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Commit 27ec4407 authored by Ingo Molnar's avatar Ingo Molnar
Browse files

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: default avatarIngo Molnar <mingo@elte.hu>
parent 018d6db4

kernel/sched.c

+49 −3
Original line number Diff line number Diff line
@@ -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