drivers/md/dm-ps-historical-service-time.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Historical Service Time
   4  *
   5  *  Keeps a time-weighted exponential moving average of the historical
   6  *  service time. Estimates future service time based on the historical
   7  *  service time and the number of outstanding requests.
   8  *
   9  *  Marks paths stale if they have not finished within hst *
  10  *  num_paths. If a path is stale and unused, we will send a single
  11  *  request to probe in case the path has improved. This situation
  12  *  generally arises if the path is so much worse than others that it
  13  *  will never have the best estimated service time, or if the entire
  14  *  multipath device is unused. If a path is stale and in use, limit the
  15  *  number of requests it can receive with the assumption that the path
  16  *  has become degraded.
  17  *
  18  *  To avoid repeatedly calculating exponents for time weighting, times
  19  *  are split into HST_WEIGHT_COUNT buckets each (1 >> HST_BUCKET_SHIFT)
  20  *  ns, and the weighting is pre-calculated.
  21  *
  22  */
  23
  24 #include "dm.h"
  25 #include "dm-path-selector.h"
  26
  27 #include <linux/blkdev.h>
  28 #include <linux/slab.h>
  29 #include <linux/module.h>
  30
  31
  32 #define DM_MSG_PREFIX   "multipath historical-service-time"
  33 #define HST_MIN_IO 1
  34 #define HST_VERSION "0.1.1"
  35
  36 #define HST_FIXED_SHIFT 10  /* 10 bits of decimal precision */
  37 #define HST_FIXED_MAX (ULLONG_MAX >> HST_FIXED_SHIFT)
  38 #define HST_FIXED_1 (1 << HST_FIXED_SHIFT)
  39 #define HST_FIXED_95 972
  40 #define HST_MAX_INFLIGHT HST_FIXED_1
  41 #define HST_BUCKET_SHIFT 24 /* Buckets are ~ 16ms */
  42 #define HST_WEIGHT_COUNT 64ULL
  43
  44 struct selector {
  45         struct list_head valid_paths;
  46         struct list_head failed_paths;
  47         int valid_count;
  48         spinlock_t lock;
  49
  50         unsigned int weights[HST_WEIGHT_COUNT];
  51         unsigned int threshold_multiplier;
  52 };
  53
  54 struct path_info {
  55         struct list_head list;
  56         struct dm_path *path;
  57         unsigned int repeat_count;
  58
  59         spinlock_t lock;
  60
  61         u64 historical_service_time; /* Fixed point */
  62
  63         u64 stale_after;
  64         u64 last_finish;
  65
  66         u64 outstanding;
  67 };
  68
  69 /**
  70  * fixed_power - compute: x^n, in O(log n) time
  71  *
  72  * @x:         base of the power
  73  * @frac_bits: fractional bits of @x
  74  * @n:         power to raise @x to.
  75  *
  76  * By exploiting the relation between the definition of the natural power
  77  * function: x^n := x*x*...*x (x multiplied by itself for n times), and
  78  * the binary encoding of numbers used by computers: n := \Sum n_i * 2^i,
  79  * (where: n_i \elem {0, 1}, the binary vector representing n),
  80  * we find: x^n := x^(\Sum n_i * 2^i) := \Prod x^(n_i * 2^i), which is
  81  * of course trivially computable in O(log_2 n), the length of our binary
  82  * vector.
