/*
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* sched_clock() for unstable CPU clocks
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*
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* Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra
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*
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* Updates and enhancements:
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* Copyright (C) 2008 Red Hat, Inc. Steven Rostedt <srostedt@redhat.com>
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*
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* Based on code by:
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* Ingo Molnar <mingo@redhat.com>
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* Guillaume Chazarain <guichaz@gmail.com>
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*
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*
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* What this file implements:
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*
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* cpu_clock(i) provides a fast (execution time) high resolution
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* clock with bounded drift between CPUs. The value of cpu_clock(i)
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* is monotonic for constant i. The timestamp returned is in nanoseconds.
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*
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* ######################### BIG FAT WARNING ##########################
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* # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
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* # go backwards !! #
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* ####################################################################
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*
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* There is no strict promise about the base, although it tends to start
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* at 0 on boot (but people really shouldn't rely on that).
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*
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* cpu_clock(i) -- can be used from any context, including NMI.
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* local_clock() -- is cpu_clock() on the current CPU.
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*
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* sched_clock_cpu(i)
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*
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* How it is implemented:
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*
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* The implementation either uses sched_clock() when
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* !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
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* sched_clock() is assumed to provide these properties (mostly it means
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* the architecture provides a globally synchronized highres time source).
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*
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* Otherwise it tries to create a semi stable clock from a mixture of other
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* clocks, including:
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*
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* - GTOD (clock monotomic)
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* - sched_clock()
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* - explicit idle events
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*
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* We use GTOD as base and use sched_clock() deltas to improve resolution. The
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* deltas are filtered to provide monotonicity and keeping it within an
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* expected window.
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*
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* Furthermore, explicit sleep and wakeup hooks allow us to account for time
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* that is otherwise invisible (TSC gets stopped).
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*
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*/
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#include "sched.h"
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#include <linux/sched_clock.h>
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/*
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* Scheduler clock - returns current time in nanosec units.
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* This is default implementation.
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* Architectures and sub-architectures can override this.
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*/
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unsigned long long __weak sched_clock(void)
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{
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return (unsigned long long)(jiffies - INITIAL_JIFFIES)
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* (NSEC_PER_SEC / HZ);
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}
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EXPORT_SYMBOL_GPL(sched_clock);
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static DEFINE_STATIC_KEY_FALSE(sched_clock_running);
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#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
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/*
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* We must start with !__sched_clock_stable because the unstable -> stable
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* transition is accurate, while the stable -> unstable transition is not.
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*
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* Similarly we start with __sched_clock_stable_early, thereby assuming we
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* will become stable, such that there's only a single 1 -> 0 transition.
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*/
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static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
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static int __sched_clock_stable_early = 1;
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/*
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* We want: ktime_get_ns() + __gtod_offset == sched_clock() + __sched_clock_offset
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*/
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__read_mostly u64 __sched_clock_offset;
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static __read_mostly u64 __gtod_offset;
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struct sched_clock_data {
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u64 tick_raw;
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u64 tick_gtod;
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u64 clock;
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};
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static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
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static inline struct sched_clock_data *this_scd(void)
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{
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return this_cpu_ptr(&sched_clock_data);
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}
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static inline struct sched_clock_data *cpu_sdc(int cpu)
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{
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return &per_cpu(sched_clock_data, cpu);
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}
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int sched_clock_stable(void)
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{
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return static_branch_likely(&__sched_clock_stable);
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}
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static void __scd_stamp(struct sched_clock_data *scd)
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{
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scd->tick_gtod = ktime_get_ns();
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scd->tick_raw = sched_clock();
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}
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static void __set_sched_clock_stable(void)
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{
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struct sched_clock_data *scd;
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/*
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* Since we're still unstable and the tick is already running, we have
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* to disable IRQs in order to get a consistent scd->tick* reading.
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*/
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local_irq_disable();
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scd = this_scd();
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/*
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* Attempt to make the (initial) unstable->stable transition continuous.
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*/
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__sched_clock_offset = (scd->tick_gtod + __gtod_offset) - (scd->tick_raw);
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local_irq_enable();
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printk(KERN_INFO "sched_clock: Marking stable (%lld, %lld)->(%lld, %lld)\n",
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scd->tick_gtod, __gtod_offset,
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scd->tick_raw, __sched_clock_offset);
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static_branch_enable(&__sched_clock_stable);
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tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
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}
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/*
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* If we ever get here, we're screwed, because we found out -- typically after
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* the fact -- that TSC wasn't good. This means all our clocksources (including
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* ktime) could have reported wrong values.
