2 * sched_clock for unstable cpu clocks
4 * Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
6 * Updates and enhancements:
7 * Copyright (C) 2008 Red Hat, Inc. Steven Rostedt <srostedt@redhat.com>
10 * Ingo Molnar <mingo@redhat.com>
11 * Guillaume Chazarain <guichaz@gmail.com>
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.
20 * ######################### BIG FAT WARNING ##########################
21 * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
23 * ####################################################################
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).
28 * cpu_clock(i) -- can be used from any context, including NMI.
29 * local_clock() -- is cpu_clock() on the current cpu.
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).
40 * Otherwise it tries to create a semi stable clock from a mixture of other
43 * - GTOD (clock monotomic)
45 * - explicit idle events
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
51 * Furthermore, explicit sleep and wakeup hooks allow us to account for time
52 * that is otherwise invisible (TSC gets stopped).
55 #include <linux/spinlock.h>
56 #include <linux/hardirq.h>
57 #include <linux/export.h>
58 #include <linux/percpu.h>
59 #include <linux/ktime.h>
60 #include <linux/sched.h>
61 #include <linux/static_key.h>
62 #include <linux/workqueue.h>
65 * Scheduler clock - returns current time in nanosec units.
66 * This is default implementation.
67 * Architectures and sub-architectures can override this.
69 unsigned long long __attribute__((weak)) sched_clock(void)
71 return (unsigned long long)(jiffies - INITIAL_JIFFIES)
72 * (NSEC_PER_SEC / HZ);
74 EXPORT_SYMBOL_GPL(sched_clock);
76 __read_mostly int sched_clock_running;
78 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
79 static struct static_key __sched_clock_stable = STATIC_KEY_INIT;
81 int sched_clock_stable(void)
83 if (static_key_false(&__sched_clock_stable))
88 void set_sched_clock_stable(void)
90 if (!sched_clock_stable())
91 static_key_slow_dec(&__sched_clock_stable);
94 static void __clear_sched_clock_stable(struct work_struct *work)
96 /* XXX worry about clock continuity */
97 if (sched_clock_stable())
98 static_key_slow_inc(&__sched_clock_stable);
101 static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);
103 void clear_sched_clock_stable(void)
106 schedule_work(&sched_clock_work);
108 __clear_sched_clock_stable(&sched_clock_work);
111 struct sched_clock_data {
117 static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
119 static inline struct sched_clock_data *this_scd(void)
121 return &__get_cpu_var(sched_clock_data);
124 static inline struct sched_clock_data *cpu_sdc(int cpu)
126 return &per_cpu(sched_clock_data, cpu);
129 void sched_clock_init(void)
131 u64 ktime_now = ktime_to_ns(ktime_get());
134 for_each_possible_cpu(cpu) {
135 struct sched_clock_data *scd = cpu_sdc(cpu);
138 scd->tick_gtod = ktime_now;
139 scd->clock = ktime_now;
142 sched_clock_running = 1;
146 * min, max except they take wrapping into account
149 static inline u64 wrap_min(u64 x, u64 y)
151 return (s64)(x - y) < 0 ? x : y;
154 static inline u64 wrap_max(u64 x, u64 y)
156 return (s64)(x - y) > 0 ? x : y;
160 * update the percpu scd from the raw @now value
162 * - filter out backward motion
163 * - use the GTOD tick value to create a window to filter crazy TSC values
165 static u64 sched_clock_local(struct sched_clock_data *scd)
167 u64 now, clock, old_clock, min_clock, max_clock;
172 delta = now - scd->tick_raw;
173 if (unlikely(delta < 0))
176 old_clock = scd->clock;
179 * scd->clock = clamp(scd->tick_gtod + delta,
180 * max(scd->tick_gtod, scd->clock),
181 * scd->tick_gtod + TICK_NSEC);
184 clock = scd->tick_gtod + delta;
185 min_clock = wrap_max(scd->tick_gtod, old_clock);
186 max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
188 clock = wrap_max(clock, min_clock);
189 clock = wrap_min(clock, max_clock);
191 if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
197 static u64 sched_clock_remote(struct sched_clock_data *scd)
199 struct sched_clock_data *my_scd = this_scd();
200 u64 this_clock, remote_clock;
201 u64 *ptr, old_val, val;
203 #if BITS_PER_LONG != 64
206 * Careful here: The local and the remote clock values need to
207 * be read out atomic as we need to compare the values and
208 * then update either the local or the remote side. So the
209 * cmpxchg64 below only protects one readout.
