| 1 | /* |
| 2 | * sched_clock() for unstable CPU clocks |
| 3 | * |
| 4 | * Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra |
| 5 | * |
| 6 | * Updates and enhancements: |
| 7 | * Copyright (C) 2008 Red Hat, Inc. Steven Rostedt <srostedt@redhat.com> |
| 8 | * |
| 9 | * Based on code by: |
| 10 | * Ingo Molnar <mingo@redhat.com> |
| 11 | * Guillaume Chazarain <guichaz@gmail.com> |
| 12 | * |
| 13 | * |
| 14 | * What this file implements: |
| 15 | * |
| 16 | * cpu_clock(i) provides a fast (execution time) high resolution |
| 17 | * clock with bounded drift between CPUs. The value of cpu_clock(i) |
| 18 | * is monotonic for constant i. The timestamp returned is in nanoseconds. |
| 19 | * |
| 20 | * ######################### BIG FAT WARNING ########################## |
| 21 | * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can # |
| 22 | * # go backwards !! # |
| 23 | * #################################################################### |
| 24 | * |
| 25 | * There is no strict promise about the base, although it tends to start |
| 26 | * at 0 on boot (but people really shouldn't rely on that). |
| 27 | * |
| 28 | * cpu_clock(i) -- can be used from any context, including NMI. |
| 29 | * local_clock() -- is cpu_clock() on the current CPU. |
| 30 | * |
| 31 | * sched_clock_cpu(i) |
| 32 | * |
| 33 | * How it is implemented: |
| 34 | * |
| 35 | * The implementation either uses sched_clock() when |
| 36 | * !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the |
| 37 | * sched_clock() is assumed to provide these properties (mostly it means |
| 38 | * the architecture provides a globally synchronized highres time source). |
| 39 | * |
| 40 | * Otherwise it tries to create a semi stable clock from a mixture of other |
| 41 | * clocks, including: |
| 42 | * |
| 43 | * - GTOD (clock monotomic) |
| 44 | * - sched_clock() |
| 45 | * - explicit idle events |
| 46 | * |
| 47 | * We use GTOD as base and use sched_clock() deltas to improve resolution. The |
| 48 | * deltas are filtered to provide monotonicity and keeping it within an |
| 49 | * expected window. |
| 50 | * |
| 51 | * Furthermore, explicit sleep and wakeup hooks allow us to account for time |
| 52 | * that is otherwise invisible (TSC gets stopped). |
| 53 | * |
| 54 | */ |
| 55 | #include "sched.h" |
| 56 | #include <linux/sched_clock.h> |
| 57 | |
| 58 | /* |
| 59 | * Scheduler clock - returns current time in nanosec units. |
| 60 | * This is default implementation. |
| 61 | * Architectures and sub-architectures can override this. |
| 62 | */ |
| 63 | unsigned long long __weak sched_clock(void) |
| 64 | { |
| 65 | return (unsigned long long)(jiffies - INITIAL_JIFFIES) |
| 66 | * (NSEC_PER_SEC / HZ); |
| 67 | } |
| 68 | EXPORT_SYMBOL_GPL(sched_clock); |
| 69 | |
| 70 | static DEFINE_STATIC_KEY_FALSE(sched_clock_running); |
| 71 | |
| 72 | #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK |
| 73 | /* |
| 74 | * We must start with !__sched_clock_stable because the unstable -> stable |
| 75 | * transition is accurate, while the stable -> unstable transition is not. |
| 76 | * |
| 77 | * Similarly we start with __sched_clock_stable_early, thereby assuming we |
| 78 | * will become stable, such that there's only a single 1 -> 0 transition. |
| 79 | */ |
