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

[Wolfgang Grandegger]

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.
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