2 * sched_clock for unstable cpu clocks
4 * Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra
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/sched/clock.h>
62 #include <linux/static_key.h>
63 #include <linux/workqueue.h>
64 #include <linux/compiler.h>
65 #include <linux/tick.h>
68 * Scheduler clock - returns current time in nanosec units.
69 * This is default implementation.
70 * Architectures and sub-architectures can override this.
72 unsigned long long __weak
sched_clock(void)
74 return (unsigned long long)(jiffies
- INITIAL_JIFFIES
)
75 * (NSEC_PER_SEC
/ HZ
);
77 EXPORT_SYMBOL_GPL(sched_clock
);
79 __read_mostly
int sched_clock_running
;
81 void sched_clock_init(void)
83 sched_clock_running
= 1;
86 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
88 * We must start with !__sched_clock_stable because the unstable -> stable
89 * transition is accurate, while the stable -> unstable transition is not.
91 * Similarly we start with __sched_clock_stable_early, thereby assuming we
92 * will become stable, such that there's only a single 1 -> 0 transition.
94 static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable
);
95 static int __sched_clock_stable_early
= 1;
98 * We want: ktime_get_ns() + gtod_offset == sched_clock() + raw_offset
100 static __read_mostly u64 raw_offset
;
101 static __read_mostly u64 gtod_offset
;
103 struct sched_clock_data
{
109 static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data
, sched_clock_data
);
111 static inline struct sched_clock_data
*this_scd(void)
113 return this_cpu_ptr(&sched_clock_data
);
116 static inline struct sched_clock_data
*cpu_sdc(int cpu
)
118 return &per_cpu(sched_clock_data
, cpu
);
121 int sched_clock_stable(void)
123 return static_branch_likely(&__sched_clock_stable
);
126 static void __set_sched_clock_stable(void)
128 struct sched_clock_data
*scd
= this_scd();
131 * Attempt to make the (initial) unstable->stable transition continuous.
133 raw_offset
= (scd
->tick_gtod
+ gtod_offset
) - (scd
->tick_raw
);
135 printk(KERN_INFO
"sched_clock: Marking stable (%lld, %lld)->(%lld, %lld)\n",
136 scd
->tick_gtod
, gtod_offset
,
137 scd
->tick_raw
, raw_offset
);
139 static_branch_enable(&__sched_clock_stable
);
140 tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE
);
143 static void __clear_sched_clock_stable(struct work_struct
*work
)
145 struct sched_clock_data
*scd
= this_scd();
148 * Attempt to make the stable->unstable transition continuous.
150 * Trouble is, this is typically called from the TSC watchdog
151 * timer, which is late per definition. This means the tick
152 * values can already be screwy.
154 * Still do what we can.
156 gtod_offset
= (scd
->tick_raw
+ raw_offset
) - (scd
->tick_gtod
);
158 printk(KERN_INFO
"sched_clock: Marking unstable (%lld, %lld)<-(%lld, %lld)\n",
159 scd
->tick_gtod
, gtod_offset
,
160 scd
->tick_raw
, raw_offset
);
162 static_branch_disable(&__sched_clock_stable
);
163 tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE
);
166 static DECLARE_WORK(sched_clock_work
, __clear_sched_clock_stable
);
168 void clear_sched_clock_stable(void)
170 __sched_clock_stable_early
= 0;
172 smp_mb(); /* matches sched_clock_init_late() */
174 if (sched_clock_running
== 2)
175 schedule_work(&sched_clock_work
);
178 void sched_clock_init_late(void)
180 sched_clock_running
= 2;
182 * Ensure that it is impossible to not do a static_key update.
184 * Either {set,clear}_sched_clock_stable() must see sched_clock_running
185 * and do the update, or we must see their __sched_clock_stable_early
186 * and do the update, or both.
