Re: perf/tracepoint: another fuzzer generated lockup

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 11, 2013 at 01:44:19PM +0100, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > > That said, I'm not sure what kernel you're running, but there were 
> > > some issues with time-keeping hereabouts, but more importantly that 
> > > second timing includes the printk() call of the first -- so that's 
> > > always going to be fucked.
> > 
> > It's a recent tip:master. So the delta debug printout is certainly 
> > buggy, meanwhile these lockup only happen with Vince selftests, and they 
> > trigger a lot of these NMI-too-long issues, or may be that's the other 
> > way round :)...
> > 
> > I'm trying to narrow down the issue, lets hope the lockup is not 
> > actually due to printk itself.
> 
> I'd _very_ strongly suggest to not include the printk() overhead in the 
> execution time delta! What that function wants to report is pure NMI 
> execution overhead, not problem reporting overhead.
> 
> That way any large number reported there is always a bug somewhere, 
> somehow.

-ENOPATCH :-)

You'll find that there's two levels of measuring NMI latency and the
outer will invariably include the reporting of the inner one; fixing
that is going to be hideously ugly.

That said, I would very strongly suggest to tear that printk() from the
NMI path, its just waiting to wreck someone's machine :-)