1/*
  2 * sched_clock for unstable cpu clocks
  3 *
  4 *  Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
  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:
 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 * sched_clock_cpu(i) -- must be used with local IRQs disabled (implied by NMI)
 30 * local_clock()      -- is cpu_clock() on the current cpu.
 31 *
 32 * How:
 33 *
 34 * The implementation either uses sched_clock() when
 35 * !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
 36 * sched_clock() is assumed to provide these properties (mostly it means
 37 * the architecture provides a globally synchronized highres time source).
 38 *
 39 * Otherwise it tries to create a semi stable clock from a mixture of other
 40 * clocks, including:
 41 *
 42 *  - GTOD (clock monotomic)
 43 *  - sched_clock()
 44 *  - explicit idle events
 45 *
 46 * We use GTOD as base and use sched_clock() deltas to improve resolution. The
 47 * deltas are filtered to provide monotonicity and keeping it within an
 48 * expected window.
 49 *
 50 * Furthermore, explicit sleep and wakeup hooks allow us to account for time
 51 * that is otherwise invisible (TSC gets stopped).
 52 *
 53 *
 54 * Notes:
 55 *
 56 * The !IRQ-safetly of sched_clock() and sched_clock_cpu() comes from things
 57 * like cpufreq interrupts that can change the base clock (TSC) multiplier
 58 * and cause funny jumps in time -- although the filtering provided by
 59 * sched_clock_cpu() should mitigate serious artifacts we cannot rely on it
 60 * in general since for !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK we fully rely on
 61 * sched_clock().
 62 */
 63#include <linux/spinlock.h>
 64#include <linux/hardirq.h>
 65#include <linux/module.h>
 66#include <linux/percpu.h>
 67#include <linux/ktime.h>
 68#include <linux/sched.h>
 69
 70/*
 71 * Scheduler clock - returns current time in nanosec units.
 72 * This is default implementation.
 73 * Architectures and sub-architectures can override this.
 74 */
 75unsigned long long __attribute__((weak)) sched_clock(void)
 76{
 77	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
 78					* (NSEC_PER_SEC / HZ);
 79}
 80EXPORT_SYMBOL_GPL(sched_clock);
 81
 82__read_mostly int sched_clock_running;
 83
 84#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
 85__read_mostly int sched_clock_stable;
 86
 87struct sched_clock_data {
 88	u64			tick_raw;
 89	u64			tick_gtod;
 90	u64			clock;
 91};
 92
 93static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
 94
 95static inline struct sched_clock_data *this_scd(void)
 96{
 97	return &__get_cpu_var(sched_clock_data);
 98}
 99
100static inline struct sched_clock_data *cpu_sdc(int cpu)
101{
102	return &per_cpu(sched_clock_data, cpu);
103}
104
105void sched_clock_init(void)
106{
107	u64 ktime_now = ktime_to_ns(ktime_get());
108	int cpu;
109
110	for_each_possible_cpu(cpu) {
111		struct sched_clock_data *scd = cpu_sdc(cpu);
112
113		scd->tick_raw = 0;
114		scd->tick_gtod = ktime_now;
115		scd->clock = ktime_now;
116	}
117
118	sched_clock_running = 1;
119}
120
121/*
122 * min, max except they take wrapping into account
123 */
124
125static inline u64 wrap_min(u64 x, u64 y)
126{
127	return (s64)(x - y) < 0 ? x : y;
128}
129
130static inline u64 wrap_max(u64 x, u64 y)
131{
132	return (s64)(x - y) > 0 ? x : y;
133}
134
135/*
136 * update the percpu scd from the raw @now value
137 *
138 *  - filter out backward motion
139 *  - use the GTOD tick value to create a window to filter crazy TSC values
140 */
141static u64 sched_clock_local(struct sched_clock_data *scd)
142{
143	u64 now, clock, old_clock, min_clock, max_clock;
144	s64 delta;
145
146again:
147	now = sched_clock();
148	delta = now - scd->tick_raw;
149	if (unlikely(delta < 0))
150		delta = 0;
151
152	old_clock = scd->clock;
153
154	/*
155	 * scd->clock = clamp(scd->tick_gtod + delta,
156	 *		      max(scd->tick_gtod, scd->clock),
157	 *		      scd->tick_gtod + TICK_NSEC);
158	 */
159
160	clock = scd->tick_gtod + delta;
161	min_clock = wrap_max(scd->tick_gtod, old_clock);
162	max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
163
164	clock = wrap_max(clock, min_clock);
165	clock = wrap_min(clock, max_clock);
166
167	if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
168		goto again;
169
170	return clock;
171}
172
173static u64 sched_clock_remote(struct sched_clock_data *scd)
174{
175	struct sched_clock_data *my_scd = this_scd();
176	u64 this_clock, remote_clock;
177	u64 *ptr, old_val, val;
178
179	sched_clock_local(my_scd);
180again:
181	this_clock = my_scd->clock;
182	remote_clock = scd->clock;
183
184	/*
185	 * Use the opportunity that we have both locks
186	 * taken to couple the two clocks: we take the
187	 * larger time as the latest time for both
188	 * runqueues. (this creates monotonic movement)
189	 */
190	if (likely((s64)(remote_clock - this_clock) < 0)) {
191		ptr = &scd->clock;
192		old_val = remote_clock;
193		val = this_clock;
194	} else {
195		/*
196		 * Should be rare, but possible:
197		 */
198		ptr = &my_scd->clock;
199		old_val = this_clock;
200		val = remote_clock;
201	}
202
203	if (cmpxchg64(ptr, old_val, val) != old_val)
204		goto again;
205
206	return val;
207}
208
209/*
210 * Similar to cpu_clock(), but requires local IRQs to be disabled.
