[deliverable/linux.git] / kernel / sched / clock.c

/*
 * sched_clock for unstable cpu clocks
 *
 *  Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra
 *
 *  Updates and enhancements:
 *    Copyright (C) 2008 Red Hat, Inc. Steven Rostedt <srostedt@redhat.com>
 *
 * Based on code by:
 *   Ingo Molnar <mingo@redhat.com>
 *   Guillaume Chazarain <guichaz@gmail.com>
 *
 *
 * What:
 *
 * cpu_clock(i) provides a fast (execution time) high resolution
 * clock with bounded drift between CPUs. The value of cpu_clock(i)
 * is monotonic for constant i. The timestamp returned is in nanoseconds.
 *
 * ######################### BIG FAT WARNING ##########################
 * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
 * # go backwards !!                                                  #
 * ####################################################################
 *
 * There is no strict promise about the base, although it tends to start
 * at 0 on boot (but people really shouldn't rely on that).
 *
 * cpu_clock(i)       -- can be used from any context, including NMI.
 * local_clock()      -- is cpu_clock() on the current cpu.
 *
 * sched_clock_cpu(i)
 *
 * How:
 *
 * The implementation either uses sched_clock() when
 * !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
 * sched_clock() is assumed to provide these properties (mostly it means
 * the architecture provides a globally synchronized highres time source).
 *
 * Otherwise it tries to create a semi stable clock from a mixture of other
 * clocks, including:
 *
 *  - GTOD (clock monotomic)
 *  - sched_clock()
 *  - explicit idle events
 *
 * We use GTOD as base and use sched_clock() deltas to improve resolution. The
 * deltas are filtered to provide monotonicity and keeping it within an
 * expected window.
 *
 * Furthermore, explicit sleep and wakeup hooks allow us to account for time
 * that is otherwise invisible (TSC gets stopped).
 *
 */
#include <linux/spinlock.h>
#include <linux/hardirq.h>
#include <linux/export.h>
#include <linux/percpu.h>
#include <linux/ktime.h>
#include <linux/sched.h>
#include <linux/static_key.h>
#include <linux/workqueue.h>
#include <linux/compiler.h>
#include <linux/tick.h>

/*
 * Scheduler clock - returns current time in nanosec units.
 * This is default implementation.
 * Architectures and sub-architectures can override this.
 */
unsigned long long __weak sched_clock(void)
{
	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
					* (NSEC_PER_SEC / HZ);
}
EXPORT_SYMBOL_GPL(sched_clock);

__read_mostly int sched_clock_running;

#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
static struct static_key __sched_clock_stable = STATIC_KEY_INIT;
static int __sched_clock_stable_early;

int sched_clock_stable(void)
{
	return static_key_false(&__sched_clock_stable);
}

static void __set_sched_clock_stable(void)
{
	if (!sched_clock_stable())
		static_key_slow_inc(&__sched_clock_stable);

	tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
}

void set_sched_clock_stable(void)
{
	__sched_clock_stable_early = 1;

	smp_mb(); /* matches sched_clock_init() */

	if (!sched_clock_running)
		return;

	__set_sched_clock_stable();
}

static void __clear_sched_clock_stable(struct work_struct *work)
{
	/* XXX worry about clock continuity */
	if (sched_clock_stable())
		static_key_slow_dec(&__sched_clock_stable);

	tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE);
}

static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);

void clear_sched_clock_stable(void)
{
	__sched_clock_stable_early = 0;

	smp_mb(); /* matches sched_clock_init() */

	if (!sched_clock_running)
		return;

	schedule_work(&sched_clock_work);
}

struct sched_clock_data {
	u64			tick_raw;
	u64			tick_gtod;
	u64			clock;
};

static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);

static inline struct sched_clock_data *this_scd(void)
{
	return this_cpu_ptr(&sched_clock_data);
}

static inline struct sched_clock_data *cpu_sdc(int cpu)
{
	return &per_cpu(sched_clock_data, cpu);
}

void sched_clock_init(void)
{
	u64 ktime_now = ktime_to_ns(ktime_get());
	int cpu;

	for_each_possible_cpu(cpu) {
		struct sched_clock_data *scd = cpu_sdc(cpu);

		scd->tick_raw = 0;
		scd->tick_gtod = ktime_now;
		scd->clock = ktime_now;
	}

	sched_clock_running = 1;

