1/* 2 * kernel/sched/proc.c 3 * 4 * Kernel load calculations, forked from sched/core.c 5 */ 6 7#include <linux/export.h> 8 9#include "sched.h" 10 11unsigned long this_cpu_load(void) 12{ 13 struct rq *this = this_rq(); 14 return this->cpu_load[0]; 15} 16 17 18/* 19 * Global load-average calculations 20 * 21 * We take a distributed and async approach to calculating the global load-avg 22 * in order to minimize overhead. 23 * 24 * The global load average is an exponentially decaying average of nr_running + 25 * nr_uninterruptible. 26 * 27 * Once every LOAD_FREQ: 28 * 29 * nr_active = 0; 30 * for_each_possible_cpu(cpu) 31 * nr_active += cpu_of(cpu)->nr_running + cpu_of(cpu)->nr_uninterruptible; 32 * 33 * avenrun[n] = avenrun[0] * exp_n + nr_active * (1 - exp_n) 34 * 35 * Due to a number of reasons the above turns in the mess below: 36 * 37 * - for_each_possible_cpu() is prohibitively expensive on machines with 38 * serious number of cpus, therefore we need to take a distributed approach 39 * to calculating nr_active. 40 * 41 * \Sum_i x_i(t) = \Sum_i x_i(t) - x_i(t_0) | x_i(t_0) := 0 42 * = \Sum_i { \Sum_j=1 x_i(t_j) - x_i(t_j-1) } 43 * 44 * So assuming nr_active := 0 when we start out -- true per definition, we 45 * can simply take per-cpu deltas and fold those into a global accumulate 46 * to obtain the same result. See calc_load_fold_active(). 47 * 48 * Furthermore, in order to avoid synchronizing all per-cpu delta folding 49 * across the machine, we assume 10 ticks is sufficient time for every 50 * cpu to have completed this task. 51 * 52 * This places an upper-bound on the IRQ-off latency of the machine. Then 53 * again, being late doesn't loose the delta, just wrecks the sample. 54 * 55 * - cpu_rq()->nr_uninterruptible isn't accurately tracked per-cpu because 56 * this would add another cross-cpu cacheline miss and atomic operation 57 * to the wakeup path. Instead we increment on whatever cpu the task ran 58 * when it went into uninterruptible state and decrement on whatever cpu 59 * did the wakeup. This means that only the sum of nr_uninterruptible over 60 * all cpus yields the correct result. 61 * 62 * This covers the NO_HZ=n code, for extra head-aches, see the comment below. 63 */ 64 65/* Variables and functions for calc_load */ 66atomic_long_t calc_load_tasks; 67unsigned long calc_load_update; 68unsigned long avenrun[3]; 69EXPORT_SYMBOL(avenrun); /* should be removed */ 70 71/** 72 * get_avenrun - get the load average array 73 * @loads: pointer to dest load array 74 * @offset: offset to add 75 * @shift: shift count to shift the result left 76 * 77 * These values are estimates at best, so no need for locking. 78 */ 79void get_avenrun(unsigned long *loads, unsigned long offset, int shift) 80{ 81 loads[0] = (avenrun[0] + offset) << shift; 82 loads[1] = (avenrun[1] + offset) << shift; 83 loads[2] = (avenrun[2] + offset) << shift; 84} 85 86long calc_load_fold_active(struct rq *this_rq) 87{ 88 long nr_active, delta = 0; 89 90 nr_active = this_rq->nr_running; 91 nr_active += (long) this_rq->nr_uninterruptible; 92 93 if (nr_active != this_rq->calc_load_active) { 94 delta = nr_active - this_rq->calc_load_active; 95 this_rq->calc_load_active = nr_active; 96 } 97 98 return delta; 99} 100 101/* 102 * a1 = a0 * e + a * (1 - e) 103 */ 104static unsigned long 105calc_load(unsigned long load, unsigned long exp, unsigned long active) 106{ 107 load *= exp; 108 load += active * (FIXED_1 - exp); 109 load += 1UL << (FSHIFT - 1); 110 return load >> FSHIFT; 111} 112 113#ifdef CONFIG_NO_HZ_COMMON 114/* 115 * Handle NO_HZ for the global load-average. 