Paul E. McKenney wrote: > On Tue, Jan 29, 2008 at 02:38:04PM +0100, Wolfgang Grandegger wrote: >> Luotao Fu wrote: >>> Hi, >>> >>> Wolfgang Grandegger wrote: >>> .......... >>>> Do you still get high latencies with: >>>> >>>> CONFIG_PREEMPT_RCU_BOOST=y >>>> CONFIG_RCU_TRACE=y >>>> CONFIG_NO_HZ is not set >>>> >>>> With this setting I have not yet realized latencies > 150us. Could you >>>> please give it a try? If I change one of the parameters above, latency >>>> increases in short time. >>> I played through some combination of the RCU options and can back your >>> observation this time: With the rcu Tracer or the priority boost turned >>> off I also could measure reliably extraordinory high latencies. If they >>> are both turned on, no high latencies could be measured. Turning on the >>> dynamic ticker however doesn't seem to cause high latencies during my >>> test runs. Seemed like an rcu issue here. >> I'm just making a long test run on my TQM5200 module with my good >> settings. After more than 4.5 hours under load, cyclictest shows a >> maximum latency of 177 us. I'm going to re-check the effect of CONFIG_NO_HZ. >> >>> Further such results only appear if the target board is booted with >>> nfsrootfs. (As I already have mentioned several times before), which >>> leads my suspection to rcu usage in nfs implementation. In this case >>> this problem might even be platformindependent. I'd have to do some >>> tests on one of our arm boards later to test this. Since there're no >>> reports like this for other architecture as powerpc till now, I doubt >>> quite if this is verifiable. >> It's also my suspicion that the high latencies are related to the RCU >> usage in the network layer, where it's heavily used. What is really >> wired is that switching off CONFIG_RCU_TRACE has a negative impact on >> the latency. As I see it, it just adds some trace points, but I might >> have missed something. > > I would expect that CONFIG_RCU_TRACE=n (as in "no" rather than "module") > would have low latencies rather than high latencies. So I am quite > surprised by your result. I will dig into this more. Thanks a lot. To be clear. I need "CONFIG_PREEMPT_RCU_BOOST=y" *and* "CONFIG_RCU_TRACE=y" to achieve reasonable latencies below 180us. With CONFIG_PREEMPT_RCU_BOOST or CONFIG_RCU_TRACE not set or CONFIG_RCU_TRACE=m is rmeasure latencies up to 600us within a minute or so. Wolfgang. > Thanx, Paul > - > To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > > - To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