	/*
	 * Ensure that it is impossible to not do a static_key update.
	 *
	 * Either {set,clear}_sched_clock_stable() must see sched_clock_running
	 * and do the update, or we must see their __sched_clock_stable_early
	 * and do the update, or both.
	 */
	smp_mb(); /* matches {set,clear}_sched_clock_stable() */

	if (__sched_clock_stable_early)
		__set_sched_clock_stable();
	else
		__clear_sched_clock_stable(NULL);
}

/*
 * min, max except they take wrapping into account
 */

static inline u64 wrap_min(u64 x, u64 y)
{
	return (s64)(x - y) < 0 ? x : y;
}

static inline u64 wrap_max(u64 x, u64 y)
{
	return (s64)(x - y) > 0 ? x : y;
}

/*
 * update the percpu scd from the raw @now value
 *
 *  - filter out backward motion
 *  - use the GTOD tick value to create a window to filter crazy TSC values
 */
static u64 sched_clock_local(struct sched_clock_data *scd)
{
	u64 now, clock, old_clock, min_clock, max_clock;
	s64 delta;

again:
	now = sched_clock();
	delta = now - scd->tick_raw;
	if (unlikely(delta < 0))
		delta = 0;

	old_clock = scd->clock;

	/*
	 * scd->clock = clamp(scd->tick_gtod + delta,
	 *		      max(scd->tick_gtod, scd->clock),
	 *		      scd->tick_gtod + TICK_NSEC);
	 */

	clock = scd->tick_gtod + delta;
	min_clock = wrap_max(scd->tick_gtod, old_clock);
	max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);

	clock = wrap_max(clock, min_clock);
	clock = wrap_min(clock, max_clock);

	if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
		goto again;

	return clock;
}

static u64 sched_clock_remote(struct sched_clock_data *scd)
{
	struct sched_clock_data *my_scd = this_scd();
	u64 this_clock, remote_clock;
	u64 *ptr, old_val, val;

#if BITS_PER_LONG != 64
again:
	/*
	 * Careful here: The local and the remote clock values need to
	 * be read out atomic as we need to compare the values and
	 * then update either the local or the remote side. So the
	 * cmpxchg64 below only protects one readout.
	 *
	 * We must reread via sched_clock_local() in the retry case on
	 * 32bit as an NMI could use sched_clock_local() via the
	 * tracer and hit between the readout of
	 * the low32bit and the high 32bit portion.
	 */
	this_clock = sched_clock_local(my_scd);
	/*
	 * We must enforce atomic readout on 32bit, otherwise the
	 * update on the remote cpu can hit inbetween the readout of
	 * the low32bit and the high 32bit portion.
	 */
	remote_clock = cmpxchg64(&scd->clock, 0, 0);
#else
	/*
	 * On 64bit the read of [my]scd->clock is atomic versus the
	 * update, so we can avoid the above 32bit dance.
	 */
	sched_clock_local(my_scd);
again:
	this_clock = my_scd->clock;
	remote_clock = scd->clock;
#endif

