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
);