| 80 | static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable); |
| 81 | static int __sched_clock_stable_early = 1; |
| 82 | |
| 83 | /* |
| 84 | * We want: ktime_get_ns() + __gtod_offset == sched_clock() + __sched_clock_offset |
| 85 | */ |
| 86 | __read_mostly u64 __sched_clock_offset; |
| 87 | static __read_mostly u64 __gtod_offset; |
| 88 | |
| 89 | struct sched_clock_data { |
| 90 | u64 tick_raw; |
| 91 | u64 tick_gtod; |
| 92 | u64 clock; |
| 93 | }; |
| 94 | |
| 95 | static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data); |
| 96 | |
| 97 | static inline struct sched_clock_data *this_scd(void) |
| 98 | { |
| 99 | return this_cpu_ptr(&sched_clock_data); |
| 100 | } |
| 101 | |
| 102 | static inline struct sched_clock_data *cpu_sdc(int cpu) |
| 103 | { |
| 104 | return &per_cpu(sched_clock_data, cpu); |
| 105 | } |
| 106 | |
| 107 | int sched_clock_stable(void) |
| 108 | { |
| 109 | return static_branch_likely(&__sched_clock_stable); |
| 110 | } |
| 111 | |
| 112 | static void __scd_stamp(struct sched_clock_data *scd) |
| 113 | { |
| 114 | scd->tick_gtod = ktime_get_ns(); |
| 115 | scd->tick_raw = sched_clock(); |
| 116 | } |
| 117 | |
| 118 | static void __set_sched_clock_stable(void) |
| 119 | { |
| 120 | struct sched_clock_data *scd; |
| 121 | |
| 122 | /* |
| 123 | * Since we're still unstable and the tick is already running, we have |
| 124 | * to disable IRQs in order to get a consistent scd->tick* reading. |
| 125 | */ |
| 126 | local_irq_disable(); |
| 127 | scd = this_scd(); |
| 128 | /* |
| 129 | * Attempt to make the (initial) unstable->stable transition continuous. |
| 130 | */ |
| 131 | __sched_clock_offset = (scd->tick_gtod + __gtod_offset) - (scd->tick_raw); |
| 132 | local_irq_enable(); |
| 133 | |
| 134 | printk(KERN_INFO "sched_clock: Marking stable (%lld, %lld)->(%lld, %lld)\n", |
| 135 | scd->tick_gtod, __gtod_offset, |
| 136 | scd->tick_raw, __sched_clock_offset); |
| 137 | |
| 138 | static_branch_enable(&__sched_clock_stable); |
| 139 | tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE); |
| 140 | } |
| 141 | |
| 142 | /* |
| 143 | * If we ever get here, we're screwed, because we found out -- typically after |
| 144 | * the fact -- that TSC wasn't good. This means all our clocksources (including |
| 145 | * ktime) could have reported wrong values. |
| 146 | * |
| 147 | * What we do here is an attempt to fix up and continue sort of where we left |
| 148 | * off in a coherent manner. |
| 149 | * |
| 150 | * The only way to fully avoid random clock jumps is to boot with: |
| 151 | * "tsc=unstable". |
| 152 | */ |
| 153 | static void __sched_clock_work(struct work_struct *work) |
| 154 | { |
| 155 | struct sched_clock_data *scd; |
| 156 | int cpu; |
| 157 | |
| 158 | /* take a current timestamp and set 'now' */ |
| 159 | preempt_disable(); |
| 160 | scd = this_scd(); |
| 161 | __scd_stamp(scd); |
| 162 | scd->clock = scd->tick_gtod + __gtod_offset; |
| 163 | preempt_enable(); |
| 164 | |
| 165 | /* clone to all CPUs */ |
| 166 | for_each_possible_cpu(cpu) |
| 167 | per_cpu(sched_clock_data, cpu) = *scd; |
| 168 | |
| 169 | printk(KERN_WARNING "TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'.\n"); |
| 170 | printk(KERN_INFO "sched_clock: Marking unstable (%lld, %lld)<-(%lld, %lld)\n", |
| 171 | scd->tick_gtod, __gtod_offset, |
