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>

1 files changed, 15 insertions, 9 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index f98f75f3c708..9457106b18af 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -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;
 }