Line data Source code
1 : // SPDX-License-Identifier: GPL-2.0-only
2 : /*
3 : * sched_clock() for unstable CPU clocks
4 : *
5 : * Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra
6 : *
7 : * Updates and enhancements:
8 : * Copyright (C) 2008 Red Hat, Inc. Steven Rostedt <srostedt@redhat.com>
9 : *
10 : * Based on code by:
11 : * Ingo Molnar <mingo@redhat.com>
12 : * Guillaume Chazarain <guichaz@gmail.com>
13 : *
14 : *
15 : * What this file implements:
16 : *
17 : * cpu_clock(i) provides a fast (execution time) high resolution
18 : * clock with bounded drift between CPUs. The value of cpu_clock(i)
19 : * is monotonic for constant i. The timestamp returned is in nanoseconds.
20 : *
21 : * ######################### BIG FAT WARNING ##########################
22 : * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
23 : * # go backwards !! #
24 : * ####################################################################
25 : *
26 : * There is no strict promise about the base, although it tends to start
27 : * at 0 on boot (but people really shouldn't rely on that).
28 : *
29 : * cpu_clock(i) -- can be used from any context, including NMI.
30 : * local_clock() -- is cpu_clock() on the current CPU.
31 : *
32 : * sched_clock_cpu(i)
33 : *
34 : * How it is implemented:
35 : *
36 : * The implementation either uses sched_clock() when
37 : * !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
38 : * sched_clock() is assumed to provide these properties (mostly it means
39 : * the architecture provides a globally synchronized highres time source).
40 : *
41 : * Otherwise it tries to create a semi stable clock from a mixture of other
42 : * clocks, including:
43 : *
44 : * - GTOD (clock monotonic)
45 : * - sched_clock()
46 : * - explicit idle events
47 : *
48 : * We use GTOD as base and use sched_clock() deltas to improve resolution. The
49 : * deltas are filtered to provide monotonicity and keeping it within an
50 : * expected window.
51 : *
52 : * Furthermore, explicit sleep and wakeup hooks allow us to account for time
53 : * that is otherwise invisible (TSC gets stopped).
54 : *
55 : */
56 :
57 : /*
58 : * Scheduler clock - returns current time in nanosec units.
59 : * This is default implementation.
60 : * Architectures and sub-architectures can override this.
61 : */
62 8792 : notrace unsigned long long __weak sched_clock(void)
63 : {
64 : return (unsigned long long)(jiffies - INITIAL_JIFFIES)
65 8792 : * (NSEC_PER_SEC / HZ);
66 : }
67 : EXPORT_SYMBOL_GPL(sched_clock);
68 :
69 : static DEFINE_STATIC_KEY_FALSE(sched_clock_running);
70 :
71 : #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
72 : /*
73 : * We must start with !__sched_clock_stable because the unstable -> stable
74 : * transition is accurate, while the stable -> unstable transition is not.
75 : *
76 : * Similarly we start with __sched_clock_stable_early, thereby assuming we
77 : * will become stable, such that there's only a single 1 -> 0 transition.
78 : */
79 : static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
80 : static int __sched_clock_stable_early = 1;
81 :
82 : /*
83 : * We want: ktime_get_ns() + __gtod_offset == sched_clock() + __sched_clock_offset
84 : */
85 : __read_mostly u64 __sched_clock_offset;
86 : static __read_mostly u64 __gtod_offset;
87 :
88 : struct sched_clock_data {
89 : u64 tick_raw;
90 : u64 tick_gtod;
91 : u64 clock;
92 : };
93 :
94 : static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
95 :
96 : static __always_inline struct sched_clock_data *this_scd(void)
97 : {
98 : return this_cpu_ptr(&sched_clock_data);
99 : }
100 :
101 : notrace static inline struct sched_clock_data *cpu_sdc(int cpu)
102 : {
103 : return &per_cpu(sched_clock_data, cpu);
104 : }
105 :
106 : notrace int sched_clock_stable(void)
107 : {
108 : return static_branch_likely(&__sched_clock_stable);
109 : }
110 :
111 : notrace static void __scd_stamp(struct sched_clock_data *scd)
112 : {
113 : scd->tick_gtod = ktime_get_ns();
114 : scd->tick_raw = sched_clock();
115 : }
116 :
117 : notrace static void __set_sched_clock_stable(void)
118 : {
119 : struct sched_clock_data *scd;
120 :
121 : /*
122 : * Since we're still unstable and the tick is already running, we have
123 : * to disable IRQs in order to get a consistent scd->tick* reading.
