Re: RCU stall when using function_graph

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 2 Aug 2017 00:15:44 +0200
Daniel Lezcano <daniel.lezcano@linaro.org> wrote:

> On 02/08/2017 00:04, Paul E. McKenney wrote:
> >> Hi Paul,
> >>
> >> I have been trying to set the function_graph tracer for ftrace and each time I
> >> get a CPU stall.
> >>
> >> How to reproduce:
> >> -----------------
> >>
> >> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
> >>
> >> This error appears with v4.13-rc3 and v4.12-rc6.

Can you bisect this? It may be due to this commit:

0598e4f08 ("ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines")

-- Steve

> >>
> >> Is it something already reported ?  
> > 
> > I have seen this sort of thing, but only when actually dumping the trace
> > out, and I though those got fixed.  You are seeing this just accumulating
> > the trace?  
> 
> No, just by changing the tracer. It is the first operation I do after
> rebooting and it is reproducible each time. That happens on an ARM64
> platform.
> 
> > These RCU CPU stall warnings usually occur when something grabs hold of
> > a CPU for too long, as in 21 seconds or so.  One way that they can happen
> > is excessive lock contention, another is having the kernel run through
> > too much data at one shot.
> > 
> > Adding Steven Rostedt on CC for his thoughts.
> > 
>