/* * 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. * sched_clock_cpu(i) -- must be used with local IRQs disabled (implied by NMI) * local_clock() -- is cpu_clock() on the current cpu. * * 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). * * * Notes: * * The !IRQ-safetly of sched_clock() and sched_clock_cpu() comes from things * like cpufreq interrupts that can change the base clock (TSC) multiplier * and cause funny jumps in time -- although the filtering provided by * sched_clock_cpu() should mitigate serious artifacts we cannot rely on it * in general since for !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK we fully rely on * sched_clock(). */ #include <linux/spinlock.h> #include <linux/hardirq.h> #include <linux/module.h> #include <linux/percpu.h> #include <linux/ktime.h> #include <linux/sched.h> /* * 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 - INITIAL_JIFFIES) * (NSEC_PER_SEC / HZ); } EXPORT_SYMBOL_GPL(sched_clock); static __read_mostly int sched_clock_running; #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK __read_mostly int sched_clock_stable; 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; } /* * 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; sched_clock_local(my_scd); again: this_clock = my_scd->clock; remote_clock = scd->clock; /* * 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; WARN_ON_ONCE(!irqs_disabled()); if (sched_clock_stable) return sched_clock(); if (unlikely(!sched_clock_running)) return 0ull; scd = cpu_sdc(cpu); if (cpu != smp_processor_id()) clock = sched_clock_remote(scd); else clock = sched_clock_local(scd); 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) { u64 clock; unsigned long flags; local_irq_save(flags); clock = sched_clock_cpu(cpu); local_irq_restore(flags); return 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) { u64 clock; unsigned long flags; local_irq_save(flags); clock = sched_clock_cpu(smp_processor_id()); local_irq_restore(flags); return 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_cpu(cpu); } u64 local_clock(void) { return sched_clock_cpu(0); } #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */ EXPORT_SYMBOL_GPL(cpu_clock); EXPORT_SYMBOL_GPL(local_clock);