116 * 117 * Since the above described distributed algorithm to compute the global 118 * load-average relies on per-cpu sampling from the tick, it is affected by 119 * NO_HZ. 120 * 121 * The basic idea is to fold the nr_active delta into a global idle-delta upon 122 * entering NO_HZ state such that we can include this as an 'extra' cpu delta 123 * when we read the global state. 124 * 125 * Obviously reality has to ruin such a delightfully simple scheme: 126 * 127 * - When we go NO_HZ idle during the window, we can negate our sample 128 * contribution, causing under-accounting. 129 * 130 * We avoid this by keeping two idle-delta counters and flipping them 131 * when the window starts, thus separating old and new NO_HZ load. 132 * 133 * The only trick is the slight shift in index flip for read vs write. 134 * 135 * 0s 5s 10s 15s 136 * +10 +10 +10 +10 137 * |-|-----------|-|-----------|-|-----------|-| 138 * r:0 0 1 1 0 0 1 1 0 139 * w:0 1 1 0 0 1 1 0 0 140 * 141 * This ensures we'll fold the old idle contribution in this window while 142 * accumlating the new one. 143 * 144 * - When we wake up from NO_HZ idle during the window, we push up our 145 * contribution, since we effectively move our sample point to a known 146 * busy state. 147 * 148 * This is solved by pushing the window forward, and thus skipping the 149 * sample, for this cpu (effectively using the idle-delta for this cpu which 150 * was in effect at the time the window opened). This also solves the issue 151 * of having to deal with a cpu having been in NOHZ idle for multiple 152 * LOAD_FREQ intervals. 153 * 154 * When making the ILB scale, we should try to pull this in as well. 155 */ 156static atomic_long_t calc_load_idle[2]; 157static int calc_load_idx; 158 159static inline int calc_load_write_idx(void) 160{ 161 int idx = calc_load_idx; 162 163 /* 164 * See calc_global_nohz(), if we observe the new index, we also 165 * need to observe the new update time. 166 */ 167 smp_rmb(); 168 169 /* 170 * If the folding window started, make sure we start writing in the 171 * next idle-delta. 172 */ 173 if (!time_before(jiffies, calc_load_update)) 174 idx++; 175 176 return idx & 1; 177} 178 179static inline int calc_load_read_idx(void) 180{ 181 return calc_load_idx & 1; 182} 183 184void calc_load_enter_idle(void) 185{ 186 struct rq *this_rq = this_rq(); 187 long delta; 188 189 /* 190 * We're going into NOHZ mode, if there's any pending delta, fold it 191 * into the pending idle delta. 192 */ 193 delta = calc_load_fold_active(this_rq); 194 if (delta) { 195 int idx = calc_load_write_idx(); 196 atomic_long_add(delta, &calc_load_idle[idx]); 197 } 198} 199 200void calc_load_exit_idle(void) 201{ 202 struct rq *this_rq = this_rq(); 203 204 /* 205 * If we're still before the sample window, we're done. 206 */ 207 if (time_before(jiffies, this_rq->calc_load_update)) 208 return; 209 210 /* 211 * We woke inside or after the sample window, this means we're already 212 * accounted through the nohz accounting, so skip the entire deal and 213 * sync up for the next window. 214 */ 215 this_rq->calc_load_update = calc_load_update; 216 if (time_before(jiffies, this_rq->calc_load_update + 10)) 217 this_rq->calc_load_update += LOAD_FREQ; 218} 219 220static long calc_load_fold_idle(void) 221{ 222 int idx = calc_load_read_idx(); 223 long delta = 0; 224 225 if (atomic_long_read(&calc_load_idle[idx])) 226 delta = atomic_long_xchg(&calc_load_idle[idx], 0); 227 228 return delta; 229} 230 231/** 232 * fixed_power_int - compute: x^n, in O(log n) time 233 * 234 * @x: base of the power 235 * @frac_bits: fractional bits of @x 236 * @n: power to raise @x to. 237 * 238 * By exploiting the relation between the definition of the natural power 239 * function: x^n := x*x*...