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sched: make cpu_clock() globally synchronous 

Alexey Zaytsev reported (and bisected) that the introduction of
cpu_clock() in printk made the timestamps jump back and forth.

Make cpu_clock() more reliable while still keeping it fast when it's
called frequently.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Ingo Molnar 2008-02-28 21:00:21 +01:00
parent 018d6db4cb
commit 27ec440779
1 changed files with 49 additions and 3 deletions
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52
kernel/sched.c
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@ -632,11 +632,39 @@ int sysctl_sched_rt_runtime = 950000;
*/
#define RUNTIME_INF ((u64)~0ULL)
static const unsigned long long time_sync_thresh = 100000;
static DEFINE_PER_CPU(unsigned long long, time_offset);
static DEFINE_PER_CPU(unsigned long long, prev_cpu_time);
/*
* For kernel-internal use: high-speed (but slightly incorrect) per-cpu
* clock constructed from sched_clock():
* Global lock which we take every now and then to synchronize
* the CPUs time. This method is not warp-safe, but it's good
* enough to synchronize slowly diverging time sources and thus
* it's good enough for tracing:
*/
unsigned long long cpu_clock(int cpu)
static DEFINE_SPINLOCK(time_sync_lock);
static unsigned long long prev_global_time;
static unsigned long long __sync_cpu_clock(cycles_t time, int cpu)
{
unsigned long flags;
spin_lock_irqsave(&time_sync_lock, flags);
if (time < prev_global_time) {
per_cpu(time_offset, cpu) += prev_global_time - time;
time = prev_global_time;
} else {
prev_global_time = time;
}
spin_unlock_irqrestore(&time_sync_lock, flags);
return time;
}
static unsigned long long __cpu_clock(int cpu)
{
unsigned long long now;
unsigned long flags;
@ -657,6 +685,24 @@ unsigned long long cpu_clock(int cpu)
return now;
}
/*
* For kernel-internal use: high-speed (but slightly incorrect) per-cpu
* clock constructed from sched_clock():
*/
unsigned long long cpu_clock(int cpu)
{
unsigned long long prev_cpu_time, time, delta_time;
prev_cpu_time = per_cpu(prev_cpu_time, cpu);
time = __cpu_clock(cpu) + per_cpu(time_offset, cpu);
delta_time = time-prev_cpu_time;
if (unlikely(delta_time > time_sync_thresh))
time = __sync_cpu_clock(time, cpu);
return time;
}
EXPORT_SYMBOL_GPL(cpu_clock);
#ifndef prepare_arch_switch