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 * sched_clock_cpu(i) -- must be used with local IRQs disabled (implied by NMI)
30 * local_clock() -- is cpu_clock() on the current cpu.
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).
39 * Otherwise it tries to create a semi stable clock from a mixture of other
42 * - GTOD (clock monotomic)
44 * - explicit idle events
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
50 * Furthermore, explicit sleep and wakeup hooks allow us to account for time
51 * that is otherwise invisible (TSC gets stopped).
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
63 #include <linux/spinlock.h>
64 #include <linux/hardirq.h>
65 #include <linux/export.h>
66 #include <linux/percpu.h>
67 #include <linux/ktime.h>
68 #include <linux/sched.h>
71 * Scheduler clock - returns current time in nanosec units.
72 * This is default implementation.
73 * Architectures and sub-architectures can override this.
75 unsigned long long __attribute__((weak)) sched_clock(void)
77 return (unsigned long long)(jiffies - INITIAL_JIFFIES)
78 * (NSEC_PER_SEC / HZ);
80 EXPORT_SYMBOL_GPL(sched_clock);
82 __read_mostly int sched_clock_running;
84 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
85 __read_mostly int sched_clock_stable;
87 struct sched_clock_data {
93 static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
95 static inline struct sched_clock_data *this_scd(void)
97 return &__get_cpu_var(sched_clock_data);
100 static inline struct sched_clock_data *cpu_sdc(int cpu)
102 return &per_cpu(sched_clock_data, cpu);
105 void sched_clock_init(void)
107 u64 ktime_now = ktime_to_ns(ktime_get());
110 for_each_possible_cpu(cpu) {
111 struct sched_clock_data *scd = cpu_sdc(cpu);
114 scd->tick_gtod = ktime_now;
115 scd->clock = ktime_now;
118 sched_clock_running = 1;
122 * min, max except they take wrapping into account
125 static inline u64 wrap_min(u64 x, u64 y)
127 return (s64)(x - y) < 0 ? x : y;
130 static inline u64 wrap_max(u64 x, u64 y)
132 return (s64)(x - y) > 0 ? x : y;
136 * update the percpu scd from the raw @now value
138 * - filter out backward motion
139 * - use the GTOD tick value to create a window to filter crazy TSC values
141 static u64 sched_clock_local(struct sched_clock_data *scd)
143 u64 now, clock, old_clock, min_clock, max_clock;
148 delta = now - scd->tick_raw;
149 if (unlikely(delta < 0))
152 old_clock = scd->clock;
155 * scd->clock = clamp(scd->tick_gtod + delta,
156 * max(scd->tick_gtod, scd->clock),
157 * scd->tick_gtod + TICK_NSEC);
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);
164 clock = wrap_max(clock, min_clock);
165 clock = wrap_min(clock, max_clock);
167 if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
173 static u64 sched_clock_remote(struct sched_clock_data *scd)
175 struct sched_clock_data *my_scd = this_scd();
176 u64 this_clock, remote_clock;
177 u64 *ptr, old_val, val;
179 sched_clock_local(my_scd);
181 this_clock = my_scd->clock;
182 remote_clock = scd->clock;
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)
190 if (likely((s64)(remote_clock - this_clock) < 0)) {
192 old_val = remote_clock;
196 * Should be rare, but possible:
198 ptr = &my_scd->clock;
199 old_val = this_clock;
203 if (cmpxchg64(ptr, old_val, val) != old_val)
210 * Similar to cpu_clock(), but requires local IRQs to be disabled.
214 u64 sched_clock_cpu(int cpu)
216 struct sched_clock_data *scd;
219 WARN_ON_ONCE(!irqs_disabled());
221 if (sched_clock_stable)
222 return sched_clock();
224 if (unlikely(!sched_clock_running))
229 if (cpu != smp_processor_id())
230 clock = sched_clock_remote(scd);
232 clock = sched_clock_local(scd);
237 void sched_clock_tick(void)
239 struct sched_clock_data *scd;
242 if (sched_clock_stable)
245 if (unlikely(!sched_clock_running))
248 WARN_ON_ONCE(!irqs_disabled());
251 now_gtod = ktime_to_ns(ktime_get());
255 scd->tick_gtod = now_gtod;
256 sched_clock_local(scd);
260 * We are going deep-idle (irqs are disabled):
262 void sched_clock_idle_sleep_event(void)
264 sched_clock_cpu(smp_processor_id());
266 EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
269 * We just idled delta nanoseconds (called with irqs disabled):
271 void sched_clock_idle_wakeup_event(u64 delta_ns)
273 if (timekeeping_suspended)
277 touch_softlockup_watchdog();
279 EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
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
286 * ######################### BIG FAT WARNING ##########################
287 * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
288 * # go backwards !! #
289 * ####################################################################
291 u64 cpu_clock(int cpu)
296 local_irq_save(flags);
297 clock = sched_clock_cpu(cpu);
298 local_irq_restore(flags);
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
310 u64 local_clock(void)
315 local_irq_save(flags);
316 clock = sched_clock_cpu(smp_processor_id());
317 local_irq_restore(flags);
322 #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
324 void sched_clock_init(void)
326 sched_clock_running = 1;
329 u64 sched_clock_cpu(int cpu)
331 if (unlikely(!sched_clock_running))
334 return sched_clock();
337 u64 cpu_clock(int cpu)
339 return sched_clock_cpu(cpu);
342 u64 local_clock(void)
344 return sched_clock_cpu(0);
347 #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
349 EXPORT_SYMBOL_GPL(cpu_clock);
350 EXPORT_SYMBOL_GPL(local_clock);