Re: RCU stall when using function_graph

Daniel Lezcano
2017-08-03T14:38:08Z

On Thu, Aug 03, 2017 at 05:44:21AM -0700, Paul E. McKenney wrote:

[ ... ]

> > > BTW, function_graph tracer is the most invasive of the tracers. It's 4x
> > > slower than function tracer. I'm wondering if the tracer isn't the
> > > cause, but just slows things down enough to cause a some other race
> > > condition that triggers the bug.
> >
> > Yes, that could be true.
> >
> > I tried the following scenario:
> >
> > - cpufreq governor => userspace + max_freq (1.2GHz)
> > - function_graph set ==> OK
> >
> > - cpufreq governor => userspace + min_freq (200MHz)
> > - function_graph set ==> RCU stall
> >
> > Beside that, I realize the board is constantly processing SOF interrupts
> > every 124us, so that adds more overhead.
> >
> > Removing the USB support, thus the associated processing for the SOF
> > interrupts, I don't see anymore the RCU stall.
>
> Looks like Steve called this one! ;-)

Yep :)

> > Is it the expected behavior to have the system hang after a RCU stall
> > raises ?
>
> No, but if NMI stack traces are enabled and there are any NMI problems,
> bad things can happen. In addition, the bulk of output can cause problems
> if you have a slow console connection.

Ok, thanks.

-- Daniel

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