xref: /openbmc/linux/kernel/sched/clock.c (revision fb7d4948)
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  */
sched_clock(void)62 notrace unsigned long long __weak sched_clock(void)
63 {
64 	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
65 					* (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 
this_scd(void)96 static __always_inline struct sched_clock_data *this_scd(void)
97 {
98 	return this_cpu_ptr(&sched_clock_data);
99 }
100 
cpu_sdc(int cpu)101 notrace static inline struct sched_clock_data *cpu_sdc(int cpu)
102 {
103 	return &per_cpu(sched_clock_data, cpu);
104 }
105 
sched_clock_stable(void)106 notrace int sched_clock_stable(void)
107 {
108 	return static_branch_likely(&__sched_clock_stable);
109 }
110 
__scd_stamp(struct sched_clock_data * scd)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 
__set_sched_clock_stable(void)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  */
__sched_clock_work(struct work_struct * work)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 
__clear_sched_clock_stable(void)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 
clear_sched_clock_stable(void)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 
__sched_clock_gtod_offset(void)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 
sched_clock_init(void)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  */
sched_clock_init_late(void)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 
wrap_min(u64 x,u64 y)247 static __always_inline u64 wrap_min(u64 x, u64 y)
248 {
249 	return (s64)(x - y) < 0 ? x : y;
250 }
251 
wrap_max(u64 x,u64 y)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  */
sched_clock_local(struct sched_clock_data * scd)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_noinstr();
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 (!raw_try_cmpxchg64(&scd->clock, &old_clock, clock))
291 		goto again;
292 
293 	return clock;
294 }
295 
local_clock_noinstr(void)296 noinstr u64 local_clock_noinstr(void)
297 {
298 	u64 clock;
299 
300 	if (static_branch_likely(&__sched_clock_stable))
301 		return sched_clock_noinstr() + __sched_clock_offset;
302 
303 	if (!static_branch_likely(&sched_clock_running))
304 		return sched_clock_noinstr();
305 
306 	clock = sched_clock_local(this_scd());
307 
308 	return clock;
309 }
310 
local_clock(void)311 u64 local_clock(void)
312 {
313 	u64 now;
314 	preempt_disable_notrace();
315 	now = local_clock_noinstr();
316 	preempt_enable_notrace();
317 	return now;
318 }
319 EXPORT_SYMBOL_GPL(local_clock);
320 
sched_clock_remote(struct sched_clock_data * scd)321 static notrace u64 sched_clock_remote(struct sched_clock_data *scd)
322 {
323 	struct sched_clock_data *my_scd = this_scd();
324 	u64 this_clock, remote_clock;
325 	u64 *ptr, old_val, val;
326 
327 #if BITS_PER_LONG != 64
328 again:
329 	/*
330 	 * Careful here: The local and the remote clock values need to
331 	 * be read out atomic as we need to compare the values and
332 	 * then update either the local or the remote side. So the
333 	 * cmpxchg64 below only protects one readout.
334 	 *
335 	 * We must reread via sched_clock_local() in the retry case on
336 	 * 32-bit kernels as an NMI could use sched_clock_local() via the
337 	 * tracer and hit between the readout of
338 	 * the low 32-bit and the high 32-bit portion.
339 	 */
340 	this_clock = sched_clock_local(my_scd);
341 	/*
342 	 * We must enforce atomic readout on 32-bit, otherwise the
343 	 * update on the remote CPU can hit inbetween the readout of
344 	 * the low 32-bit and the high 32-bit portion.
345 	 */
346 	remote_clock = cmpxchg64(&scd->clock, 0, 0);
347 #else
348 	/*
349 	 * On 64-bit kernels the read of [my]scd->clock is atomic versus the
350 	 * update, so we can avoid the above 32-bit dance.