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*
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* What we do here is an attempt to fix up and continue sort of where we left
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* off in a coherent manner.
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*
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* The only way to fully avoid random clock jumps is to boot with:
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* "tsc=unstable".
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*/
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static void __sched_clock_work(struct work_struct *work)
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{
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struct sched_clock_data *scd;
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int cpu;
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/* take a current timestamp and set 'now' */
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preempt_disable();
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scd = this_scd();
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__scd_stamp(scd);
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scd->clock = scd->tick_gtod + __gtod_offset;
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preempt_enable();
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/* clone to all CPUs */
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for_each_possible_cpu(cpu)
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per_cpu(sched_clock_data, cpu) = *scd;
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printk(KERN_WARNING "TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'.\n");
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printk(KERN_INFO "sched_clock: Marking unstable (%lld, %lld)<-(%lld, %lld)\n",
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scd->tick_gtod, __gtod_offset,
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scd->tick_raw, __sched_clock_offset);
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static_branch_disable(&__sched_clock_stable);
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}
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static DECLARE_WORK(sched_clock_work, __sched_clock_work);
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static void __clear_sched_clock_stable(void)
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{
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if (!sched_clock_stable())
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return;
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tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE);
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schedule_work(&sched_clock_work);
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}
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void clear_sched_clock_stable(void)
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{
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__sched_clock_stable_early = 0;
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smp_mb(); /* matches sched_clock_init_late() */
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if (static_key_count(&sched_clock_running.key) == 2)
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__clear_sched_clock_stable();
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}
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static void __sched_clock_gtod_offset(void)
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{
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struct sched_clock_data *scd = this_scd();
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__scd_stamp(scd);
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__gtod_offset = (scd->tick_raw + __sched_clock_offset) - scd->tick_gtod;
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}
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void __init sched_clock_init(void)
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{
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/*
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* Set __gtod_offset such that once we mark sched_clock_running,
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* sched_clock_tick() continues where sched_clock() left off.
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*
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* Even if TSC is buggered, we're still UP at this point so it
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* can't really be out of sync.
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*/
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local_irq_disable();
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__sched_clock_gtod_offset();
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local_irq_enable();
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static_branch_inc(&sched_clock_running);
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}
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/*
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* We run this as late_initcall() such that it runs after all built-in drivers,
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* notably: acpi_processor and intel_idle, which can mark the TSC as unstable.
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*/
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static int __init sched_clock_init_late(void)
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{
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static_branch_inc(&sched_clock_running);
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/*
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* Ensure that it is impossible to not do a static_key update.
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*
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* Either {set,clear}_sched_clock_stable() must see sched_clock_running
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* and do the update, or we must see their __sched_clock_stable_early
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* and do the update, or both.
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*/
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smp_mb(); /* matches {set,clear}_sched_clock_stable() */
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if (__sched_clock_stable_early)
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__set_sched_clock_stable();
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return 0;
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}
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late_initcall(sched_clock_init_late);
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/*
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* min, max except they take wrapping into account
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*/
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static inline u64 wrap_min(u64 x, u64 y)
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{
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return (s64)(x - y) < 0 ? x : y;
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}
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static inline u64 wrap_max(u64 x, u64 y)
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{
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return (s64)(x - y) > 0 ? x : y;
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}
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/*
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* update the percpu scd from the raw @now value
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*
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* - filter out backward motion
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* - use the GTOD tick value to create a window to filter crazy TSC values
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*/
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static u64 sched_clock_local(struct sched_clock_data *scd)
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{
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u64 now, clock, old_clock, min_clock, max_clock, gtod;
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s64 delta;
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again:
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now = sched_clock();
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delta = now - scd->tick_raw;
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if (unlikely(delta < 0))
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delta = 0;
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old_clock = scd->clock;
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/*
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* scd->clock = clamp(scd->tick_gtod + delta,
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* max(scd->tick_gtod, scd->clock),
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* scd->tick_gtod + TICK_NSEC);
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*/
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gtod = scd->tick_gtod + __gtod_offset;
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clock = gtod + delta;
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min_clock = wrap_max(gtod, old_clock);
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max_clock = wrap_max(old_clock, gtod + TICK_NSEC);
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clock = wrap_max(clock, min_clock);
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clock = wrap_min(clock, max_clock);
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if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
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goto again;
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return clock;
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}
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static u64 sched_clock_remote(struct sched_clock_data *scd)
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{
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struct sched_clock_data *my_scd = this_scd();
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u64 this_clock, remote_clock;
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u64 *ptr, old_val, val;
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#if BITS_PER_LONG != 64
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again:
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/*
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* Careful here: The local and the remote clock values need to
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* be read out atomic as we need to compare the values and
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* then update either the local or the remote side. So the
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* cmpxchg64 below only protects one readout.
