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#include <linux/sched/clock.h>
58#include "sched.h"
59
60/*
61 * Scheduler clock - returns current time in nanosec units.
62 * This is default implementation.
63 * Architectures and sub-architectures can override this.
64 */
65notrace unsigned long long __weak sched_clock(void)
66{
67 return (unsigned long long)(jiffies - INITIAL_JIFFIES)
68 * (NSEC_PER_SEC / HZ);
69}
70EXPORT_SYMBOL_GPL(sched_clock);
71
72static DEFINE_STATIC_KEY_FALSE(sched_clock_running);
73
74#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
75/*
76 * We must start with !__sched_clock_stable because the unstable -> stable
77 * transition is accurate, while the stable -> unstable transition is not.
78 *
79 * Similarly we start with __sched_clock_stable_early, thereby assuming we
80 * will become stable, such that there's only a single 1 -> 0 transition.
81 */
82static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
83static int __sched_clock_stable_early = 1;
84
85/*
86 * We want: ktime_get_ns() + __gtod_offset == sched_clock() + __sched_clock_offset
87 */
88__read_mostly u64 __sched_clock_offset;
89static __read_mostly u64 __gtod_offset;
90
91struct sched_clock_data {
92 u64 tick_raw;
93 u64 tick_gtod;
94 u64 clock;
95};
96
97static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
98
99static __always_inline struct sched_clock_data *this_scd(void)
100{
101 return this_cpu_ptr(&sched_clock_data);
102}
103
104notrace static inline struct sched_clock_data *cpu_sdc(int cpu)
105{
106 return &per_cpu(sched_clock_data, cpu);
107}
108
109notrace int sched_clock_stable(void)
110{
111 return static_branch_likely(&__sched_clock_stable);
112}
113
114notrace static void __scd_stamp(struct sched_clock_data *scd)
115{
116 scd->tick_gtod = ktime_get_ns();
117 scd->tick_raw = sched_clock();
118}
119
120notrace static void __set_sched_clock_stable(void)
121{
122 struct sched_clock_data *scd;
123
124 /*
125 * Since we're still unstable and the tick is already running, we have
126 * to disable IRQs in order to get a consistent scd->tick* reading.
127 */
128 local_irq_disable();
129 scd = this_scd();
130 /*
131 * Attempt to make the (initial) unstable->stable transition continuous.
132 */
133 __sched_clock_offset = (scd->tick_gtod + __gtod_offset) - (scd->tick_raw);
134 local_irq_enable();
135
136 printk(KERN_INFO "sched_clock: Marking stable (%lld, %lld)->(%lld, %lld)\n",
137 scd->tick_gtod, __gtod_offset,
138 scd->tick_raw, __sched_clock_offset);
139
140 static_branch_enable(&__sched_clock_stable);
141 tick_dep_clear(bit: TICK_DEP_BIT_CLOCK_UNSTABLE);
142}
143
144/*
145 * If we ever get here, we're screwed, because we found out -- typically after
146 * the fact -- that TSC wasn't good. This means all our clocksources (including
147 * ktime) could have reported wrong values.
148 *
149 * What we do here is an attempt to fix up and continue sort of where we left
150 * off in a coherent manner.
151 *
152 * The only way to fully avoid random clock jumps is to boot with:
153 * "tsc=unstable".
154 */
155notrace static void __sched_clock_work(struct work_struct *work)
156{
157 struct sched_clock_data *scd;
158 int cpu;
159
160 /* take a current timestamp and set 'now' */
161 preempt_disable();
162 scd = this_scd();
163 __scd_stamp(scd);
164 scd->clock = scd->tick_gtod + __gtod_offset;
165 preempt_enable();
166
167 /* clone to all CPUs */
168 for_each_possible_cpu(cpu)
169 per_cpu(sched_clock_data, cpu) = *scd;
170
171 printk(KERN_WARNING "TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'.\n");
172 printk(KERN_INFO "sched_clock: Marking unstable (%lld, %lld)<-(%lld, %lld)\n",
173 scd->tick_gtod, __gtod_offset,
174 scd->tick_raw, __sched_clock_offset);
175
176 static_branch_disable(&__sched_clock_stable);
177}
178
179static DECLARE_WORK(sched_clock_work, __sched_clock_work);
180
181notrace static void __clear_sched_clock_stable(void)
182{
183 if (!sched_clock_stable())
184 return;
185
186 tick_dep_set(bit: TICK_DEP_BIT_CLOCK_UNSTABLE);
187 schedule_work(work: &sched_clock_work);
188}
189
190notrace void clear_sched_clock_stable(void)
191{
192 __sched_clock_stable_early = 0;
193
194 smp_mb(); /* matches sched_clock_init_late() */
195
196 if (static_key_count(key: &sched_clock_running.key) == 2)
197 __clear_sched_clock_stable();
198}
199
200notrace static void __sched_clock_gtod_offset(void)
201{
202 struct sched_clock_data *scd = this_scd();
203
204 __scd_stamp(scd);
205 __gtod_offset = (scd->tick_raw + __sched_clock_offset) - scd->tick_gtod;
206}
207
208void __init sched_clock_init(void)
209{
210 /*
211 * Set __gtod_offset such that once we mark sched_clock_running,
212 * sched_clock_tick() continues where sched_clock() left off.
