sched: Fix cross-cpu clock sync on remote wakeups

Markus reported that commit 317f3941 ("sched: Move the second half
of ttwu() to the remote cpu") caused some accounting funnies on his AMD
Phenom II X4, such as weird 'top' results.

It turns out that this is due to non-synced TSC and the queued remote
wakeups stopped coupeling the two relevant cpu clocks, which leads to
wakeups seeing time jumps, which in turn lead to skewed runtime stats.

Add an explicit call to sched_clock_cpu() to couple the per-cpu clocks
to restore the normal flow of time.
Reported-and-tested-by: NMarkus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1306835745.2353.3.camel@twinsSigned-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -2600,6 +2600,7 @@ static void ttwu_queue(struct task_struct *p, int cpu)
 
 #if defined(CONFIG_SMP)
 	if (sched_feat(TTWU_QUEUE) && cpu != smp_processor_id()) {
+		sched_clock_cpu(cpu); /* sync clocks x-cpu */
 		ttwu_queue_remote(p, cpu);
 		return;
 	}