Paul E. McKenney wrote:

On Wed, Jul 02, 2008 at 01:03:27PM +0200, Wolfgang Grandegger wrote:

quoted

Wolfgang Grandegger wrote:

quoted

Steven Rostedt wrote:
There seems to be something in trace_mark() keeping latency low:
  http://lxr.linux.no/linux+v2.6.24.4/include/linux/marker.h#L52
I will follow your suggestions.

I removed all "likely" and "unlikely" macros, but the latencies did not 
improve. Then I added

	preempt_disable();
	preempt_enable();

at the two locations mentioned above, like trace_mark() does,  and disabled 
CONFIG_RCU_TRACE. That helped to keep the latencies low, and it did for 
2.6.25.8-rt7 as well. As I see it, adding preemption points seem to prevent 
high latencies.

Interesting.  Perhaps a preempt_enable_no_resched() somewhere that
should be preempt_enable()?

FYI, Thomas fixed the real problem a week ago and the patch went into 
2.6.24.7-rt16. See his mail with the subject "2.6.24.7-rt16".

Wolfgang.