  83  *
  84  * (see: kernel/sched/loadavg.c)
  85  */
  86 static u64 fixed_power(u64 x, unsigned int frac_bits, unsigned int n)
  87 {
  88         unsigned long result = 1UL << frac_bits;
  89
  90         if (n) {
  91                 for (;;) {
  92                         if (n & 1) {
  93                                 result *= x;
  94                                 result += 1UL << (frac_bits - 1);
  95                                 result >>= frac_bits;
  96                         }
  97                         n >>= 1;
  98                         if (!n)
  99                                 break;
 100                         x *= x;
 101                         x += 1UL << (frac_bits - 1);
 102                         x >>= frac_bits;
 103                 }
 104         }
 105
 106         return result;
 107 }
 108
 109 /*
 110  * Calculate the next value of an exponential moving average
 111  * a_1 = a_0 * e + a * (1 - e)
 112  *
 113  * @last: [0, ULLONG_MAX >> HST_FIXED_SHIFT]
 114  * @next: [0, ULLONG_MAX >> HST_FIXED_SHIFT]
 115  * @weight: [0, HST_FIXED_1]
 116  *
 117  * Note:
 118  *   To account for multiple periods in the same calculation,
 119  *   a_n = a_0 * e^n + a * (1 - e^n),
 120  *   so call fixed_ema(last, next, pow(weight, N))
 121  */
 122 static u64 fixed_ema(u64 last, u64 next, u64 weight)
 123 {
 124         last *= weight;
 125         last += next * (HST_FIXED_1 - weight);
 126         last += 1ULL << (HST_FIXED_SHIFT - 1);
 127         return last >> HST_FIXED_SHIFT;
 128 }
 129
 130 static struct selector *alloc_selector(void)
 131 {
 132         struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
 133
 134         if (s) {
 135                 INIT_LIST_HEAD(&s->valid_paths);
 136                 INIT_LIST_HEAD(&s->failed_paths);
 137                 spin_lock_init(&s->lock);
 138                 s->valid_count = 0;
 139         }
 140
 141         return s;
 142 }
 143
 144 /*
 145  * Get the weight for a given time span.
 146  */
 147 static u64 hst_weight(struct path_selector *ps, u64 delta)
 148 {
 149         struct selector *s = ps->context;
 150         int bucket = clamp(delta >> HST_BUCKET_SHIFT, 0ULL,
 151                            HST_WEIGHT_COUNT - 1);
 152
 153         return s->weights[bucket];
 154 }
 155
 156 /*
 157  * Set up the weights array.
 158  *
 159  * weights[len-1] = 0
 160  * weights[n] = base ^ (n + 1)
 161  */
 162 static void hst_set_weights(struct path_selector *ps, unsigned int base)
 163 {
 164         struct selector *s = ps->context;
 165         int i;
 166
 167         if (base >= HST_FIXED_1)
 168                 return;
 169
 170         for (i = 0; i < HST_WEIGHT_COUNT - 1; i++)
 171                 s->weights[i] = fixed_power(base, HST_FIXED_SHIFT, i + 1);
 172         s->weights[HST_WEIGHT_COUNT - 1] = 0;
 173 }
 174
 175 static int hst_create(struct path_selector *ps, unsigned int argc, char **argv)
 176 {
 177         struct selector *s;
 178         unsigned int base_weight = HST_FIXED_95;
 179         unsigned int threshold_multiplier = 0;
 180         char dummy;
 181
 182         /*
 183          * Arguments: [<base_weight> [<threshold_multiplier>]]
 184          *   <base_weight>: Base weight for ema [0, 1024) 10-bit fixed point. A
 185          *                  value of 0 will completely ignore any history.
 186          *                  If not given, default (HST_FIXED_95) is used.
 187          *   <threshold_multiplier>: Minimum threshold multiplier for paths to
 188          *                  be considered different. That is, a path is
 189          *                  considered different iff (p1 > N * p2) where p1
 190          *                  is the path with higher service time. A threshold
 191          *                  of 1 or 0 has no effect. Defaults to 0.
