Messages in this thread | | | Date | Sun, 06 Jul 2008 11:41:28 +0200 | From | Wolfgang Grandegger <> | Subject | Re: 2.6.24-rc8-rt1: Strange latencies on mpc5200 powerpc - RCU issue? |
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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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