211 * We must reread via sched_clock_local() in the retry case on
212 * 32bit as an NMI could use sched_clock_local() via the
213 * tracer and hit between the readout of
214 * the low32bit and the high 32bit portion.
216 this_clock = sched_clock_local(my_scd);
218 * We must enforce atomic readout on 32bit, otherwise the
219 * update on the remote cpu can hit inbetween the readout of
220 * the low32bit and the high 32bit portion.
222 remote_clock = cmpxchg64(&scd->clock, 0, 0);
225 * On 64bit the read of [my]scd->clock is atomic versus the
226 * update, so we can avoid the above 32bit dance.
228 sched_clock_local(my_scd);
230 this_clock = my_scd->clock;
231 remote_clock = scd->clock;
235 * Use the opportunity that we have both locks
236 * taken to couple the two clocks: we take the
237 * larger time as the latest time for both
238 * runqueues. (this creates monotonic movement)
240 if (likely((s64)(remote_clock - this_clock) < 0)) {
242 old_val = remote_clock;
246 * Should be rare, but possible:
248 ptr = &my_scd->clock;
249 old_val = this_clock;
253 if (cmpxchg64(ptr, old_val, val) != old_val)
260 * Similar to cpu_clock(), but requires local IRQs to be disabled.
264 u64 sched_clock_cpu(int cpu)
266 struct sched_clock_data *scd;
269 if (sched_clock_stable())
270 return sched_clock();
272 if (unlikely(!sched_clock_running))
278 if (cpu != smp_processor_id())
279 clock = sched_clock_remote(scd);
281 clock = sched_clock_local(scd);
287 void sched_clock_tick(void)
289 struct sched_clock_data *scd;
292 if (sched_clock_stable())
295 if (unlikely(!sched_clock_running))
298 WARN_ON_ONCE(!irqs_disabled());
301 now_gtod = ktime_to_ns(ktime_get());
305 scd->tick_gtod = now_gtod;
306 sched_clock_local(scd);
310 * We are going deep-idle (irqs are disabled):
312 void sched_clock_idle_sleep_event(void)
314 sched_clock_cpu(smp_processor_id());
316 EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
319 * We just idled delta nanoseconds (called with irqs disabled):
321 void sched_clock_idle_wakeup_event(u64 delta_ns)
323 if (timekeeping_suspended)
327 touch_softlockup_watchdog();
329 EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
332 * As outlined at the top, provides a fast, high resolution, nanosecond
333 * time source that is monotonic per cpu argument and has bounded drift
336 * ######################### BIG FAT WARNING ##########################
337 * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
338 * # go backwards !! #
339 * ####################################################################
341 u64 cpu_clock(int cpu)
343 if (static_key_false(&__sched_clock_stable))
344 return sched_clock_cpu(cpu);
346 return sched_clock();
350 * Similar to cpu_clock() for the current cpu. Time will only be observed
351 * to be monotonic if care is taken to only compare timestampt taken on the
356 u64 local_clock(void)
358 if (static_key_false(&__sched_clock_stable))
359 return sched_clock_cpu(raw_smp_processor_id());
361 return sched_clock();
364 #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
366 void sched_clock_init(void)
368 sched_clock_running = 1;
371 u64 sched_clock_cpu(int cpu)
373 if (unlikely(!sched_clock_running))
376 return sched_clock();
379 u64 cpu_clock(int cpu)
381 return sched_clock();
384 u64 local_clock(void)
386 return sched_clock();
389 #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
391 EXPORT_SYMBOL_GPL(cpu_clock);
392 EXPORT_SYMBOL_GPL(local_clock);