	/*
	 * Use the opportunity that we have both locks
	 * taken to couple the two clocks: we take the
	 * larger time as the latest time for both
	 * runqueues. (this creates monotonic movement)
	 */
	if (likely((s64)(remote_clock - this_clock) < 0)) {
		ptr = &scd->clock;
		old_val = remote_clock;
		val = this_clock;
	} else {
		/*
		 * Should be rare, but possible:
		 */
		ptr = &my_scd->clock;
		old_val = this_clock;
		val = remote_clock;
	}

	if (cmpxchg64(ptr, old_val, val) != old_val)
		goto again;

	return val;
}

/*
 * Similar to cpu_clock(), but requires local IRQs to be disabled.
 *
 * See cpu_clock().
 */
u64 sched_clock_cpu(int cpu)
{
	struct sched_clock_data *scd;
	u64 clock;

	if (sched_clock_stable())
		return sched_clock();

	if (unlikely(!sched_clock_running))
		return 0ull;

	preempt_disable_notrace();
	scd = cpu_sdc(cpu);

	if (cpu != smp_processor_id())
		clock = sched_clock_remote(scd);
	else
		clock = sched_clock_local(scd);
	preempt_enable_notrace();

	return clock;
}

void sched_clock_tick(void)
{
	struct sched_clock_data *scd;
	u64 now, now_gtod;

	if (sched_clock_stable())
		return;

	if (unlikely(!sched_clock_running))
		return;

	WARN_ON_ONCE(!irqs_disabled());

	scd = this_scd();
	now_gtod = ktime_to_ns(ktime_get());
	now = sched_clock();

	scd->tick_raw = now;
	scd->tick_gtod = now_gtod;
	sched_clock_local(scd);
}

/*
 * We are going deep-idle (irqs are disabled):
 */
void sched_clock_idle_sleep_event(void)
{
	sched_clock_cpu(smp_processor_id());
}
EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);

/*
 * We just idled delta nanoseconds (called with irqs disabled):
 */
void sched_clock_idle_wakeup_event(u64 delta_ns)
{
	if (timekeeping_suspended)
		return;

	sched_clock_tick();
	touch_softlockup_watchdog_sched();
}
EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);

/*
 * As outlined at the top, provides a fast, high resolution, nanosecond
 * time source that is monotonic per cpu argument and has bounded drift
 * between cpus.
 *
 * ######################### BIG FAT WARNING ##########################
 * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
 * # go backwards !!                                                  #
 * ####################################################################
 */
u64 cpu_clock(int cpu)
{
	if (!sched_clock_stable())
		return sched_clock_cpu(cpu);

	return sched_clock();
}

/*
 * Similar to cpu_clock() for the current cpu. Time will only be observed
 * to be monotonic if care is taken to only compare timestampt taken on the
 * same CPU.
 *
 * See cpu_clock().
 */
u64 local_clock(void)
{
	if (!sched_clock_stable())
		return sched_clock_cpu(raw_smp_processor_id());

	return sched_clock();
}

#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */

void sched_clock_init(void)
{
	sched_clock_running = 1;
}

u64 sched_clock_cpu(int cpu)
{
	if (unlikely(!sched_clock_running))
		return 0;

	return sched_clock();
}

u64 cpu_clock(int cpu)
{
	return sched_clock();
}

u64 local_clock(void)
{
	return sched_clock();
}

#endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */

EXPORT_SYMBOL_GPL(cpu_clock);
EXPORT_SYMBOL_GPL(local_clock);