 192          */
 193         if (argc > 2)
 194                 return -EINVAL;
 195
 196         if (argc && (sscanf(argv[0], "%u%c", &base_weight, &dummy) != 1 ||
 197              base_weight >= HST_FIXED_1)) {
 198                 return -EINVAL;
 199         }
 200
 201         if (argc > 1 && (sscanf(argv[1], "%u%c",
 202                                 &threshold_multiplier, &dummy) != 1)) {
 203                 return -EINVAL;
 204         }
 205
 206         s = alloc_selector();
 207         if (!s)
 208                 return -ENOMEM;
 209
 210         ps->context = s;
 211
 212         hst_set_weights(ps, base_weight);
 213         s->threshold_multiplier = threshold_multiplier;
 214         return 0;
 215 }
 216
 217 static void free_paths(struct list_head *paths)
 218 {
 219         struct path_info *pi, *next;
 220
 221         list_for_each_entry_safe(pi, next, paths, list) {
 222                 list_del(&pi->list);
 223                 kfree(pi);
 224         }
 225 }
 226
 227 static void hst_destroy(struct path_selector *ps)
 228 {
 229         struct selector *s = ps->context;
 230
 231         free_paths(&s->valid_paths);
 232         free_paths(&s->failed_paths);
 233         kfree(s);
 234         ps->context = NULL;
 235 }
 236
 237 static int hst_status(struct path_selector *ps, struct dm_path *path,
 238                      status_type_t type, char *result, unsigned int maxlen)
 239 {
 240         unsigned int sz = 0;
 241         struct path_info *pi;
 242
 243         if (!path) {
 244                 struct selector *s = ps->context;
 245
 246                 DMEMIT("2 %u %u ", s->weights[0], s->threshold_multiplier);
 247         } else {
 248                 pi = path->pscontext;
 249
 250                 switch (type) {
 251                 case STATUSTYPE_INFO:
 252                         DMEMIT("%llu %llu %llu ", pi->historical_service_time,
 253                                pi->outstanding, pi->stale_after);
 254                         break;
 255                 case STATUSTYPE_TABLE:
 256                         DMEMIT("0 ");
 257                         break;
 258                 }
 259         }
 260
 261         return sz;
 262 }
 263
 264 static int hst_add_path(struct path_selector *ps, struct dm_path *path,
 265                        int argc, char **argv, char **error)
 266 {
 267         struct selector *s = ps->context;
 268         struct path_info *pi;
 269         unsigned int repeat_count = HST_MIN_IO;
 270         char dummy;
 271         unsigned long flags;
 272
 273         /*
 274          * Arguments: [<repeat_count>]
 275          *   <repeat_count>: The number of I/Os before switching path.
 276          *                   If not given, default (HST_MIN_IO) is used.
 277          */
 278         if (argc > 1) {
 279                 *error = "historical-service-time ps: incorrect number of arguments";
 280                 return -EINVAL;
 281         }
 282
 283         if (argc && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
 284                 *error = "historical-service-time ps: invalid repeat count";
 285                 return -EINVAL;
 286         }
 287
 288         /* allocate the path */
 289         pi = kmalloc(sizeof(*pi), GFP_KERNEL);
 290         if (!pi) {
 291                 *error = "historical-service-time ps: Error allocating path context";
 292                 return -ENOMEM;
 293         }
 294
 295         pi->path = path;
 296         pi->repeat_count = repeat_count;
 297
 298         pi->historical_service_time = HST_FIXED_1;
 299
 300         spin_lock_init(&pi->lock);
 301         pi->outstanding = 0;
 302
 303         pi->stale_after = 0;
 304         pi->last_finish = 0;
 305
 306         path->pscontext = pi;
 307
 308         spin_lock_irqsave(&s->lock, flags);
 309         list_add_tail(&pi->list, &s->valid_paths);
 310         s->valid_count++;
 311         spin_unlock_irqrestore(&s->lock, flags);
 312
 313         return 0;
 314 }
 315
 316 static void hst_fail_path(struct path_selector *ps, struct dm_path *path)
 317 {
 318         struct selector *s = ps->context;
 319         struct path_info *pi = path->pscontext;
 320         unsigned long flags;
 321
 322         spin_lock_irqsave(&s->lock, flags);
 323         list_move(&pi->list, &s->failed_paths);
 324         s->valid_count--;
 325         spin_unlock_irqrestore(&s->lock, flags);
 326 }
 327
 328 static int hst_reinstate_path(struct path_selector *ps, struct dm_path *path)
 329 {
 330         struct selector *s = ps->context;
 331         struct path_info *pi = path->pscontext;
 332         unsigned long flags;
 333
 334         spin_lock_irqsave(&s->lock, flags);
 335         list_move_tail(&pi->list, &s->valid_paths);
 336         s->valid_count++;
 337         spin_unlock_irqrestore(&s->lock, flags);
 338
 339         return 0;
 340 }
 341
 342 static void hst_fill_compare(struct path_info *pi, u64 *hst,
 343                              u64 *out, u64 *stale)
 344 {
 345         unsigned long flags;
 346
 347         spin_lock_irqsave(&pi->lock, flags);
 348         *hst = pi->historical_service_time;
 349         *out = pi->outstanding;
 350         *stale = pi->stale_after;
 351         spin_unlock_irqrestore(&pi->lock, flags);
 352 }
 353
 354 /*
 355  * Compare the estimated service time of 2 paths, pi1 and pi2,
 356  * for the incoming I/O.