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*
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* We must reread via sched_clock_local() in the retry case on
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* 32-bit kernels as an NMI could use sched_clock_local() via the
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* tracer and hit between the readout of
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* the low 32-bit and the high 32-bit portion.
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*/
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this_clock = sched_clock_local(my_scd);
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/*
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* We must enforce atomic readout on 32-bit, otherwise the
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* update on the remote CPU can hit inbetween the readout of
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* the low 32-bit and the high 32-bit portion.
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*/
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remote_clock = cmpxchg64(&scd->clock, 0, 0);
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#else
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/*
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* On 64-bit kernels the read of [my]scd->clock is atomic versus the
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* update, so we can avoid the above 32-bit dance.
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*/
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sched_clock_local(my_scd);
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again:
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this_clock = my_scd->clock;
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remote_clock = scd->clock;
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#endif
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/*
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* Use the opportunity that we have both locks
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* taken to couple the two clocks: we take the
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* larger time as the latest time for both
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* runqueues. (this creates monotonic movement)
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*/
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if (likely((s64)(remote_clock - this_clock) < 0)) {
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ptr = &scd->clock;
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old_val = remote_clock;
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val = this_clock;
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} else {
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/*
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* Should be rare, but possible:
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*/
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ptr = &my_scd->clock;
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old_val = this_clock;
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val = remote_clock;
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}
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if (cmpxchg64(ptr, old_val, val) != old_val)
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goto again;
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return val;
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}
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/*
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* Similar to cpu_clock(), but requires local IRQs to be disabled.
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*
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* See cpu_clock().
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*/
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u64 sched_clock_cpu(int cpu)
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{
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struct sched_clock_data *scd;
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u64 clock;
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if (sched_clock_stable())
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return sched_clock() + __sched_clock_offset;
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if (!static_branch_unlikely(&sched_clock_running))
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return sched_clock();
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preempt_disable_notrace();
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scd = cpu_sdc(cpu);
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if (cpu != smp_processor_id())
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clock = sched_clock_remote(scd);
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else
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clock = sched_clock_local(scd);
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preempt_enable_notrace();
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return clock;
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}
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EXPORT_SYMBOL_GPL(sched_clock_cpu);
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void sched_clock_tick(void)
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{
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struct sched_clock_data *scd;
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if (sched_clock_stable())
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return;
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if (!static_branch_unlikely(&sched_clock_running))
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return;
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lockdep_assert_irqs_disabled();
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scd = this_scd();
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__scd_stamp(scd);
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sched_clock_local(scd);
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}
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void sched_clock_tick_stable(void)
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{
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if (!sched_clock_stable())
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return;
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/*
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* Called under watchdog_lock.
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*
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* The watchdog just found this TSC to (still) be stable, so now is a
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* good moment to update our __gtod_offset. Because once we find the
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* TSC to be unstable, any computation will be computing crap.
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*/
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local_irq_disable();
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__sched_clock_gtod_offset();
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local_irq_enable();
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}
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/*
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* We are going deep-idle (irqs are disabled):
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*/
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void sched_clock_idle_sleep_event(void)
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{
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sched_clock_cpu(smp_processor_id());
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}
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EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
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/*
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* We just idled; resync with ktime.
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*/
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void sched_clock_idle_wakeup_event(void)
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{
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unsigned long flags;
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if (sched_clock_stable())
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return;
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if (unlikely(timekeeping_suspended))
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return;
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local_irq_save(flags);
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sched_clock_tick();
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local_irq_restore(flags);
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}
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EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
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#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
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void __init sched_clock_init(void)
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{
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static_branch_inc(&sched_clock_running);
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local_irq_disable();
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generic_sched_clock_init();
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local_irq_enable();
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}
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u64 sched_clock_cpu(int cpu)
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{
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if (!static_branch_unlikely(&sched_clock_running))
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return 0;
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return sched_clock();
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}
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#endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
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/*
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* Running clock - returns the time that has elapsed while a guest has been
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* running.
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* On a guest this value should be local_clock minus the time the guest was
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* suspended by the hypervisor (for any reason).
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* On bare metal this function should return the same as local_clock.
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* Architectures and sub-architectures can override this.
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*/
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u64 __weak running_clock(void)
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{
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return local_clock();
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}
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