213 *
214 * Even if TSC is buggered, we're still UP at this point so it
215 * can't really be out of sync.
216 */
217 local_irq_disable();
218 __sched_clock_gtod_offset();
219 local_irq_enable();
220
221 static_branch_inc(&sched_clock_running);
222}
223/*
224 * We run this as late_initcall() such that it runs after all built-in drivers,
225 * notably: acpi_processor and intel_idle, which can mark the TSC as unstable.
226 */
227static int __init sched_clock_init_late(void)
228{
229 static_branch_inc(&sched_clock_running);
230 /*
231 * Ensure that it is impossible to not do a static_key update.
232 *
233 * Either {set,clear}_sched_clock_stable() must see sched_clock_running
234 * and do the update, or we must see their __sched_clock_stable_early
235 * and do the update, or both.
236 */
237 smp_mb(); /* matches {set,clear}_sched_clock_stable() */
238
239 if (__sched_clock_stable_early)
240 __set_sched_clock_stable();
241
242 return 0;
243}
244late_initcall(sched_clock_init_late);
245
246/*
247 * min, max except they take wrapping into account
248 */
249
250static __always_inline u64 wrap_min(u64 x, u64 y)
251{
252 return (s64)(x - y) < 0 ? x : y;
253}
254
255static __always_inline u64 wrap_max(u64 x, u64 y)
256{
257 return (s64)(x - y) > 0 ? x : y;
258}
259
260/*
261 * update the percpu scd from the raw @now value
262 *
263 * - filter out backward motion
264 * - use the GTOD tick value to create a window to filter crazy TSC values
265 */
266static __always_inline u64 sched_clock_local(struct sched_clock_data *scd)
267{
268 u64 now, clock, old_clock, min_clock, max_clock, gtod;
269 s64 delta;
270
271again:
272 now = sched_clock_noinstr();
273 delta = now - scd->tick_raw;
274 if (unlikely(delta < 0))
275 delta = 0;
276
277 old_clock = scd->clock;
278
279 /*
280 * scd->clock = clamp(scd->tick_gtod + delta,
281 * max(scd->tick_gtod, scd->clock),
282 * scd->tick_gtod + TICK_NSEC);
283 */
284
285 gtod = scd->tick_gtod + __gtod_offset;
286 clock = gtod + delta;
287 min_clock = wrap_max(x: gtod, y: old_clock);
288 max_clock = wrap_max(x: old_clock, y: gtod + TICK_NSEC);
289
290 clock = wrap_max(x: clock, y: min_clock);
291 clock = wrap_min(x: clock, y: max_clock);
292
293 if (!raw_try_cmpxchg64(&scd->clock, &old_clock, clock))
294 goto again;
295
296 return clock;
297}
298
299noinstr u64 local_clock_noinstr(void)
300{
301 u64 clock;
302
303 if (static_branch_likely(&__sched_clock_stable))
304 return sched_clock_noinstr() + __sched_clock_offset;
305
306 if (!static_branch_likely(&sched_clock_running))
307 return sched_clock_noinstr();
308
309 clock = sched_clock_local(scd: this_scd());
310
311 return clock;
312}
313
314u64 local_clock(void)
315{
316 u64 now;
317 preempt_disable_notrace();
318 now = local_clock_noinstr();
319 preempt_enable_notrace();
320 return now;
321}
322EXPORT_SYMBOL_GPL(local_clock);
323
324static notrace u64 sched_clock_remote(struct sched_clock_data *scd)
325{
326 struct sched_clock_data *my_scd = this_scd();
327 u64 this_clock, remote_clock;
328 u64 *ptr, old_val, val;
329
330#if BITS_PER_LONG != 64
331again:
332 /*
333 * Careful here: The local and the remote clock values need to
334 * be read out atomic as we need to compare the values and
335 * then update either the local or the remote side. So the
336 * cmpxchg64 below only protects one readout.