351 	 */
352 	sched_clock_local(my_scd);
353 again:
354 	this_clock = my_scd->clock;
355 	remote_clock = scd->clock;
356 #endif
357 
358 	/*
359 	 * Use the opportunity that we have both locks
360 	 * taken to couple the two clocks: we take the
361 	 * larger time as the latest time for both
362 	 * runqueues. (this creates monotonic movement)
363 	 */
364 	if (likely((s64)(remote_clock - this_clock) < 0)) {
365 		ptr = &scd->clock;
366 		old_val = remote_clock;
367 		val = this_clock;
368 	} else {
369 		/*
370 		 * Should be rare, but possible:
371 		 */
372 		ptr = &my_scd->clock;
373 		old_val = this_clock;
374 		val = remote_clock;
375 	}
376 
377 	if (!try_cmpxchg64(ptr, &old_val, val))
378 		goto again;
379 
380 	return val;
381 }
382 
383 /*
384  * Similar to cpu_clock(), but requires local IRQs to be disabled.
385  *
386  * See cpu_clock().
387  */
sched_clock_cpu(int cpu)388 notrace u64 sched_clock_cpu(int cpu)
389 {
390 	struct sched_clock_data *scd;
391 	u64 clock;
392 
393 	if (sched_clock_stable())
394 		return sched_clock() + __sched_clock_offset;
395 
396 	if (!static_branch_likely(&sched_clock_running))
397 		return sched_clock();
398 
399 	preempt_disable_notrace();
400 	scd = cpu_sdc(cpu);
401 
402 	if (cpu != smp_processor_id())
403 		clock = sched_clock_remote(scd);
404 	else
405 		clock = sched_clock_local(scd);
406 	preempt_enable_notrace();
407 
408 	return clock;
409 }
410 EXPORT_SYMBOL_GPL(sched_clock_cpu);
411 
sched_clock_tick(void)412 notrace void sched_clock_tick(void)
413 {
414 	struct sched_clock_data *scd;
415 
416 	if (sched_clock_stable())
417 		return;
418 
419 	if (!static_branch_likely(&sched_clock_running))
420 		return;
421 
422 	lockdep_assert_irqs_disabled();
423 
424 	scd = this_scd();
425 	__scd_stamp(scd);
426 	sched_clock_local(scd);
427 }
428 
sched_clock_tick_stable(void)429 notrace void sched_clock_tick_stable(void)
430 {
431 	if (!sched_clock_stable())
432 		return;
433 
434 	/*
435 	 * Called under watchdog_lock.
436 	 *
437 	 * The watchdog just found this TSC to (still) be stable, so now is a
438 	 * good moment to update our __gtod_offset. Because once we find the
439 	 * TSC to be unstable, any computation will be computing crap.
440 	 */
441 	local_irq_disable();
442 	__sched_clock_gtod_offset();
443 	local_irq_enable();
444 }
445 
446 /*
447  * We are going deep-idle (irqs are disabled):
448  */
sched_clock_idle_sleep_event(void)449 notrace void sched_clock_idle_sleep_event(void)
450 {
451 	sched_clock_cpu(smp_processor_id());
452 }
453 EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
454 
455 /*
456  * We just idled; resync with ktime.
457  */
sched_clock_idle_wakeup_event(void)458 notrace void sched_clock_idle_wakeup_event(void)
459 {
460 	unsigned long flags;
461 
462 	if (sched_clock_stable())
463 		return;
464 
465 	if (unlikely(timekeeping_suspended))
466 		return;
467 
468 	local_irq_save(flags);
469 	sched_clock_tick();
470 	local_irq_restore(flags);
471 }
472 EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
473 
474 #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
475 
sched_clock_init(void)476 void __init sched_clock_init(void)
477 {
478 	static_branch_inc(&sched_clock_running);
479 	local_irq_disable();
480 	generic_sched_clock_init();
481 	local_irq_enable();
482 }
483 
sched_clock_cpu(int cpu)484 notrace u64 sched_clock_cpu(int cpu)
485 {
486 	if (!static_branch_likely(&sched_clock_running))
487 		return 0;
488 
489 	return sched_clock();
490 }
491 
492 #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
493 
494 /*
495  * Running clock - returns the time that has elapsed while a guest has been
496  * running.
497  * On a guest this value should be local_clock minus the time the guest was
498  * suspended by the hypervisor (for any reason).
499  * On bare metal this function should return the same as local_clock.
500  * Architectures and sub-architectures can override this.
501  */
running_clock(void)502 notrace u64 __weak running_clock(void)
503 {
504 	return local_clock();
505 }
506