| 172 | scd->tick_raw, __sched_clock_offset); |
| 173 | |
| 174 | static_branch_disable(&__sched_clock_stable); |
| 175 | } |
| 176 | |
| 177 | static DECLARE_WORK(sched_clock_work, __sched_clock_work); |
| 178 | |
| 179 | static void __clear_sched_clock_stable(void) |
| 180 | { |
| 181 | if (!sched_clock_stable()) |
| 182 | return; |
| 183 | |
| 184 | tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE); |
| 185 | schedule_work(&sched_clock_work); |
| 186 | } |
| 187 | |
| 188 | void clear_sched_clock_stable(void) |
| 189 | { |
| 190 | __sched_clock_stable_early = 0; |
| 191 | |
| 192 | smp_mb(); /* matches sched_clock_init_late() */ |
| 193 | |
| 194 | if (static_key_count(&sched_clock_running.key) == 2) |
| 195 | __clear_sched_clock_stable(); |
| 196 | } |
| 197 | |
| 198 | static void __sched_clock_gtod_offset(void) |
| 199 | { |
| 200 | struct sched_clock_data *scd = this_scd(); |
| 201 | |
| 202 | __scd_stamp(scd); |
| 203 | __gtod_offset = (scd->tick_raw + __sched_clock_offset) - scd->tick_gtod; |
| 204 | } |
| 205 | |
| 206 | void __init sched_clock_init(void) |
| 207 | { |
| 208 | /* |
| 209 | * Set __gtod_offset such that once we mark sched_clock_running, |
| 210 | * sched_clock_tick() continues where sched_clock() left off. |
| 211 | * |
| 212 | * Even if TSC is buggered, we're still UP at this point so it |
| 213 | * can't really be out of sync. |
| 214 | */ |
| 215 | local_irq_disable(); |
| 216 | __sched_clock_gtod_offset(); |
| 217 | local_irq_enable(); |
| 218 | |
| 219 | static_branch_inc(&sched_clock_running); |
| 220 | } |
| 221 | /* |
| 222 | * We run this as late_initcall() such that it runs after all built-in drivers, |
| 223 | * notably: acpi_processor and intel_idle, which can mark the TSC as unstable. |
| 224 | */ |
| 225 | static int __init sched_clock_init_late(void) |
| 226 | { |
| 227 | static_branch_inc(&sched_clock_running); |
| 228 | /* |
| 229 | * Ensure that it is impossible to not do a static_key update. |
| 230 | * |
| 231 | * Either {set,clear}_sched_clock_stable() must see sched_clock_running |
| 232 | * and do the update, or we must see their __sched_clock_stable_early |
| 233 | * and do the update, or both. |
| 234 | */ |
| 235 | smp_mb(); /* matches {set,clear}_sched_clock_stable() */ |
| 236 | |
| 237 | if (__sched_clock_stable_early) |
| 238 | __set_sched_clock_stable(); |
| 239 | |
| 240 | return 0; |
| 241 | } |
| 242 | late_initcall(sched_clock_init_late); |
| 243 | |
| 244 | /* |
| 245 | * min, max except they take wrapping into account |
| 246 | */ |
| 247 | |
| 248 | static inline u64 wrap_min(u64 x, u64 y) |
| 249 | { |
| 250 | return (s64)(x - y) < 0 ? x : y; |
| 251 | } |
| 252 | |
| 253 | static inline u64 wrap_max(u64 x, u64 y) |
| 254 | { |
| 255 | return (s64)(x - y) > 0 ? x : y; |
| 256 | } |
| 257 | |
| 258 | /* |
| 259 | * update the percpu scd from the raw @now value |
| 260 | * |
| 261 | * - filter out backward motion |
| 262 | * - use the GTOD tick value to create a window to filter crazy TSC values |
| 263 | */ |
| 264 | static u64 sched_clock_local(struct sched_clock_data *scd) |
| 265 | { |
| 266 | u64 now, clock, old_clock, min_clock, max_clock, gtod; |
| 267 | s64 delta; |
| 268 | |
| 269 | again: |
| 270 | now = sched_clock(); |
| 271 | delta = now - scd->tick_raw; |
