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

[Wolfgang Grandegger <wg@grandegger.com>]

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.

Thanks,

Wolfgang.