124 : */
125 : local_irq_disable();
126 : scd = this_scd();
127 : /*
128 : * Attempt to make the (initial) unstable->stable transition continuous.
129 : */
130 : __sched_clock_offset = (scd->tick_gtod + __gtod_offset) - (scd->tick_raw);
131 : local_irq_enable();
132 :
133 : printk(KERN_INFO "sched_clock: Marking stable (%lld, %lld)->(%lld, %lld)\n",
134 : scd->tick_gtod, __gtod_offset,
135 : scd->tick_raw, __sched_clock_offset);
136 :
137 : static_branch_enable(&__sched_clock_stable);
138 : tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
139 : }
140 :
141 : /*
142 : * If we ever get here, we're screwed, because we found out -- typically after
143 : * the fact -- that TSC wasn't good. This means all our clocksources (including
144 : * ktime) could have reported wrong values.
145 : *
146 : * What we do here is an attempt to fix up and continue sort of where we left
147 : * off in a coherent manner.
148 : *
149 : * The only way to fully avoid random clock jumps is to boot with:
150 : * "tsc=unstable".
151 : */
152 : notrace static void __sched_clock_work(struct work_struct *work)
153 : {
154 : struct sched_clock_data *scd;
155 : int cpu;
156 :
157 : /* take a current timestamp and set 'now' */
158 : preempt_disable();
159 : scd = this_scd();
160 : __scd_stamp(scd);
161 : scd->clock = scd->tick_gtod + __gtod_offset;
162 : preempt_enable();
163 :
164 : /* clone to all CPUs */
165 : for_each_possible_cpu(cpu)
166 : per_cpu(sched_clock_data, cpu) = *scd;
167 :
168 : printk(KERN_WARNING "TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'.\n");
169 : printk(KERN_INFO "sched_clock: Marking unstable (%lld, %lld)<-(%lld, %lld)\n",
170 : scd->tick_gtod, __gtod_offset,
171 : scd->tick_raw, __sched_clock_offset);
172 :
173 : static_branch_disable(&__sched_clock_stable);
174 : }
175 :
176 : static DECLARE_WORK(sched_clock_work, __sched_clock_work);
177 :
178 : notrace static void __clear_sched_clock_stable(void)
179 : {
180 : if (!sched_clock_stable())
181 : return;
182 :
183 : tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE);
184 : schedule_work(&sched_clock_work);
185 : }
186 :
187 : notrace void clear_sched_clock_stable(void)
188 : {
189 : __sched_clock_stable_early = 0;
190 :
191 : smp_mb(); /* matches sched_clock_init_late() */
192 :
193 : if (static_key_count(&sched_clock_running.key) == 2)
194 : __clear_sched_clock_stable();
195 : }
196 :
197 : notrace static void __sched_clock_gtod_offset(void)
198 : {
199 : struct sched_clock_data *scd = this_scd();
200 :
201 : __scd_stamp(scd);
202 : __gtod_offset = (scd->tick_raw + __sched_clock_offset) - scd->tick_gtod;
203 : }
204 :
205 : void __init sched_clock_init(void)
206 : {
207 : /*
208 : * Set __gtod_offset such that once we mark sched_clock_running,
209 : * sched_clock_tick() continues where sched_clock() left off.
210 : *
211 : * Even if TSC is buggered, we're still UP at this point so it
212 : * can't really be out of sync.
213 : */
214 : local_irq_disable();
215 : __sched_clock_gtod_offset();
216 : local_irq_enable();
217 :
218 : static_branch_inc(&sched_clock_running);
219 : }
220 : /*
221 : * We run this as late_initcall() such that it runs after all built-in drivers,
222 : * notably: acpi_processor and intel_idle, which can mark the TSC as unstable.
223 : */
224 : static int __init sched_clock_init_late(void)
225 : {
226 : static_branch_inc(&sched_clock_running);
227 : /*
228 : * Ensure that it is impossible to not do a static_key update.
229 : *
230 : * Either {set,clear}_sched_clock_stable() must see sched_clock_running
231 : * and do the update, or we must see their __sched_clock_stable_early
232 : * and do the update, or both.
