* Peter Zijlstra <pet...@infradead.org> wrote:
> On Mon, Nov 11, 2013 at 01:44:19PM +0100, Ingo Molnar wrote:
> >
> > * Frederic Weisbecker <fweis...@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 :-)
So why not just write the value somewhere and printk once at the end of
the NMI sequence, once everything is said and done?
Thanks,
Ingo
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