Steven Rostedt wrote:

On Wed, 2 Jul 2008, Wolfgang Grandegger wrote:

quoted

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.

This to me sounds like we have preempt_enable_noresched someplace that
shouldn't. In otherwords, we enabled preemption without checking if
preemption is needed, which is a serious bug in -rt.

Yep, in 2.6.25.8-rt7 trace_mark() is not used any more, maybe for that 
reason. Nevertheless, this "bug" kept the latency on my MPC5200 box low.
I will try with the old tracer to understand what the real cause of the 
high latencies is (with CONFIG_RCU_TRACE disabled).

Wolfgang.