| 272 | if (unlikely(delta < 0)) |
| 273 | delta = 0; |
| 274 | |
| 275 | old_clock = scd->clock; |
| 276 | |
| 277 | /* |
| 278 | * scd->clock = clamp(scd->tick_gtod + delta, |
| 279 | * max(scd->tick_gtod, scd->clock), |
| 280 | * scd->tick_gtod + TICK_NSEC); |
| 281 | */ |
| 282 | |
| 283 | gtod = scd->tick_gtod + __gtod_offset; |
| 284 | clock = gtod + delta; |
| 285 | min_clock = wrap_max(gtod, old_clock); |
| 286 | max_clock = wrap_max(old_clock, gtod + TICK_NSEC); |
| 287 | |
| 288 | clock = wrap_max(clock, min_clock); |
| 289 | clock = wrap_min(clock, max_clock); |
| 290 | |
| 291 | if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock) |
| 292 | goto again; |
| 293 | |
| 294 | return clock; |
| 295 | } |
| 296 | |
| 297 | static u64 sched_clock_remote(struct sched_clock_data *scd) |
| 298 | { |
| 299 | struct sched_clock_data *my_scd = this_scd(); |
| 300 | u64 this_clock, remote_clock; |
| 301 | u64 *ptr, old_val, val; |
| 302 | |
| 303 | #if BITS_PER_LONG != 64 |
| 304 | again: |
| 305 | /* |
| 306 | * Careful here: The local and the remote clock values need to |
| 307 | * be read out atomic as we need to compare the values and |
| 308 | * then update either the local or the remote side. So the |
| 309 | * cmpxchg64 below only protects one readout. |
| 310 | * |
| 311 | * We must reread via sched_clock_local() in the retry case on |
| 312 | * 32-bit kernels as an NMI could use sched_clock_local() via the |
| 313 | * tracer and hit between the readout of |
| 314 | * the low 32-bit and the high 32-bit portion. |
| 315 | */ |
| 316 | this_clock = sched_clock_local(my_scd); |
| 317 | /* |
| 318 | * We must enforce atomic readout on 32-bit, otherwise the |
| 319 | * update on the remote CPU can hit inbetween the readout of |
| 320 | * the low 32-bit and the high 32-bit portion. |
| 321 | */ |
| 322 | remote_clock = cmpxchg64(&scd->clock, 0, 0); |
| 323 | #else |
| 324 | /* |
| 325 | * On 64-bit kernels the read of [my]scd->clock is atomic versus the |
| 326 | * update, so we can avoid the above 32-bit dance. |
| 327 | */ |
| 328 | sched_clock_local(my_scd); |
| 329 | again: |
| 330 | this_clock = my_scd->clock; |
| 331 | remote_clock = scd->clock; |
| 332 | #endif |
| 333 | |
| 334 | /* |
| 335 | * Use the opportunity that we have both locks |
| 336 | * taken to couple the two clocks: we take the |
| 337 | * larger time as the latest time for both |
| 338 | * runqueues. (this creates monotonic movement) |
| 339 | */ |
| 340 | if (likely((s64)(remote_clock - this_clock) < 0)) { |
| 341 | ptr = &scd->clock; |
| 342 | old_val = remote_clock; |
| 343 | val = this_clock; |
| 344 | } else { |
| 345 | /* |
| 346 | * Should be rare, but possible: |
| 347 | */ |
| 348 | ptr = &my_scd->clock; |
| 349 | old_val = this_clock; |
| 350 | val = remote_clock; |
| 351 | } |
| 352 | |
| 353 | if (cmpxchg64(ptr, old_val, val) != old_val) |
| 354 | goto again; |
| 355 | |
| 356 | return val; |
| 357 | } |
| 358 | |
| 359 | /* |
| 360 | * Similar to cpu_clock(), but requires local IRQs to be disabled. |
| 361 | * |
| 362 | * See cpu_clock(). |
| 363 | */ |
| 364 | u64 sched_clock_cpu(int cpu) |
| 365 | { |
| 366 | struct sched_clock_data *scd; |
| 367 | u64 clock; |
| 368 | |
| 369 | if (sched_clock_stable()) |