337 *
338 * We must reread via sched_clock_local() in the retry case on
339 * 32-bit kernels as an NMI could use sched_clock_local() via the
340 * tracer and hit between the readout of
341 * the low 32-bit and the high 32-bit portion.
342 */
343 this_clock = sched_clock_local(my_scd);
344 /*
345 * We must enforce atomic readout on 32-bit, otherwise the
346 * update on the remote CPU can hit in between the readout of
347 * the low 32-bit and the high 32-bit portion.
348 */
349 remote_clock = cmpxchg64(&scd->clock, 0, 0);
350#else
351 /*
352 * On 64-bit kernels the read of [my]scd->clock is atomic versus the
353 * update, so we can avoid the above 32-bit dance.
354 */
355 sched_clock_local(scd: my_scd);
356again:
357 this_clock = my_scd->clock;
358 remote_clock = scd->clock;
359#endif
360
361 /*
362 * Use the opportunity that we have both locks
363 * taken to couple the two clocks: we take the
364 * larger time as the latest time for both
365 * runqueues. (this creates monotonic movement)
366 */
367 if (likely((s64)(remote_clock - this_clock) < 0)) {
368 ptr = &scd->clock;
369 old_val = remote_clock;
370 val = this_clock;
371 } else {
372 /*
373 * Should be rare, but possible:
374 */
375 ptr = &my_scd->clock;
376 old_val = this_clock;
377 val = remote_clock;
378 }
379
380 if (!try_cmpxchg64(ptr, &old_val, val))
381 goto again;
382
383 return val;
384}
385
386/*
387 * Similar to cpu_clock(), but requires local IRQs to be disabled.
388 *
389 * See cpu_clock().
390 */
391notrace u64 sched_clock_cpu(int cpu)
392{
393 struct sched_clock_data *scd;
394 u64 clock;
395
396 if (sched_clock_stable())
397 return sched_clock() + __sched_clock_offset;
398
399 if (!static_branch_likely(&sched_clock_running))
400 return sched_clock();
401
402 preempt_disable_notrace();
403 scd = cpu_sdc(cpu);
404
405 if (cpu != smp_processor_id())
406 clock = sched_clock_remote(scd);
407 else
408 clock = sched_clock_local(scd);
409 preempt_enable_notrace();
410
411 return clock;
412}
413EXPORT_SYMBOL_GPL(sched_clock_cpu);
414
415notrace void sched_clock_tick(void)
416{
417 struct sched_clock_data *scd;
418
419 if (sched_clock_stable())
420 return;
421
422 if (!static_branch_likely(&sched_clock_running))
423 return;
424
425 lockdep_assert_irqs_disabled();
426
427 scd = this_scd();
428 __scd_stamp(scd);
429 sched_clock_local(scd);
430}
431
432notrace void sched_clock_tick_stable(void)
433{
434 if (!sched_clock_stable())
435 return;
436
437 /*
438 * Called under watchdog_lock.
439 *
440 * The watchdog just found this TSC to (still) be stable, so now is a
441 * good moment to update our __gtod_offset. Because once we find the
442 * TSC to be unstable, any computation will be computing crap.
443 */
444 local_irq_disable();
445 __sched_clock_gtod_offset();
446 local_irq_enable();
447}
448
449/*
450 * We are going deep-idle (IRQs are disabled):
451 */
452notrace void sched_clock_idle_sleep_event(void)
453{
454 sched_clock_cpu(smp_processor_id());
455}
456EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
457
458/*
459 * We just idled; resync with ktime.
460 */
461notrace void sched_clock_idle_wakeup_event(void)
462{
463 unsigned long flags;
464
465 if (sched_clock_stable())
466 return;
467
468 if (unlikely(timekeeping_suspended))
469 return;
470
471 local_irq_save(flags);
472 sched_clock_tick();
473 local_irq_restore(flags);
474}
475EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
476
477#else /* !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK: */
478
479void __init sched_clock_init(void)
480{
481 static_branch_inc(&sched_clock_running);
482 local_irq_disable();
483 generic_sched_clock_init();
484 local_irq_enable();
485}
486
487notrace u64 sched_clock_cpu(int cpu)
488{
489 if (!static_branch_likely(&sched_clock_running))
490 return 0;
491
492 return sched_clock();
493}
494
495#endif /* !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
496
497/*
498 * Running clock - returns the time that has elapsed while a guest has been
499 * running.
500 * On a guest this value should be local_clock minus the time the guest was
501 * suspended by the hypervisor (for any reason).
502 * On bare metal this function should return the same as local_clock.
503 * Architectures and sub-architectures can override this.
504 */
505notrace u64 __weak running_clock(void)
506{
507 return local_clock();
508}
509