188 smp_mb(); /* matches {set,clear}_sched_clock_stable() */
190 if (__sched_clock_stable_early
)
191 __set_sched_clock_stable();
195 * min, max except they take wrapping into account
198 static inline u64
wrap_min(u64 x
, u64 y
)
200 return (s64
)(x
- y
) < 0 ? x
: y
;
203 static inline u64
wrap_max(u64 x
, u64 y
)
205 return (s64
)(x
- y
) > 0 ? x
: y
;
209 * update the percpu scd from the raw @now value
211 * - filter out backward motion
212 * - use the GTOD tick value to create a window to filter crazy TSC values
214 static u64
sched_clock_local(struct sched_clock_data
*scd
)
216 u64 now
, clock
, old_clock
, min_clock
, max_clock
;
221 delta
= now
- scd
->tick_raw
;
222 if (unlikely(delta
< 0))
225 old_clock
= scd
->clock
;
228 * scd->clock = clamp(scd->tick_gtod + delta,
229 * max(scd->tick_gtod, scd->clock),
230 * scd->tick_gtod + TICK_NSEC);
233 clock
= scd
->tick_gtod
+ gtod_offset
+ delta
;
234 min_clock
= wrap_max(scd
->tick_gtod
, old_clock
);
235 max_clock
= wrap_max(old_clock
, scd
->tick_gtod
+ TICK_NSEC
);
237 clock
= wrap_max(clock
, min_clock
);
238 clock
= wrap_min(clock
, max_clock
);
240 if (cmpxchg64(&scd
->clock
, old_clock
, clock
) != old_clock
)
246 static u64
sched_clock_remote(struct sched_clock_data
*scd
)
248 struct sched_clock_data
*my_scd
= this_scd();
249 u64 this_clock
, remote_clock
;
250 u64
*ptr
, old_val
, val
;
252 #if BITS_PER_LONG != 64
255 * Careful here: The local and the remote clock values need to
256 * be read out atomic as we need to compare the values and
257 * then update either the local or the remote side. So the
258 * cmpxchg64 below only protects one readout.
260 * We must reread via sched_clock_local() in the retry case on
261 * 32bit as an NMI could use sched_clock_local() via the
262 * tracer and hit between the readout of
263 * the low32bit and the high 32bit portion.
265 this_clock
= sched_clock_local(my_scd
);
267 * We must enforce atomic readout on 32bit, otherwise the
268 * update on the remote cpu can hit inbetween the readout of
269 * the low32bit and the high 32bit portion.
271 remote_clock
= cmpxchg64(&scd
->clock
, 0, 0);
274 * On 64bit the read of [my]scd->clock is atomic versus the
275 * update, so we can avoid the above 32bit dance.
277 sched_clock_local(my_scd
);
279 this_clock
= my_scd
->clock
;
280 remote_clock
= scd
->clock
;
284 * Use the opportunity that we have both locks
285 * taken to couple the two clocks: we take the
286 * larger time as the latest time for both
287 * runqueues. (this creates monotonic movement)
289 if (likely((s64
)(remote_clock
- this_clock
) < 0)) {
291 old_val
= remote_clock
;
295 * Should be rare, but possible:
297 ptr
= &my_scd
->clock
;
298 old_val
= this_clock
;
302 if (cmpxchg64(ptr
, old_val
, val
) != old_val
)
309 * Similar to cpu_clock(), but requires local IRQs to be disabled.
313 u64
sched_clock_cpu(int cpu
)
315 struct sched_clock_data
*scd
;
318 if (sched_clock_stable())
319 return sched_clock() + raw_offset
;
321 if (unlikely(!sched_clock_running
))
324 preempt_disable_notrace();
327 if (cpu
!= smp_processor_id())
328 clock
= sched_clock_remote(scd
);
330 clock
= sched_clock_local(scd
);
331 preempt_enable_notrace();
335 EXPORT_SYMBOL_GPL(sched_clock_cpu
);
337 void sched_clock_tick(void)
339 struct sched_clock_data
*scd
;
341 WARN_ON_ONCE(!irqs_disabled());
344 * Update these values even if sched_clock_stable(), because it can
345 * become unstable at any point in time at which point we need some
346 * values to fall back on.
348 * XXX arguably we can skip this if we expose tsc_clocksource_reliable
351 scd
->tick_raw
= sched_clock();
352 scd
->tick_gtod
= ktime_get_ns();
354 if (!sched_clock_stable() && likely(sched_clock_running
))
355 sched_clock_local(scd
);
359 * We are going deep-idle (irqs are disabled):
361 void sched_clock_idle_sleep_event(void)
363 sched_clock_cpu(smp_processor_id());
365 EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event
);
368 * We just idled delta nanoseconds (called with irqs disabled):
370 void sched_clock_idle_wakeup_event(u64 delta_ns
)
372 if (timekeeping_suspended
)
376 touch_softlockup_watchdog_sched();
378 EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event
);
380 #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
382 u64
sched_clock_cpu(int cpu
)
384 if (unlikely(!sched_clock_running
))
387 return sched_clock();
390 #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
393 * Running clock - returns the time that has elapsed while a guest has been
395 * On a guest this value should be local_clock minus the time the guest was
396 * suspended by the hypervisor (for any reason).
397 * On bare metal this function should return the same as local_clock.
398 * Architectures and sub-architectures can override this.
400 u64 __weak
running_clock(void)
402 return local_clock();