/*
 * Running clock - returns the time that has elapsed while a guest has been
 * running.
 * On a guest this value should be local_clock minus the time the guest was
 * suspended by the hypervisor (for any reason).
 * On bare metal this function should return the same as local_clock.
 * Architectures and sub-architectures can override this.
 */
u64 __weak running_clock(void)
{
	return local_clock();
}
Commit	Line	Data
	1	/*
	2	* sched_clock for unstable cpu clocks
	3	*
	4	* Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra
	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	* local_clock() -- is cpu_clock() on the current cpu.
	30	*
	31	* sched_clock_cpu(i)
	32	*
	33	* How:
	34	*
	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).
	39	*
	40	* Otherwise it tries to create a semi stable clock from a mixture of other
	41	* clocks, including:
	42	*
	43	* - GTOD (clock monotomic)
	44	* - sched_clock()
	45	* - explicit idle events
	46	*
	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
	49	* expected window.
	50	*
	51	* Furthermore, explicit sleep and wakeup hooks allow us to account for time
	52	* that is otherwise invisible (TSC gets stopped).
	53	*
	54	*/
	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>
	63	#include <linux/compiler.h>
	64	#include <linux/tick.h>
	65
	66	/*
	67	* Scheduler clock - returns current time in nanosec units.
	68	* This is default implementation.
	69	* Architectures and sub-architectures can override this.
	70	*/
	71	unsigned long long __weak sched_clock(void)
	72	{
	73	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
	74	* (NSEC_PER_SEC / HZ);
	75	}
	76	EXPORT_SYMBOL_GPL(sched_clock);
	77
	78	__read_mostly int sched_clock_running;
	79
	80	#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
	81	static struct static_key __sched_clock_stable = STATIC_KEY_INIT;
	82	static int __sched_clock_stable_early;
	83
	84	int sched_clock_stable(void)
	85	{
	86	return static_key_false(&__sched_clock_stable);
	87	}
	88
	89	static void __set_sched_clock_stable(void)
	90	{
	91	if (!sched_clock_stable())
	92	static_key_slow_inc(&__sched_clock_stable);
	93
	94	tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
	95	}
	96
	97	void set_sched_clock_stable(void)
	98	{
	99	__sched_clock_stable_early = 1;
	100
	101	smp_mb(); /* matches sched_clock_init() */
	102
	103	if (!sched_clock_running)
	104	return;
	105
	106	__set_sched_clock_stable();
	107	}
	108
	109	static void __clear_sched_clock_stable(struct work_struct *work)
	110	{
	111	/* XXX worry about clock continuity */
	112	if (sched_clock_stable())
	113	static_key_slow_dec(&__sched_clock_stable);
	114
	115	tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE);
	116	}
	117
	118	static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);
	119
	120	void clear_sched_clock_stable(void)
	121	{
	122	__sched_clock_stable_early = 0;
	123
	124	smp_mb(); /* matches sched_clock_init() */
	125
	126	if (!sched_clock_running)
	127	return;
	128
	129	schedule_work(&sched_clock_work);
	130	}
	131
	132	struct sched_clock_data {
	133	u64 tick_raw;
	134	u64 tick_gtod;
	135	u64 clock;
	136	};
	137
	138	static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
	139
	140	static inline struct sched_clock_data *this_scd(void)
	141	{
	142	return this_cpu_ptr(&sched_clock_data);
	143	}
	144
	145	static inline struct sched_clock_data *cpu_sdc(int cpu)
	146	{
	147	return &per_cpu(sched_clock_data, cpu);
	148	}
	149
	150	void sched_clock_init(void)
	151	{
	152	u64 ktime_now = ktime_to_ns(ktime_get());
	153	int cpu;
	154
	155	for_each_possible_cpu(cpu) {
	156	struct sched_clock_data *scd = cpu_sdc(cpu);
	157
	158	scd->tick_raw = 0;
