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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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.

							Thanx, Paul