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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: David Miller <davem@davemloft.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Ingo Molnar 2008-04-23 09:24:06 +02:00
parent cb4ad1ffc7
commit dfbf4a1bc3
1 changed files with 15 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
kernel/sched.c
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@ -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;
}