| 370 | return sched_clock() + __sched_clock_offset; |
| 371 | |
| 372 | if (!static_branch_unlikely(&sched_clock_running)) |
| 373 | return sched_clock(); |
| 374 | |
| 375 | preempt_disable_notrace(); |
| 376 | scd = cpu_sdc(cpu); |
| 377 | |
| 378 | if (cpu != smp_processor_id()) |
| 379 | clock = sched_clock_remote(scd); |
| 380 | else |
| 381 | clock = sched_clock_local(scd); |
| 382 | preempt_enable_notrace(); |
| 383 | |
| 384 | return clock; |
| 385 | } |
| 386 | EXPORT_SYMBOL_GPL(sched_clock_cpu); |
| 387 | |
| 388 | void sched_clock_tick(void) |
| 389 | { |
| 390 | struct sched_clock_data *scd; |
| 391 | |
| 392 | if (sched_clock_stable()) |
| 393 | return; |
| 394 | |
| 395 | if (!static_branch_unlikely(&sched_clock_running)) |
| 396 | return; |
| 397 | |
| 398 | lockdep_assert_irqs_disabled(); |
| 399 | |
| 400 | scd = this_scd(); |
| 401 | __scd_stamp(scd); |
| 402 | sched_clock_local(scd); |
| 403 | } |
| 404 | |
| 405 | void sched_clock_tick_stable(void) |
| 406 | { |
| 407 | if (!sched_clock_stable()) |
| 408 | return; |
| 409 | |
| 410 | /* |
| 411 | * Called under watchdog_lock. |
| 412 | * |
| 413 | * The watchdog just found this TSC to (still) be stable, so now is a |
| 414 | * good moment to update our __gtod_offset. Because once we find the |
| 415 | * TSC to be unstable, any computation will be computing crap. |
| 416 | */ |
| 417 | local_irq_disable(); |
| 418 | __sched_clock_gtod_offset(); |
| 419 | local_irq_enable(); |
| 420 | } |
| 421 | |
| 422 | /* |
| 423 | * We are going deep-idle (irqs are disabled): |
| 424 | */ |
| 425 | void sched_clock_idle_sleep_event(void) |
| 426 | { |
| 427 | sched_clock_cpu(smp_processor_id()); |
| 428 | } |
| 429 | EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event); |
| 430 | |
| 431 | /* |
| 432 | * We just idled; resync with ktime. |
| 433 | */ |
| 434 | void sched_clock_idle_wakeup_event(void) |
| 435 | { |
| 436 | unsigned long flags; |
| 437 | |
| 438 | if (sched_clock_stable()) |
| 439 | return; |
| 440 | |
| 441 | if (unlikely(timekeeping_suspended)) |
| 442 | return; |
| 443 | |
| 444 | local_irq_save(flags); |
| 445 | sched_clock_tick(); |
| 446 | local_irq_restore(flags); |
| 447 | } |
| 448 | EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event); |
| 449 | |
| 450 | #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */ |
| 451 | |
| 452 | void __init sched_clock_init(void) |
| 453 | { |
| 454 | static_branch_inc(&sched_clock_running); |
| 455 | generic_sched_clock_init(); |
| 456 | } |
| 457 | |
| 458 | u64 sched_clock_cpu(int cpu) |
| 459 | { |
| 460 | if (!static_branch_unlikely(&sched_clock_running)) |
| 461 | return 0; |
| 462 | |
| 463 | return sched_clock(); |
| 464 | } |
| 465 | |
| 466 | #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */ |
| 467 | |
| 468 | /* |
| 469 | * Running clock - returns the time that has elapsed while a guest has been |
| 470 | * running. |
| 471 | * On a guest this value should be local_clock minus the time the guest was |
| 472 | * suspended by the hypervisor (for any reason). |
| 473 | * On bare metal this function should return the same as local_clock. |
| 474 | * Architectures and sub-architectures can override this. |
| 475 | */ |
| 476 | u64 __weak running_clock(void) |
| 477 | { |
| 478 | return local_clock(); |
| 479 | } |