Re: 2.6.24-rc8-rt1: Strange latencies on mpc5200 powerpc - RCU issue?

[Wolfgang Grandegger <wg@grandegger.com>]

Steven Rostedt wrote:
> On Wed, 2 Jul 2008, Wolfgang Grandegger wrote:
>> 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.