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

[Wolfgang Grandegger]

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
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