/* * 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 * * Based on code by: * Ingo Molnar * Guillaume Chazarain * * * 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 #include #include #include #include #include #include #include #include #include #include #include /* * 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; void sched_clock_init(void) { sched_clock_running = 1; } #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK /* * We must start with !__sched_clock_stable because the unstable -> stable * transition is accurate, while the stable -> unstable transition is not. * * Similarly we start with __sched_clock_stable_early, thereby assuming we * will become stable, such that there's only a single 1 -> 0 transition. */ static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable); static int __sched_clock_stable_early = 1; /* * We want: ktime_get_ns() + __gtod_offset == sched_clock() + __sched_clock_offset */ __read_mostly u64 __sched_clock_offset; static __read_mostly u64 __gtod_offset; 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); } int sched_clock_stable(void) { return static_branch_likely(&__sched_clock_stable); } static void __scd_stamp(struct sched_clock_data *scd) { scd->tick_gtod = ktime_get_ns(); scd->tick_raw = sched_clock(); } static void __set_sched_clock_stable(void) { struct sched_clock_data *scd = this_scd(); /* * Attempt to make the (initial) unstable->stable transition continuous. */ __sched_clock_offset = (scd->tick_gtod + __gtod_offset) - (scd->tick_raw); printk(KERN_INFO "sched_clock: Marking stable (%lld, %lld)->(%lld, %lld)\n", scd->tick_gtod, __gtod_offset, scd->tick_raw, __sched_clock_offset); static_branch_enable(&__sched_clock_stable); tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE); } /* * If we ever get here, we're screwed, because we found out -- typically after * the fact -- that TSC wasn't good. This means all our clocksources (including * ktime) could have reported wrong values. * * What we do here is an attempt to fix up and continue sort of where we left * off in a coherent manner. * * The only way to fully avoid random clock jumps is to boot with: * "tsc=unstable". */ static void __sched_clock_work(struct work_struct *work) { struct sched_clock_data *scd; int cpu; /* take a current timestamp and set 'now' */ preempt_disable(); scd = this_scd(); __scd_stamp(scd); scd->clock = scd->tick_gtod + __gtod_offset; preempt_enable(); /* clone to all CPUs */ for_each_possible_cpu(cpu) per_cpu(sched_clock_data, cpu) = *scd; printk(KERN_INFO "sched_clock: Marking unstable (%lld, %lld)<-(%lld, %lld)\n", scd->tick_gtod, __gtod_offset, scd->tick_raw, __sched_clock_offset); static_branch_disable(&__sched_clock_stable); } static DECLARE_WORK(sched_clock_work, __sched_clock_work); static void __clear_sched_clock_stable(void) { if (!sched_clock_stable()) return; tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE); schedule_work(&sched_clock_work); } void clear_sched_clock_stable(void) { __sched_clock_stable_early = 0; smp_mb(); /* matches sched_clock_init_late() */ if (sched_clock_running == 2) __clear_sched_clock_stable(); } void sched_clock_init_late(void) { sched_clock_running = 2; /* * 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(); } /* * 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, gtod; 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); */ gtod = scd->tick_gtod + __gtod_offset; clock = gtod + delta; min_clock = wrap_max(gtod, old_clock); max_clock = wrap_max(old_clock, 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() + __sched_clock_offset; 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; } EXPORT_SYMBOL_GPL(sched_clock_cpu); void sched_clock_tick(void) { struct sched_clock_data *scd; WARN_ON_ONCE(!irqs_disabled()); /* * Update these values even if sched_clock_stable(), because it can * become unstable at any point in time at which point we need some * values to fall back on. * * XXX arguably we can skip this if we expose tsc_clocksource_reliable */ scd = this_scd(); __scd_stamp(scd); if (!sched_clock_stable() && likely(sched_clock_running)) 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); #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */ u64 sched_clock_cpu(int cpu) { if (unlikely(!sched_clock_running)) return 0; return sched_clock(); } #endif /* CONFIG_HAVE_UNSTABLE_SCHED_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(); }