sched_clock.c

/*
 * sched_clock for unstable cpu clocks
 *
 *  Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
 *
 * Based on code by:
 *   Ingo Molnar <mingo@redhat.com>
 *   Guillaume Chazarain <guichaz@gmail.com>
 *
 * Create a semi stable clock from a mixture of other events, including:
 *  - gtod
 *  - jiffies
 *  - sched_clock()
 *  - explicit idle events
 *
 * We use gtod as base and the unstable clock deltas. The deltas are filtered,
 * making it monotonic and keeping it within an expected window.  This window
 * is set up using jiffies.
 *
 * Furthermore, explicit sleep and wakeup hooks allow us to account for time
 * that is otherwise invisible (TSC gets stopped).
 *
 * The clock: sched_clock_cpu() is monotonic per cpu, and should be somewhat
 * consistent between cpus (never more than 1 jiffies difference).
 */
#include <linux/sched.h>
#include <linux/percpu.h>
#include <linux/spinlock.h>
#include <linux/ktime.h>
#include <linux/module.h>


#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK

struct sched_clock_data {
	/*
	 * Raw spinlock - this is a special case: this might be called
	 * from within instrumentation code so we dont want to do any
	 * instrumentation ourselves.
	 */
	raw_spinlock_t		lock;

	unsigned long		tick_jiffies;
	u64			prev_raw;
	u64			tick_raw;
	u64			tick_gtod;
	u64			clock;
#ifdef CONFIG_NO_HZ
	int			check_max;
#endif
};

static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);

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

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

static __read_mostly int sched_clock_running;

void sched_clock_init(void)
{
	u64 ktime_now = ktime_to_ns(ktime_get());
	unsigned long now_jiffies = jiffies;
	int cpu;

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

		scd->lock = (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
		scd->tick_jiffies = now_jiffies;
		scd->prev_raw = 0;
		scd->tick_raw = 0;
		scd->tick_gtod = ktime_now;
		scd->clock = ktime_now;
#ifdef CONFIG_NO_HZ
		scd->check_max = 1;
#endif
	}

	sched_clock_running = 1;
}

#ifdef CONFIG_NO_HZ
/*
 * The dynamic ticks makes the delta jiffies inaccurate. This
 * prevents us from checking the maximum time update.
 * Disable the maximum check during stopped ticks.
 */
void sched_clock_tick_stop(int cpu)
{
	struct sched_clock_data *scd = cpu_sdc(cpu);

	scd->check_max = 0;
}

void sched_clock_tick_start(int cpu)
{
	struct sched_clock_data *scd = cpu_sdc(cpu);

	scd->check_max = 1;
}

static int check_max(struct sched_clock_data *scd)
{
	return scd->check_max;
}
#else
static int check_max(struct sched_clock_data *scd)
{
	return 1;
}
#endif /* CONFIG_NO_HZ */

/*
 * update the percpu scd from the raw @now value
 *
 *  - filter out backward motion
 *  - use jiffies to generate a min,max window to clip the raw values
 */
static void __update_sched_clock(struct sched_clock_data *scd, u64 now, u64 *time)
{
	unsigned long now_jiffies = jiffies;
	long delta_jiffies = now_jiffies - scd->tick_jiffies;
	u64 clock = scd->clock;
	u64 min_clock, max_clock;
	s64 delta = now - scd->prev_raw;

	WARN_ON_ONCE(!irqs_disabled());

	min_clock = scd->tick_gtod +
		(delta_jiffies ? delta_jiffies - 1 : 0) * TICK_NSEC;

	if (unlikely(delta < 0)) {
		clock++;
		goto out;
	}

	/*
	 * The clock must stay within a jiffie of the gtod.
	 * But since we may be at the start of a jiffy or the end of one
	 * we add another jiffy buffer.
	 */
	max_clock = scd->tick_gtod + (2 + delta_jiffies) * TICK_NSEC;

	if (unlikely(clock + delta > max_clock) && check_max(scd)) {
		if (clock < max_clock)
			clock = max_clock;
		else
			clock++;
	} else {
		clock += delta;
	}

 out:
	if (unlikely(clock < min_clock))
		clock = min_clock;

	if (time)
		*time = clock;
	else {
		scd->prev_raw = now;
		scd->clock = clock;
	}
}

static void lock_double_clock(struct sched_clock_data *data1,
				struct sched_clock_data *data2)
{
	if (data1 < data2) {
		__raw_spin_lock(&data1->lock);
		__raw_spin_lock(&data2->lock);
	} else {
		__raw_spin_lock(&data2->lock);
		__raw_spin_lock(&data1->lock);
	}
}

u64 sched_clock_cpu(int cpu)
{
	struct sched_clock_data *scd = cpu_sdc(cpu);
	u64 now, clock;

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

	WARN_ON_ONCE(!irqs_disabled());
	now = sched_clock();

	if (cpu != raw_smp_processor_id()) {
		/*
		 * in order to update a remote cpu's clock based on our
		 * unstable raw time rebase it against:
		 *   tick_raw		(offset between raw counters)
		 *   tick_gotd          (tick offset between cpus)
		 */
		struct sched_clock_data *my_scd = this_scd();

		lock_double_clock(scd, my_scd);

		now -= my_scd->tick_raw;
		now += scd->tick_raw;

		now += my_scd->tick_gtod;
		now -= scd->tick_gtod;

		__raw_spin_unlock(&my_scd->lock);

		__update_sched_clock(scd, now, &clock);

		__raw_spin_unlock(&scd->lock);

	} else {
		__raw_spin_lock(&scd->lock);
		__update_sched_clock(scd, now, NULL);
		clock = scd->clock;
		__raw_spin_unlock(&scd->lock);
	}

	return clock;
}

void sched_clock_tick(void)
{
	struct sched_clock_data *scd = this_scd();
	unsigned long now_jiffies = jiffies;
	u64 now, now_gtod;

	if (unlikely(!sched_clock_running))
		return;

	WARN_ON_ONCE(!irqs_disabled());

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

	__raw_spin_lock(&scd->lock);
	__update_sched_clock(scd, now, NULL);
	/*
	 * update tick_gtod after __update_sched_clock() because that will
	 * already observe 1 new jiffy; adding a new tick_gtod to that would
	 * increase the clock 2 jiffies.
	 */
	scd->tick_jiffies = now_jiffies;
	scd->tick_raw = now;
	scd->tick_gtod = now_gtod;
	__raw_spin_unlock(&scd->lock);
}

/*
 * 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)
{
	struct sched_clock_data *scd = this_scd();
	u64 now = sched_clock();

	/*
	 * Override the previous timestamp and ignore all
	 * sched_clock() deltas that occured while we idled,
	 * and use the PM-provided delta_ns to advance the
	 * rq clock:
	 */
	__raw_spin_lock(&scd->lock);
	scd->prev_raw = now;
	scd->clock += delta_ns;
	__raw_spin_unlock(&scd->lock);

	touch_softlockup_watchdog();
}
EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);

#endif

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