	159	scd->tick_gtod = ktime_now;
	160	scd->clock = ktime_now;
	161	}
	162
	163	sched_clock_running = 1;
	164
	165	/*
	166	* Ensure that it is impossible to not do a static_key update.
	167	*
	168	* Either {set,clear}_sched_clock_stable() must see sched_clock_running
	169	* and do the update, or we must see their __sched_clock_stable_early
	170	* and do the update, or both.
	171	*/
	172	smp_mb(); /* matches {set,clear}_sched_clock_stable() */
	173
	174	if (__sched_clock_stable_early)
	175	__set_sched_clock_stable();
	176	else
	177	__clear_sched_clock_stable(NULL);
	178	}
	179
	180	/*
	181	* min, max except they take wrapping into account
	182	*/
	183
	184	static inline u64 wrap_min(u64 x, u64 y)
	185	{
	186	return (s64)(x - y) < 0 ? x : y;
	187	}
	188
	189	static inline u64 wrap_max(u64 x, u64 y)
	190	{
	191	return (s64)(x - y) > 0 ? x : y;
	192	}
	193
	194	/*
	195	* update the percpu scd from the raw @now value
	196	*
	197	* - filter out backward motion
	198	* - use the GTOD tick value to create a window to filter crazy TSC values
	199	*/
	200	static u64 sched_clock_local(struct sched_clock_data *scd)
	201	{
	202	u64 now, clock, old_clock, min_clock, max_clock;
	203	s64 delta;
	204
	205	again:
	206	now = sched_clock();
	207	delta = now - scd->tick_raw;
	208	if (unlikely(delta < 0))
	209	delta = 0;
	210
	211	old_clock = scd->clock;
	212
	213	/*
	214	* scd->clock = clamp(scd->tick_gtod + delta,
	215	* max(scd->tick_gtod, scd->clock),
	216	* scd->tick_gtod + TICK_NSEC);
	217	*/
	218
	219	clock = scd->tick_gtod + delta;
	220	min_clock = wrap_max(scd->tick_gtod, old_clock);
	221	max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
	222
	223	clock = wrap_max(clock, min_clock);
	224	clock = wrap_min(clock, max_clock);
	225
	226	if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
	227	goto again;
	228
	229	return clock;
	230	}
	231
	232	static u64 sched_clock_remote(struct sched_clock_data *scd)
	233	{
	234	struct sched_clock_data *my_scd = this_scd();
	235	u64 this_clock, remote_clock;
	236	u64 *ptr, old_val, val;
	237
	238	#if BITS_PER_LONG != 64
	239	again:
	240	/*
	241	* Careful here: The local and the remote clock values need to
	242	* be read out atomic as we need to compare the values and
	243	* then update either the local or the remote side. So the
	244	* cmpxchg64 below only protects one readout.
	245	*
	246	* We must reread via sched_clock_local() in the retry case on
	247	* 32bit as an NMI could use sched_clock_local() via the
	248	* tracer and hit between the readout of
	249	* the low32bit and the high 32bit portion.
	250	*/
	251	this_clock = sched_clock_local(my_scd);
	252	/*
	253	* We must enforce atomic readout on 32bit, otherwise the
	254	* update on the remote cpu can hit inbetween the readout of
	255	* the low32bit and the high 32bit portion.
	256	*/
	257	remote_clock = cmpxchg64(&scd->clock, 0, 0);
	258	#else
	259	/*
	260	* On 64bit the read of [my]scd->clock is atomic versus the
	261	* update, so we can avoid the above 32bit dance.
	262	*/
	263	sched_clock_local(my_scd);
	264	again:
	265	this_clock = my_scd->clock;
	266	remote_clock = scd->clock;
	267	#endif
	268
	269	/*
	270	* Use the opportunity that we have both locks
	271	* taken to couple the two clocks: we take the
	272	* larger time as the latest time for both
	273	* runqueues. (this creates monotonic movement)
	274	*/
	275	if (likely((s64)(remote_clock - this_clock) < 0)) {
	276	ptr = &scd->clock;
	277	old_val = remote_clock;
	278	val = this_clock;
	279	} else {
	280	/*
	281	* Should be rare, but possible:
	282	*/
	283	ptr = &my_scd->clock;
	284	old_val = this_clock;
	285	val = remote_clock;
	286	}
	287
	288	if (cmpxchg64(ptr, old_val, val) != old_val)
