sched_clock: fix calculation of other CPU

The algorithm to calculate the 'now' of another CPU is not correct.
At each scheduler tick, each CPU records the last sched_clock and
gtod (tick_raw and tick_gtod respectively). If the TSC is somewhat the
same in speed between two clocks the algorithm would be:

  tick_gtod1 + (now1 - tick_raw1) = tick_gtod2 + (now2 - tick_raw2)

To calculate now2 we would have:

  now2 = (tick_gtod1 - tick_gtod2) + (tick_raw2 - tick_raw1) + now1

Currently the algorithm is:

  now2 = (tick_gtod1 - tick_gtod2) + (tick_raw1 - tick_raw2) + now1

This solves most of the rest of the issues I've had with timestamps in
ftace.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_clock.c

@@ -203,8 +203,8 @@ u64 sched_clock_cpu(int cpu)
 		now -= my_scd->tick_raw;
 		now += scd->tick_raw;
 
-		now -= my_scd->tick_gtod;
-		now += scd->tick_gtod;
+		now += my_scd->tick_gtod;
+		now -= scd->tick_gtod;
 
 		__raw_spin_unlock(&my_scd->lock);
 	} else {