 357  *
 358  * Returns:
 359  * < 0 : pi1 is better
 360  * 0   : no difference between pi1 and pi2
 361  * > 0 : pi2 is better
 362  *
 363  */
 364 static long long hst_compare(struct path_info *pi1, struct path_info *pi2,
 365                              u64 time_now, struct path_selector *ps)
 366 {
 367         struct selector *s = ps->context;
 368         u64 hst1, hst2;
 369         long long out1, out2, stale1, stale2;
 370         int pi2_better, over_threshold;
 371
 372         hst_fill_compare(pi1, &hst1, &out1, &stale1);
 373         hst_fill_compare(pi2, &hst2, &out2, &stale2);
 374
 375         /* Check here if estimated latency for two paths are too similar.
 376          * If this is the case, we skip extra calculation and just compare
 377          * outstanding requests. In this case, any unloaded paths will
 378          * be preferred.
 379          */
 380         if (hst1 > hst2)
 381                 over_threshold = hst1 > (s->threshold_multiplier * hst2);
 382         else
 383                 over_threshold = hst2 > (s->threshold_multiplier * hst1);
 384
 385         if (!over_threshold)
 386                 return out1 - out2;
 387
 388         /*
 389          * If an unloaded path is stale, choose it. If both paths are unloaded,
 390          * choose path that is the most stale.
 391          * (If one path is loaded, choose the other)
 392          */
 393         if ((!out1 && stale1 < time_now) || (!out2 && stale2 < time_now) ||
 394             (!out1 && !out2))
 395                 return (!out2 * stale1) - (!out1 * stale2);
 396
 397         /* Compare estimated service time. If outstanding is the same, we
 398          * don't need to multiply
 399          */
 400         if (out1 == out2) {
 401                 pi2_better = hst1 > hst2;
 402         } else {
 403                 /* Potential overflow with out >= 1024 */
 404                 if (unlikely(out1 >= HST_MAX_INFLIGHT ||
 405                              out2 >= HST_MAX_INFLIGHT)) {
 406                         /* If over 1023 in-flights, we may overflow if hst
 407                          * is at max. (With this shift we still overflow at
 408                          * 1048576 in-flights, which is high enough).
 409                          */
 410                         hst1 >>= HST_FIXED_SHIFT;
 411                         hst2 >>= HST_FIXED_SHIFT;
 412                 }
 413                 pi2_better = (1 + out1) * hst1 > (1 + out2) * hst2;
 414         }
 415
 416         /* In the case that the 'winner' is stale, limit to equal usage. */
 417         if (pi2_better) {
 418                 if (stale2 < time_now)
 419                         return out1 - out2;
 420                 return 1;
 421         }
 422         if (stale1 < time_now)
 423                 return out1 - out2;
 424         return -1;
 425 }
 426
 427 static struct dm_path *hst_select_path(struct path_selector *ps,
 428                                        size_t nr_bytes)
 429 {
 430         struct selector *s = ps->context;
 431         struct path_info *pi = NULL, *best = NULL;
 432         u64 time_now = sched_clock();
 433         struct dm_path *ret = NULL;
 434         unsigned long flags;
 435
 436         spin_lock_irqsave(&s->lock, flags);
 437         if (list_empty(&s->valid_paths))
 438                 goto out;
 439
 440         list_for_each_entry(pi, &s->valid_paths, list) {
 441                 if (!best || (hst_compare(pi, best, time_now, ps) < 0))
 442                         best = pi;
 443         }
 444
 445         if (!best)
 446                 goto out;
 447
 448         /* Move last used path to end (least preferred in case of ties) */
 449         list_move_tail(&best->list, &s->valid_paths);
 450
 451         ret = best->path;
 452
 453 out:
 454         spin_unlock_irqrestore(&s->lock, flags);
 455         return ret;
 456 }
 457
 458 static int hst_start_io(struct path_selector *ps, struct dm_path *path,
 459                         size_t nr_bytes)
 460 {
 461         struct path_info *pi = path->pscontext;
 462         unsigned long flags;
 463
 464         spin_lock_irqsave(&pi->lock, flags);
 465         pi->outstanding++;
 466         spin_unlock_irqrestore(&pi->lock, flags);
 467
 468         return 0;
 469 }
 470
 471 static u64 path_service_time(struct path_info *pi, u64 start_time)
 472 {
 473         u64 sched_now = ktime_get_ns();
 474
 475         /* if a previous disk request has finished after this IO was
 476          * sent to the hardware, pretend the submission happened
 477          * serially.