	289	goto again;
	290
	291	return val;
	292	}
	293
	294	/*
	295	* Similar to cpu_clock(), but requires local IRQs to be disabled.
	296	*
	297	* See cpu_clock().
	298	*/
	299	u64 sched_clock_cpu(int cpu)
	300	{
	301	struct sched_clock_data *scd;
	302	u64 clock;
	303
	304	if (sched_clock_stable())
	305	return sched_clock();
	306
	307	if (unlikely(!sched_clock_running))
	308	return 0ull;
	309
	310	preempt_disable_notrace();
	311	scd = cpu_sdc(cpu);
	312
	313	if (cpu != smp_processor_id())
	314	clock = sched_clock_remote(scd);
	315	else
	316	clock = sched_clock_local(scd);
	317	preempt_enable_notrace();
	318
	319	return clock;
	320	}
	321
	322	void sched_clock_tick(void)
	323	{
	324	struct sched_clock_data *scd;
	325	u64 now, now_gtod;
	326
	327	if (sched_clock_stable())
	328	return;
	329
	330	if (unlikely(!sched_clock_running))
	331	return;
	332
	333	WARN_ON_ONCE(!irqs_disabled());
	334
	335	scd = this_scd();
	336	now_gtod = ktime_to_ns(ktime_get());
	337	now = sched_clock();
	338
	339	scd->tick_raw = now;
	340	scd->tick_gtod = now_gtod;
	341	sched_clock_local(scd);
	342	}
	343
	344	/*
	345	* We are going deep-idle (irqs are disabled):
	346	*/
	347	void sched_clock_idle_sleep_event(void)
	348	{
	349	sched_clock_cpu(smp_processor_id());
	350	}
	351	EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
	352
	353	/*
	354	* We just idled delta nanoseconds (called with irqs disabled):
	355	*/
	356	void sched_clock_idle_wakeup_event(u64 delta_ns)
	357	{
	358	if (timekeeping_suspended)
	359	return;
	360
	361	sched_clock_tick();
	362	touch_softlockup_watchdog_sched();
	363	}
	364	EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
	365
	366	/*
	367	* As outlined at the top, provides a fast, high resolution, nanosecond
	368	* time source that is monotonic per cpu argument and has bounded drift
	369	* between cpus.
	370	*
	371	* ######################### BIG FAT WARNING ##########################
	372	* # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
	373	* # go backwards !! #
	374	* ####################################################################
	375	*/
	376	u64 cpu_clock(int cpu)
	377	{
	378	if (!sched_clock_stable())
	379	return sched_clock_cpu(cpu);
	380
	381	return sched_clock();
	382	}
	383
	384	/*
	385	* Similar to cpu_clock() for the current cpu. Time will only be observed
	386	* to be monotonic if care is taken to only compare timestampt taken on the
	387	* same CPU.
	388	*
	389	* See cpu_clock().
	390	*/
	391	u64 local_clock(void)
	392	{
	393	if (!sched_clock_stable())
	394	return sched_clock_cpu(raw_smp_processor_id());
	395
	396	return sched_clock();
	397	}
	398
	399	#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
	400
	401	void sched_clock_init(void)
	402	{
	403	sched_clock_running = 1;
	404	}
	405
	406	u64 sched_clock_cpu(int cpu)
	407	{
	408	if (unlikely(!sched_clock_running))
	409	return 0;
	410
	411	return sched_clock();
	412	}
	413
	414	u64 cpu_clock(int cpu)
	415	{
	416	return sched_clock();
	417	}
	418
	419	u64 local_clock(void)
	420	{
	421	return sched_clock();
	422	}
	423
	424	#endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
	425
	426	EXPORT_SYMBOL_GPL(cpu_clock);
	427	EXPORT_SYMBOL_GPL(local_clock);
	428
	429	/*
	430	* Running clock - returns the time that has elapsed while a guest has been
	431	* running.
	432	* On a guest this value should be local_clock minus the time the guest was
	433	* suspended by the hypervisor (for any reason).
	434	* On bare metal this function should return the same as local_clock.
	435	* Architectures and sub-architectures can override this.
	436	*/
	437	u64 __weak running_clock(void)
	438	{
	439	return local_clock();
	440	}