Re: linux-next 20111025: warnings inrcu_idle_exit_common()/rcu_idle_enter_common()

[Steven Rostedt]

On Mon, 2011-10-31 at 05:19 -0700, Paul E. McKenney wrote:
> On Mon, Oct 31, 2011 at 07:41:42PM +0800, Wu Fengguang wrote:
> > On Mon, Oct 31, 2011 at 06:43:25PM +0800, Wu Fengguang wrote:
> > > On Mon, Oct 31, 2011 at 05:51:52PM +0800, Paul E. McKenney wrote:
> > > > On Mon, Oct 31, 2011 at 04:26:34PM +0800, Wu Fengguang wrote:
> > > > > Hi Paul,
> > > > >
> > > > > I got two warnings in rcutree.c. The last working kernels are
> > > > > linux-next 20111014 and linux v3.1.
> > > > 
> > > > Interesting.  Could you please enable RCU event tracing at boot?
> > > 
> > > Sorry I cannot...possibly due to another ftrace bug.
> > > 
> > > > The RCU event tracing is at tracing/events/rcu/enable relative to
> > > > the debugfs mount point at runtime, if that helps.
> > > 
> > > It's exactly that linux next 20111025 (comparing to 20111014) no
> > > longer produces all the trace events that made me looking into the
> > > dmesg and find the warning from RCU (rather than the expected warning
> > > from ftrace).
> > > 
> > > The trace output is now:
> > > 
> > >         # tracer: nop
> > >         #
> > >         # WARNING: FUNCTION TRACING IS CORRUPTED
> > >         #          MAY BE MISSING FUNCTION EVENTS
> > >         #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
> > >         #              | |       |          |         |
> > > (nothing more)
> > 
> > I checked the other test box and got the same warnings. Below is the
> > full dmesg.
> > 
> > No single trace output again..
> 
> Hmmm...  I wonder if it is too early during boot for tracing to work
> correctly.
> 
> Gah!  I have rcu/next set ahead to commits that are not supposed to go
> upstream yet.  I reset it back to match the stuff that is targeted for
> the current merge window.  Still need to find the bug, of course.
> 
> Anyone have any idea why the kworker thread might be trying to enter
> the idle loop?  The idle_cpu(smp_processor_id()) call believes that
> this is not the idle task.  Or does x86 allow non-idle tasks to enter
> the idle loop?  Or to be migrated off-CPU?


It's not. Carsten Emde noticed what looked like a bug in ftrace last
week at LinuxCon, and looking deeper at it, I found that the swapper
task for all but CPU0 is named kworker.  That's because kworker creates
the idle task for all other CPUs besides CPU 0 and the idle task takes
on kworker name.

Carsten posted a patch last week too:

https://lkml.org/lkml/2011/10/26/313

I'm glad that this bug shows up outside of just ftrace :)

-- Steve



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