*x (x multiplied by itself for n times), and 240 * the binary encoding of numbers used by computers: n := \Sum n_i * 2^i, 241 * (where: n_i \elem {0, 1}, the binary vector representing n), 242 * we find: x^n := x^(\Sum n_i * 2^i) := \Prod x^(n_i * 2^i), which is 243 * of course trivially computable in O(log_2 n), the length of our binary 244 * vector. 245 */ 246static unsigned long 247fixed_power_int(unsigned long x, unsigned int frac_bits, unsigned int n) 248{ 249 unsigned long result = 1UL << frac_bits; 250 251 if (n) for (;;) { 252 if (n & 1) { 253 result *= x; 254 result += 1UL << (frac_bits - 1); 255 result >>= frac_bits; 256 } 257 n >>= 1; 258 if (!n) 259 break; 260 x *= x; 261 x += 1UL << (frac_bits - 1); 262 x >>= frac_bits; 263 } 264 265 return result; 266} 267 268/* 269 * a1 = a0 * e + a * (1 - e) 270 * 271 * a2 = a1 * e + a * (1 - e) 272 * = (a0 * e + a * (1 - e)) * e + a * (1 - e) 273 * = a0 * e^2 + a * (1 - e) * (1 + e) 274 * 275 * a3 = a2 * e + a * (1 - e) 276 * = (a0 * e^2 + a * (1 - e) * (1 + e)) * e + a * (1 - e) 277 * = a0 * e^3 + a * (1 - e) * (1 + e + e^2) 278 * 279 * ... 280 * 281 * an = a0 * e^n + a * (1 - e) * (1 + e + ... + e^n-1) [1] 282 * = a0 * e^n + a * (1 - e) * (1 - e^n)/(1 - e) 283 * = a0 * e^n + a * (1 - e^n) 284 * 285 * [1] application of the geometric series: 286 * 287 * n 1 - x^(n+1) 288 * S_n := \Sum x^i = ------------- 289 * i=0 1 - x 290 */ 291static unsigned long 292calc_load_n(unsigned long load, unsigned long exp, 293 unsigned long active, unsigned int n) 294{ 295 296 return calc_load(load, fixed_power_int(exp, FSHIFT, n), active); 297} 298 299/* 300 * NO_HZ can leave us missing all per-cpu ticks calling 301 * calc_load_account_active(), but since an idle CPU folds its delta into 302 * calc_load_tasks_idle per calc_load_account_idle(), all we need to do is fold 303 * in the pending idle delta if our idle period crossed a load cycle boundary. 304 * 305 * Once we've updated the global active value, we need to apply the exponential 306 * weights adjusted to the number of cycles missed. 307 */ 308static void calc_global_nohz(void) 309{ 310 long delta, active, n; 311 312 if (!time_before(jiffies, calc_load_update + 10)) { 313 /* 314 * Catch-up, fold however many we are behind still 315 */ 316 delta = jiffies - calc_load_update - 10; 317 n = 1 + (delta / LOAD_FREQ); 318 319 active = atomic_long_read(&calc_load_tasks); 320 active = active > 0 ? active * FIXED_1 : 0; 321 322 avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n); 323 avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n); 324 avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n); 325 326 calc_load_update += n * LOAD_FREQ; 327 } 328 329 /* 330 * Flip the idle index... 331 * 332 * Make sure we first write the new time then flip the index, so that 333 * calc_load_write_idx() will see the new time when it reads the new 334 * index, this avoids a double flip messing things up. 335 */ 336 smp_wmb(); 337 calc_load_idx++; 338} 339#else /* !CONFIG_NO_HZ_COMMON */ 340 341static inline long calc_load_fold_idle(void) { return 0; } 342static inline void calc_global_nohz(void) { } 343 344#endif /* CONFIG_NO_HZ_COMMON */ 345 346/* 347 * calc_load - update the avenrun load estimates 10 ticks after the 348 * CPUs have updated calc_load_tasks. 