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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