 478          */
 479         if (time_after64(pi->last_finish, start_time))
 480                 start_time = pi->last_finish;
 481
 482         pi->last_finish = sched_now;
 483         if (time_before64(sched_now, start_time))
 484                 return 0;
 485
 486         return sched_now - start_time;
 487 }
 488
 489 static int hst_end_io(struct path_selector *ps, struct dm_path *path,
 490                       size_t nr_bytes, u64 start_time)
 491 {
 492         struct path_info *pi = path->pscontext;
 493         struct selector *s = ps->context;
 494         unsigned long flags;
 495         u64 st;
 496
 497         spin_lock_irqsave(&pi->lock, flags);
 498
 499         st = path_service_time(pi, start_time);
 500         pi->outstanding--;
 501         pi->historical_service_time =
 502                 fixed_ema(pi->historical_service_time,
 503                           min(st * HST_FIXED_1, HST_FIXED_MAX),
 504                           hst_weight(ps, st));
 505
 506         /*
 507          * On request end, mark path as fresh. If a path hasn't
 508          * finished any requests within the fresh period, the estimated
 509          * service time is considered too optimistic and we limit the
 510          * maximum requests on that path.
 511          */
 512         pi->stale_after = pi->last_finish +
 513                 (s->valid_count * (pi->historical_service_time >> HST_FIXED_SHIFT));
 514
 515         spin_unlock_irqrestore(&pi->lock, flags);
 516
 517         return 0;
 518 }
 519
 520 static struct path_selector_type hst_ps = {
 521         .name           = "historical-service-time",
 522         .module         = THIS_MODULE,
 523         .table_args     = 1,
 524         .info_args      = 3,
 525         .create         = hst_create,
 526         .destroy        = hst_destroy,
 527         .status         = hst_status,
 528         .add_path       = hst_add_path,
 529         .fail_path      = hst_fail_path,
 530         .reinstate_path = hst_reinstate_path,
 531         .select_path    = hst_select_path,
 532         .start_io       = hst_start_io,
 533         .end_io         = hst_end_io,
 534 };
 535
 536 static int __init dm_hst_init(void)
 537 {
 538         int r = dm_register_path_selector(&hst_ps);
 539
 540         if (r < 0)
 541                 DMERR("register failed %d", r);
 542
 543         DMINFO("version " HST_VERSION " loaded");
 544
 545         return r;
 546 }
 547
 548 static void __exit dm_hst_exit(void)
 549 {
 550         int r = dm_unregister_path_selector(&hst_ps);
 551
 552         if (r < 0)
 553                 DMERR("unregister failed %d", r);
 554 }
 555
 556 module_init(dm_hst_init);
 557 module_exit(dm_hst_exit);
 558
 559 MODULE_DESCRIPTION(DM_NAME " measured service time oriented path selector");
 560 MODULE_AUTHOR("Khazhismel Kumykov <khazhy@google.com>");
 561 MODULE_LICENSE("GPL");