349 */ 350void calc_global_load(unsigned long ticks) 351{ 352 long active, delta; 353 354 if (time_before(jiffies, calc_load_update + 10)) 355 return; 356 357 /* 358 * Fold the 'old' idle-delta to include all NO_HZ cpus. 359 */ 360 delta = calc_load_fold_idle(); 361 if (delta) 362 atomic_long_add(delta, &calc_load_tasks); 363 364 active = atomic_long_read(&calc_load_tasks); 365 active = active > 0 ? active * FIXED_1 : 0; 366 367 avenrun[0] = calc_load(avenrun[0], EXP_1, active); 368 avenrun[1] = calc_load(avenrun[1], EXP_5, active); 369 avenrun[2] = calc_load(avenrun[2], EXP_15, active); 370 371 calc_load_update += LOAD_FREQ; 372 373 /* 374 * In case we idled for multiple LOAD_FREQ intervals, catch up in bulk. 375 */ 376 calc_global_nohz(); 377} 378 379/* 380 * Called from update_cpu_load() to periodically update this CPU's 381 * active count. 382 */ 383static void calc_load_account_active(struct rq *this_rq) 384{ 385 long delta; 386 387 if (time_before(jiffies, this_rq->calc_load_update)) 388 return; 389 390 delta = calc_load_fold_active(this_rq); 391 if (delta) 392 atomic_long_add(delta, &calc_load_tasks); 393 394 this_rq->calc_load_update += LOAD_FREQ; 395} 396 397/* 398 * End of global load-average stuff 399 */ 400 401/* 402 * The exact cpuload at various idx values, calculated at every tick would be 403 * load = (2^idx - 1) / 2^idx * load + 1 / 2^idx * cur_load 404 * 405 * If a cpu misses updates for n-1 ticks (as it was idle) and update gets called 406 * on nth tick when cpu may be busy, then we have: 407 * load = ((2^idx - 1) / 2^idx)^(n-1) * load 408 * load = (2^idx - 1) / 2^idx) * load + 1 / 2^idx * cur_load 409 * 410 * decay_load_missed() below does efficient calculation of 411 * load = ((2^idx - 1) / 2^idx)^(n-1) * load 412 * avoiding 0..n-1 loop doing load = ((2^idx - 1) / 2^idx) * load 413 * 414 * The calculation is approximated on a 128 point scale. 415 * degrade_zero_ticks is the number of ticks after which load at any 416 * particular idx is approximated to be zero. 417 * degrade_factor is a precomputed table, a row for each load idx. 418 * Each column corresponds to degradation factor for a power of two ticks, 419 * based on 128 point scale. 420 * Example: 421 * row 2, col 3 (=12) says that the degradation at load idx 2 after 422 * 8 ticks is 12/128 (which is an approximation of exact factor 3^8/4^8). 423 * 424 * With this power of 2 load factors, we can degrade the load n times 425 * by looking at 1 bits in n and doing as many mult/shift instead of 426 * n mult/shifts needed by the exact degradation. 427 */ 428#define DEGRADE_SHIFT 7 429static const unsigned char 430 degrade_zero_ticks[CPU_LOAD_IDX_MAX] = {0, 8, 32, 64, 128}; 431static const unsigned char 432 degrade_factor[CPU_LOAD_IDX_MAX][DEGRADE_SHIFT + 1] = { 433 {0, 0, 0, 0, 0, 0, 0, 0}, 434 {64, 32, 8, 0, 0, 0, 0, 0}, 435 {96, 72, 40, 12, 1, 0, 0}, 436 {112, 98, 75, 43, 15, 1, 0}, 437 {120, 112, 98, 76, 45, 16, 2} }; 438 439/* 440 * Update cpu_load for any missed ticks, due to tickless idle. The backlog 441 * would be when CPU is idle and so we just decay the old load without 442 * adding any new load. 443 */ 444static unsigned long 445decay_load_missed(unsigned long load, unsigned long missed_updates, int idx) 446{ 447 int j = 0; 448 449 if (!missed_updates) 450 return load; 451 452 if (missed_updates >= degrade_zero_ticks[idx]) 453 return 0; 454 455 if (idx == 1) 456 return load >> missed_updates; 457 458 while (missed_updates) { 459 if (missed_updates % 2) 460 load = (load * degrade_factor[idx][j]) >> DEGRADE_SHIFT; 461 462 missed_updates >>= 1; 463 j++; 464 } 465 return load; 466} 467 468/* 469 * Update rq->cpu_load[] statistics. This function is usually called every 470 * scheduler tick (TICK_NSEC). With tickless idle this will not be called 471 * every tick. We fix it up based on jiffies. 