Re: RCU stall when using function_graph
From: Daniel Lezcano
Date: Thu Aug 03 2017 - 10:38:14 EST
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! ;-)
> > 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.
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