Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)

Steven Rostedt
2009-01-20T17:53:53Z

On Tue, 20 Jan 2009, Ingo Molnar wrote:

>
> * Steven Rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org> wrote:
>
> > On Tue, 20 Jan 2009, Ingo Molnar wrote:
> > > Another test would be to build the scheduler latency tracer into your
> > > kernel:
> > >
> > > CONFIG_SCHED_TRACER=y
> > >
> > > And enable it via:
> > >
> > > echo wakeup > /debug/tracing/current_tracer
> > >
> > > and you should be seeing the worst-case scheduling latency traces in
> > > /debug/tracing/trace, and the largest observed latency will be in
> > > /debug/tracing/tracing_max_latency [in microseconds].
> >
> > Note, the wakeup latency only tests realtime threads, since other
> > threads can have other issues for wakeup. I could change the wakeup
> > tracer as wakeup_rt, and make a new "wakeup" that tests all threads, but
> > it may be difficult to get something accurate.
>
> hm, that's a significant regression then. The latency tracer used to
> measure the highest-prio task in the system - be that RT or non-rt.

Well, it is a regression from what was in -rt yes. But not from what ever
was in mainline.

But I needed to change this to detect the problem that we
solved with push and pull of rt tasks. The wake up of a non-rt tasks
always took longer than an -rt task, and by tracing all tasks, I never got
the wake up latency of an rt task.

As I mentioned earlier, I can make a wakeup-rt to do the rt tracing, and
make wakeup do all tasks.

-- Steve