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

[Wolfgang Grandegger]

Paul E. McKenney wrote:
> On Wed, Jan 30, 2008 at 11:45:01AM +0100, Wolfgang Grandegger wrote:
>> Paul E. McKenney wrote:
>>> On Wed, Jan 30, 2008 at 09:18:49AM +0100, Wolfgang Grandegger wrote:
>>>> 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.
>>> OK, thank you for the confirmation.
>>>
>>> The large latencies were from cyclictest, correct?  Did other tests
>>> also show these latencies?  In either case, could you please send the
>>> exact command line you used to run the test?
>> Yes, I used "$ cyclictest -n -t1 -p80 -i1000" to measure the latency. So
>> far, I have not done other tests. Any recommendation?
>> As no-rt load I used "while ls; do ls /bin; done" in one telnet window
>> and "while ./hackbench 10; do ./calibrator 400 32M cali; sleep 30; done"
>> in another. But already "while ls; do ls /bin; done" is enough to
>> trigger the high latencies quickly. Note also, that I work on a root
>> files-ystem mounted via NFS resulting in a lot of network traffic and
>> utilization.
> 
> I have to ask...
> 
> Did you see large latencies when -not- running on NFS?

I have not made test without NFS, but Fu has and he said:

"Further such results only appear if the target board is booted with
nfsrootfs."

Wolfgang.

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