[Added comment about not being able to take the xtime lock in
get_monotonic_cycles - suggested by John Stultz]
The latency tracer on SMP was given crazy results. It was found that the
get_monotonic_cycles that it uses was not returning a monotonic counter.
The cause of this was that clock->cycles_raw and clock->cycles_last can
be updated on another CPU and make the cycles_now variable out-of-date.
So the delta that was calculated from cycles_now - cycles_last was
incorrect.
This patch adds a loop to make sure that the cycles_raw and cycles_last
are consistent through out the calculation (otherwise it performs the
loop again).
With this patch the latency_tracer can produce normal results again.
Signed-off-by: Steven Rostedt <[EMAIL PROTECTED]>
Index: linux-2.6-rt/kernel/time/timekeeping.c
===================================================================
--- linux-2.6-rt.orig/kernel/time/timekeeping.c 2007-08-24 13:41:28.000000000
-0400
+++ linux-2.6-rt/kernel/time/timekeeping.c 2007-08-24 14:58:02.000000000
-0400
@@ -75,15 +75,35 @@ s64 __get_nsec_offset(void)
cycle_t notrace get_monotonic_cycles(void)
{
- cycle_t cycle_now, cycle_delta;
+ cycle_t cycle_now, cycle_delta, cycle_raw, cycle_last;
- /* read clocksource: */
- cycle_now = clocksource_read(clock);
+ do {
+ /*
+ * cycle_raw and cycle_last can change on
+ * another CPU and we need the delta calculation
+ * of cycle_now and cycle_last happen atomic, as well
+ * as the adding to cycle_raw. We don't need to grab
+ * any locks, we just keep trying until get all the
+ * calculations together in one state.
+ *
+ * In fact, we __cant__ grab any locks. This
+ * function is called from the latency_tracer which can
+ * be called anywhere. To grab any locks (including
+ * seq_locks) we risk putting ourselves into a deadlock.
+ */
+ cycle_raw = clock->cycle_raw;
+ cycle_last = clock->cycle_last;
+
+ /* read clocksource: */
+ cycle_now = clocksource_read(clock);
+
+ /* calculate the delta since the last update_wall_time: */
+ cycle_delta = (cycle_now - cycle_last) & clock->mask;
- /* calculate the delta since the last update_wall_time: */
- cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
+ } while (cycle_raw != clock->cycle_raw ||
+ cycle_last != clock->cycle_last);
- return clock->cycle_raw + cycle_delta;
+ return cycle_raw + cycle_delta;
}
unsigned long notrace cycles_to_usecs(cycle_t cycles)
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
