Re: [patch] voluntary-preempt-2.6.8-rc3-O5

[Ingo Molnar <mingo@elte.hu>]

* Florian Schmidt <mista.tapas@gmx.net> wrote:

> i think that the mlockall and client/jackd startup xruns often do not
> seem to correspond to a critical timing report.. Try the following:
> turn off xrun_debug but leave the preempt-timing stuff on. On my
> system, the mlockall_test provokes an xrun in jackd's output but i do
> not get a preempt-timing report (thresh = 500). 
> 
> OTOH when the xrun_debug is on, the xrun_debug report actually seems
> to trigger the preempt-timing report.

this later phenomenon is expected and unrelated: printk-ing to the
console (as the ALSA xrun kernel message does) is quite expensive,
especially with a full stack dump included. So this fact alone doesnt
tell us much about why the xrun itself happened.

if there is no preempt-timing report when the ALSA xrun debugging is
disabled it strongly suggests that whatever causes the xrun, it's not
due to the length of a non-preemptible critical section, but some other
phenomenon (either in userspace or in kernelspace) that causes an xrun.

irqs being left disabled by accident is one such possibility - the
preempt-timing patch does not (yet) track irqs-off sections.

> I think many of the jackd xruns are really jacks business. But maybe i
> misinterpret the symptom.

either jack's or the kernel's - but it's less likely to be the classical
non-preemptible sections we were focused on so far. (it could still be a
bug in the preempt-timing patch producing a false negative - but the
likelyhood is low i'd say.)

	Ingo