211 *
212 * See cpu_clock().
213 */
214u64 sched_clock_cpu(int cpu)
215{
216	struct sched_clock_data *scd;
217	u64 clock;
218
219	WARN_ON_ONCE(!irqs_disabled());
220
221	if (sched_clock_stable)
222		return sched_clock();
223
224	if (unlikely(!sched_clock_running))
225		return 0ull;
226
227	scd = cpu_sdc(cpu);
228
229	if (cpu != smp_processor_id())
230		clock = sched_clock_remote(scd);
231	else
232		clock = sched_clock_local(scd);
233
234	return clock;
235}
236
237void sched_clock_tick(void)
238{
239	struct sched_clock_data *scd;
240	u64 now, now_gtod;
241
242	if (sched_clock_stable)
243		return;
244
245	if (unlikely(!sched_clock_running))
246		return;
247
248	WARN_ON_ONCE(!irqs_disabled());
249
250	scd = this_scd();
251	now_gtod = ktime_to_ns(ktime_get());
252	now = sched_clock();
253
254	scd->tick_raw = now;
255	scd->tick_gtod = now_gtod;
256	sched_clock_local(scd);
257}
258
259/*
260 * We are going deep-idle (irqs are disabled):
261 */
262void sched_clock_idle_sleep_event(void)
263{
264	sched_clock_cpu(smp_processor_id());
265}
266EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
267
268/*
269 * We just idled delta nanoseconds (called with irqs disabled):
270 */
271void sched_clock_idle_wakeup_event(u64 delta_ns)
272{
273	if (timekeeping_suspended)
274		return;
275
276	sched_clock_tick();
277	touch_softlockup_watchdog();
278}
279EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
280
281/*
282 * As outlined at the top, provides a fast, high resolution, nanosecond
283 * time source that is monotonic per cpu argument and has bounded drift
284 * between cpus.
285 *
286 * ######################### BIG FAT WARNING ##########################
287 * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
288 * # go backwards !!                                                  #
289 * ####################################################################
290 */
291u64 cpu_clock(int cpu)
292{
293	u64 clock;
294	unsigned long flags;
295
296	local_irq_save(flags);
297	clock = sched_clock_cpu(cpu);
298	local_irq_restore(flags);
299
300	return clock;
301}
302
303/*
304 * Similar to cpu_clock() for the current cpu. Time will only be observed
305 * to be monotonic if care is taken to only compare timestampt taken on the
306 * same CPU.
307 *
308 * See cpu_clock().
309 */
310u64 local_clock(void)
311{
312	u64 clock;
313	unsigned long flags;
314
315	local_irq_save(flags);
316	clock = sched_clock_cpu(smp_processor_id());
317	local_irq_restore(flags);
318
319	return clock;
320}
321
322#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
323
324void sched_clock_init(void)
325{
326	sched_clock_running = 1;
327}
328
329u64 sched_clock_cpu(int cpu)
330{
331	if (unlikely(!sched_clock_running))
332		return 0;
333
334	return sched_clock();
335}
336
337u64 cpu_clock(int cpu)
338{
339	return sched_clock_cpu(cpu);
340}
341
342u64 local_clock(void)
343{
344	return sched_clock_cpu(0);
345}
346
347#endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
348
349EXPORT_SYMBOL_GPL(cpu_clock);
350EXPORT_SYMBOL_GPL(local_clock);