233 : */
234 : smp_mb(); /* matches {set,clear}_sched_clock_stable() */
235 :
236 : if (__sched_clock_stable_early)
237 : __set_sched_clock_stable();
238 :
239 : return 0;
240 : }
241 : late_initcall(sched_clock_init_late);
242 :
243 : /*
244 : * min, max except they take wrapping into account
245 : */
246 :
247 : static __always_inline u64 wrap_min(u64 x, u64 y)
248 : {
249 : return (s64)(x - y) < 0 ? x : y;
250 : }
251 :
252 : static __always_inline u64 wrap_max(u64 x, u64 y)
253 : {
254 : return (s64)(x - y) > 0 ? x : y;
255 : }
256 :
257 : /*
258 : * update the percpu scd from the raw @now value
259 : *
260 : * - filter out backward motion
261 : * - use the GTOD tick value to create a window to filter crazy TSC values
262 : */
263 : static __always_inline u64 sched_clock_local(struct sched_clock_data *scd)
264 : {
265 : u64 now, clock, old_clock, min_clock, max_clock, gtod;
266 : s64 delta;
267 :
268 : again:
269 : now = sched_clock();
270 : delta = now - scd->tick_raw;
271 : if (unlikely(delta < 0))
272 : delta = 0;
273 :
274 : old_clock = scd->clock;
275 :
276 : /*
277 : * scd->clock = clamp(scd->tick_gtod + delta,
278 : * max(scd->tick_gtod, scd->clock),
279 : * scd->tick_gtod + TICK_NSEC);
280 : */
281 :
282 : gtod = scd->tick_gtod + __gtod_offset;
283 : clock = gtod + delta;
284 : min_clock = wrap_max(gtod, old_clock);
285 : max_clock = wrap_max(old_clock, gtod + TICK_NSEC);
286 :
287 : clock = wrap_max(clock, min_clock);
288 : clock = wrap_min(clock, max_clock);
289 :
290 : if (!arch_try_cmpxchg64(&scd->clock, &old_clock, clock))
291 : goto again;
292 :
293 : return clock;
294 : }
295 :
296 : noinstr u64 local_clock(void)
297 : {
298 : u64 clock;
299 :
300 : if (static_branch_likely(&__sched_clock_stable))
301 : return sched_clock() + __sched_clock_offset;
302 :
303 : if (!static_branch_likely(&sched_clock_running))
304 : return sched_clock();
305 :
306 : preempt_disable_notrace();
307 : clock = sched_clock_local(this_scd());
308 : preempt_enable_notrace();
309 :
310 : return clock;
311 : }
312 : EXPORT_SYMBOL_GPL(local_clock);
313 :
314 : static notrace u64 sched_clock_remote(struct sched_clock_data *scd)
315 : {
316 : struct sched_clock_data *my_scd = this_scd();
317 : u64 this_clock, remote_clock;
318 : u64 *ptr, old_val, val;
319 :
320 : #if BITS_PER_LONG != 64
321 : again:
322 : /*
323 : * Careful here: The local and the remote clock values need to
324 : * be read out atomic as we need to compare the values and
325 : * then update either the local or the remote side. So the
326 : * cmpxchg64 below only protects one readout.
327 : *
328 : * We must reread via sched_clock_local() in the retry case on
329 : * 32-bit kernels as an NMI could use sched_clock_local() via the
330 : * tracer and hit between the readout of
331 : * the low 32-bit and the high 32-bit portion.
332 : */
333 : this_clock = sched_clock_local(my_scd);
334 : /*
335 : * We must enforce atomic readout on 32-bit, otherwise the
336 : * update on the remote CPU can hit inbetween the readout of
337 : * the low 32-bit and the high 32-bit portion.
338 : */
339 : remote_clock = cmpxchg64(&scd->clock, 0, 0);
340 : #else
341 : /*
342 : * On 64-bit kernels the read of [my]scd->clock is atomic versus the
343 : * update, so we can avoid the above 32-bit dance.
