_st_init);
 358 module_exit(dm_st_exit);
 359 
 360 MODULE_DESCRIPTION(DM_NAME " throughput oriented path selector");
 361 MODULE_AUTHOR("Kiyoshi Ueda <k-ueda@ct.jp.nec.com>");
 362 MODULE_LICENSE("GPL");