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![Peter Zijlstra](/assets/img/avatar_default.png)
The code would assume sched_clock_stable() and switch to !stable
later, this switch brings a discontinuity in time.
The discontinuity on switching from stable to unstable was always
present, but previously we would set stable/unstable before
initializing TSC and usually stick to the one we start out with.
So the static_key bits brought an extra switch where there previously
wasn't one.
Things are further complicated by the fact that we cannot use
static_key as early as we usually call set_sched_clock_stable().
Fix things by tracking the stable state in a regular variable and only
set the static_key to the right state on sched_clock_init(), which is
ran right after late_time_init->tsc_init().
Before this we would not be using the TSC anyway.
Reported-and-Tested-by: Sasha Levin <sasha.levin@oracle.com>
Reported-by: dyoung@redhat.com
Fixes: 35af99e646
("sched/clock, x86: Use a static_key for sched_clock_stable")
Cc: jacob.jun.pan@linux.intel.com
Cc: Mike Galbraith <bitbucket@online.de>
Cc: hpa@zytor.com
Cc: paulmck@linux.vnet.ibm.com
Cc: John Stultz <john.stultz@linaro.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: lenb@kernel.org
Cc: rjw@rjwysocki.net
Cc: Eliezer Tamir <eliezer.tamir@linux.intel.com>
Cc: rui.zhang@intel.com
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140122115918.GG3694@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
421 lines
9.7 KiB
C
421 lines
9.7 KiB
C
/*
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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 <pzijlstr@redhat.com>
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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:
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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:
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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 <linux/spinlock.h>
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#include <linux/hardirq.h>
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#include <linux/export.h>
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#include <linux/percpu.h>
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#include <linux/ktime.h>
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#include <linux/sched.h>
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#include <linux/static_key.h>
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#include <linux/workqueue.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 __attribute__((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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__read_mostly int sched_clock_running;
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#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
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static struct static_key __sched_clock_stable = STATIC_KEY_INIT;
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static int __sched_clock_stable_early;
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int sched_clock_stable(void)
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{
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return static_key_false(&__sched_clock_stable);
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}
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static void __set_sched_clock_stable(void)
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{
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if (!sched_clock_stable())
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static_key_slow_inc(&__sched_clock_stable);
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}
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void set_sched_clock_stable(void)
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{
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__sched_clock_stable_early = 1;
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smp_mb(); /* matches sched_clock_init() */
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if (!sched_clock_running)
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return;
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__set_sched_clock_stable();
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}
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static void __clear_sched_clock_stable(struct work_struct *work)
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{
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/* XXX worry about clock continuity */
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if (sched_clock_stable())
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static_key_slow_dec(&__sched_clock_stable);
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}
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static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);
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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() */
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if (!sched_clock_running)
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return;
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schedule_work(&sched_clock_work);
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}
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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 &__get_cpu_var(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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void sched_clock_init(void)
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{
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u64 ktime_now = ktime_to_ns(ktime_get());
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int cpu;
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for_each_possible_cpu(cpu) {
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struct sched_clock_data *scd = cpu_sdc(cpu);
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scd->tick_raw = 0;
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scd->tick_gtod = ktime_now;
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scd->clock = ktime_now;
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}
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sched_clock_running = 1;
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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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else
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__clear_sched_clock_stable(NULL);
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}
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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;
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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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clock = scd->tick_gtod + delta;
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min_clock = wrap_max(scd->tick_gtod, old_clock);
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max_clock = wrap_max(old_clock, scd->tick_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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* 32bit 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 low32bit and the high 32bit 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 32bit, otherwise the
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* update on the remote cpu can hit inbetween the readout of
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* the low32bit and the high 32bit 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 64bit the read of [my]scd->clock is atomic versus the
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* update, so we can avoid the above 32bit 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();
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if (unlikely(!sched_clock_running))
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return 0ull;
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preempt_disable();
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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();
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return clock;
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}
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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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u64 now, now_gtod;
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if (sched_clock_stable())
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return;
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if (unlikely(!sched_clock_running))
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return;
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WARN_ON_ONCE(!irqs_disabled());
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scd = this_scd();
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now_gtod = ktime_to_ns(ktime_get());
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now = sched_clock();
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scd->tick_raw = now;
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scd->tick_gtod = now_gtod;
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sched_clock_local(scd);
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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 delta nanoseconds (called with irqs disabled):
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*/
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void sched_clock_idle_wakeup_event(u64 delta_ns)
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{
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if (timekeeping_suspended)
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return;
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sched_clock_tick();
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touch_softlockup_watchdog();
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}
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EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
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/*
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* As outlined at the top, provides a fast, high resolution, nanosecond
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* time source that is monotonic per cpu argument and has bounded drift
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* between cpus.
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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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u64 cpu_clock(int cpu)
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{
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if (!sched_clock_stable())
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return sched_clock_cpu(cpu);
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return sched_clock();
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}
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/*
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* Similar to cpu_clock() for the current cpu. Time will only be observed
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* to be monotonic if care is taken to only compare timestampt taken on the
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* same CPU.
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*
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* See cpu_clock().
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*/
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u64 local_clock(void)
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{
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if (!sched_clock_stable())
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return sched_clock_cpu(raw_smp_processor_id());
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return sched_clock();
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}
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#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
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void sched_clock_init(void)
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{
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sched_clock_running = 1;
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}
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u64 sched_clock_cpu(int cpu)
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{
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if (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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u64 cpu_clock(int cpu)
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{
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return sched_clock();
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}
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u64 local_clock(void)
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{
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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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EXPORT_SYMBOL_GPL(cpu_clock);
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EXPORT_SYMBOL_GPL(local_clock);
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