344 : */
345 : sched_clock_local(my_scd);
346 : again:
347 : this_clock = my_scd->clock;
348 : remote_clock = scd->clock;
349 : #endif
350 :
351 : /*
352 : * Use the opportunity that we have both locks
353 : * taken to couple the two clocks: we take the
354 : * larger time as the latest time for both
355 : * runqueues. (this creates monotonic movement)
356 : */
357 : if (likely((s64)(remote_clock - this_clock) < 0)) {
358 : ptr = &scd->clock;
359 : old_val = remote_clock;
360 : val = this_clock;
361 : } else {
362 : /*
363 : * Should be rare, but possible:
364 : */
365 : ptr = &my_scd->clock;
366 : old_val = this_clock;
367 : val = remote_clock;
368 : }
369 :
370 : if (!try_cmpxchg64(ptr, &old_val, val))
371 : goto again;
372 :
373 : return val;
374 : }
375 :
376 : /*
377 : * Similar to cpu_clock(), but requires local IRQs to be disabled.
378 : *
379 : * See cpu_clock().
380 : */
381 : notrace u64 sched_clock_cpu(int cpu)
382 : {
383 : struct sched_clock_data *scd;
384 : u64 clock;
385 :
386 : if (sched_clock_stable())
387 : return sched_clock() + __sched_clock_offset;
388 :
389 : if (!static_branch_likely(&sched_clock_running))
390 : return sched_clock();
391 :
392 : preempt_disable_notrace();
393 : scd = cpu_sdc(cpu);
394 :
395 : if (cpu != smp_processor_id())
396 : clock = sched_clock_remote(scd);
397 : else
398 : clock = sched_clock_local(scd);
399 : preempt_enable_notrace();
400 :
401 : return clock;
402 : }
403 : EXPORT_SYMBOL_GPL(sched_clock_cpu);
404 :
405 : notrace void sched_clock_tick(void)
406 : {
407 : struct sched_clock_data *scd;
408 :
409 : if (sched_clock_stable())
410 : return;
411 :
412 : if (!static_branch_likely(&sched_clock_running))
413 : return;
414 :
415 : lockdep_assert_irqs_disabled();
416 :
417 : scd = this_scd();
418 : __scd_stamp(scd);
419 : sched_clock_local(scd);
420 : }
421 :
422 : notrace void sched_clock_tick_stable(void)
423 : {
424 : if (!sched_clock_stable())
425 : return;
426 :
427 : /*
428 : * Called under watchdog_lock.
429 : *
430 : * The watchdog just found this TSC to (still) be stable, so now is a
431 : * good moment to update our __gtod_offset. Because once we find the
432 : * TSC to be unstable, any computation will be computing crap.
433 : */
434 : local_irq_disable();
435 : __sched_clock_gtod_offset();
436 : local_irq_enable();
437 : }
438 :
439 : /*
440 : * We are going deep-idle (irqs are disabled):
441 : */
442 : notrace void sched_clock_idle_sleep_event(void)
443 : {
444 : sched_clock_cpu(smp_processor_id());
445 : }
446 : EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
447 :
448 : /*
449 : * We just idled; resync with ktime.
450 : */
451 : notrace void sched_clock_idle_wakeup_event(void)
452 : {
453 : unsigned long flags;
454 :
455 : if (sched_clock_stable())
456 : return;
457 :
458 : if (unlikely(timekeeping_suspended))
459 : return;
460 :
461 : local_irq_save(flags);
462 : sched_clock_tick();
463 : local_irq_restore(flags);
464 : }
465 : EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
466 :
467 : #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
468 :
469 1 : void __init sched_clock_init(void)
470 : {
471 1 : static_branch_inc(&sched_clock_running);
472 : local_irq_disable();
473 : generic_sched_clock_init();
474 : local_irq_enable();
475 1 : }
476 :
477 7826 : notrace u64 sched_clock_cpu(int cpu)
478 : {
479 7826 : if (!static_branch_likely(&sched_clock_running))
480 : return 0;
481 :
482 7826 : return sched_clock();
483 : }
484 :
485 : #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
486 :
487 : /*
488 : * Running clock - returns the time that has elapsed while a guest has been
489 : * running.
490 : * On a guest this value should be local_clock minus the time the guest was
491 : * suspended by the hypervisor (for any reason).
492 : * On bare metal this function should return the same as local_clock.
493 : * Architectures and sub-architectures can override this.
494 : */
495 0 : notrace u64 __weak running_clock(void)
496 : {
497 0 : return local_clock();
498 : }
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