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/*
* sched_clock for unstable cpu clocks
*
* Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
*
* 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>

/*
* 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);
}

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);
}

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 &__get_cpu_var(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();
}
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);
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