472 */ 473static void __update_cpu_load(struct rq *this_rq, unsigned long this_load, 474 unsigned long pending_updates) 475{ 476 int i, scale; 477 478 this_rq->nr_load_updates++; 479 480 /* Update our load: */ 481 this_rq->cpu_load[0] = this_load; /* Fasttrack for idx 0 */ 482 for (i = 1, scale = 2; i < CPU_LOAD_IDX_MAX; i++, scale += scale) { 483 unsigned long old_load, new_load; 484 485 /* scale is effectively 1 << i now, and >> i divides by scale */ 486 487 old_load = this_rq->cpu_load[i]; 488 old_load = decay_load_missed(old_load, pending_updates - 1, i); 489 new_load = this_load; 490 /* 491 * Round up the averaging division if load is increasing. This 492 * prevents us from getting stuck on 9 if the load is 10, for 493 * example. 494 */ 495 if (new_load > old_load) 496 new_load += scale - 1; 497 498 this_rq->cpu_load[i] = (old_load * (scale - 1) + new_load) >> i; 499 } 500 501 sched_avg_update(this_rq); 502} 503 504#ifdef CONFIG_SMP 505static inline unsigned long get_rq_runnable_load(struct rq *rq) 506{ 507 return rq->cfs.runnable_load_avg; 508} 509#else 510static inline unsigned long get_rq_runnable_load(struct rq *rq) 511{ 512 return rq->load.weight; 513} 514#endif 515 516#ifdef CONFIG_NO_HZ_COMMON 517/* 518 * There is no sane way to deal with nohz on smp when using jiffies because the 519 * cpu doing the jiffies update might drift wrt the cpu doing the jiffy reading 520 * causing off-by-one errors in observed deltas; {0,2} instead of {1,1}. 521 * 522 * Therefore we cannot use the delta approach from the regular tick since that 523 * would seriously skew the load calculation. However we'll make do for those 524 * updates happening while idle (nohz_idle_balance) or coming out of idle 525 * (tick_nohz_idle_exit). 526 * 527 * This means we might still be one tick off for nohz periods. 528 */ 529 530/* 531 * Called from nohz_idle_balance() to update the load ratings before doing the 532 * idle balance. 533 */ 534void update_idle_cpu_load(struct rq *this_rq) 535{ 536 unsigned long curr_jiffies = ACCESS_ONCE(jiffies); 537 unsigned long load = get_rq_runnable_load(this_rq); 538 unsigned long pending_updates; 539 540 /* 541 * bail if there's load or we're actually up-to-date. 542 */ 543 if (load || curr_jiffies == this_rq->last_load_update_tick) 544 return; 545 546 pending_updates = curr_jiffies - this_rq->last_load_update_tick; 547 this_rq->last_load_update_tick = curr_jiffies; 548 549 __update_cpu_load(this_rq, load, pending_updates); 550} 551 552/* 553 * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed. 554 */ 555void update_cpu_load_nohz(void) 556{ 557 struct rq *this_rq = this_rq(); 558 unsigned long curr_jiffies = ACCESS_ONCE(jiffies); 559 unsigned long pending_updates; 560 561 if (curr_jiffies == this_rq->last_load_update_tick) 562 return; 563 564 raw_spin_lock(&this_rq->lock); 565 pending_updates = curr_jiffies - this_rq->last_load_update_tick; 566 if (pending_updates) { 567 this_rq->last_load_update_tick = curr_jiffies; 568 /* 569 * We were idle, this means load 0, the current load might be 570 * !0 due to remote wakeups and the sort. 571 */ 572 __update_cpu_load(this_rq, 0, pending_updates); 573 } 574 raw_spin_unlock(&this_rq->lock); 575} 576#endif /* CONFIG_NO_HZ */ 577 578/* 579 * Called from scheduler_tick() 580 */ 581void update_cpu_load_active(struct rq *this_rq) 582{ 583 unsigned long load = get_rq_runnable_load(this_rq); 584 /* 585 * See the mess around update_idle_cpu_load() / update_cpu_load_nohz(). 586 */ 587 this_rq->last_load_update_tick = jiffies; 588 __update_cpu_load(this_rq, load, 1); 589 590 calc_load_account_active(this_rq); 591} 592