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Commit dfbf4a1b authored by Ingo Molnar's avatar Ingo Molnar
Browse files

sched: fix cpu clock 



David Miller pointed it out that nothing in cpu_clock() sets
prev_cpu_time. This caused __sync_cpu_clock() to be called
all the time - against the intention of this code.

The result was that in practice we hit a global spinlock every
time cpu_clock() is called - which - even though cpu_clock()
is used for tracing and debugging, is suboptimal.

While at it, also:

- move the irq disabling to the outest layer,
  this should make cpu_clock() warp-free when called with irqs
  enabled.

- use long long instead of cycles_t - for platforms where cycles_t
  is 32-bit.

Reported-by: default avatarDavid Miller <davem@davemloft.net>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent cb4ad1ff

kernel/sched.c

+15 −9
Original line number Diff line number Diff line
@@ -910,11 +910,14 @@ static DEFINE_PER_CPU(unsigned long long, prev_cpu_time); 
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)

static unsigned long long __sync_cpu_clock(unsigned long long time, int cpu)

{

	unsigned long flags;



	spin_lock_irqsave(&time_sync_lock, flags);

	/*

	 * We want this inlined, to not get tracer function calls

	 * in this critical section:

	 */

	spin_acquire(&time_sync_lock.dep_map, 0, 0, _THIS_IP_);

	__raw_spin_lock(&time_sync_lock.raw_lock);



	if (time < prev_global_time) {

		per_cpu(time_offset, cpu) += prev_global_time - time;
@@ -923,7 +926,8 @@ static unsigned long long __sync_cpu_clock(cycles_t time, int cpu) 
		prev_global_time = time;

	}



	spin_unlock_irqrestore(&time_sync_lock, flags);

	__raw_spin_unlock(&time_sync_lock.raw_lock);

	spin_release(&time_sync_lock.dep_map, 1, _THIS_IP_);



	return time;

}
@@ -931,7 +935,6 @@ static unsigned long long __sync_cpu_clock(cycles_t time, int cpu) 
static unsigned long long __cpu_clock(int cpu)

{

	unsigned long long now;

	unsigned long flags;

	struct rq *rq;



	/*
@@ -941,11 +944,9 @@ static unsigned long long __cpu_clock(int cpu) 
	if (unlikely(!scheduler_running))

		return 0;



	local_irq_save(flags);

	rq = cpu_rq(cpu);

	update_rq_clock(rq);

	now = rq->clock;

	local_irq_restore(flags);



	return now;

}
@@ -957,13 +958,18 @@ static unsigned long long __cpu_clock(int cpu) 
unsigned long long cpu_clock(int cpu)

{

	unsigned long long prev_cpu_time, time, delta_time;

	unsigned long flags;



	local_irq_save(flags);

	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))

	if (unlikely(delta_time > time_sync_thresh)) {

		time = __sync_cpu_clock(time, cpu);

		per_cpu(prev_cpu_time, cpu) = time;

	}

	local_irq_restore(flags);



	return time;

}