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SubjectRe: [PATCH 1/5] x86, nmi: Add new nmi type 'external'

* Don Zickus <dzickus@redhat.com> wrote:

> On Thu, May 08, 2014 at 07:35:01PM +0200, Ingo Molnar wrote:
> >
> > * Don Zickus <dzickus@redhat.com> wrote:
> >
> > > > > Again, I don't have a solution to juggle between PMI performance
> > > > > and reliable delivery. We could do away with the spinlocks and
> > > > > go back to single cpu delivery (like it used to be). Then
> > > > > devise a mechanism to switch delivery to another cpu upon
> > > > > hotplug.
> > > > >
> > > > > Thoughts?
> > > >
> > > > I'd say we should do a delayed timer that makes sure that all
> > > > possible handlers are polled after an NMI is triggered, but never
> > > > at a high rate.
> > >
> > > Hmm, I was thinking about it and wanted to avoid a poll as I hear
> > > complaints here and there about the nmi_watchdog constantly wasting
> > > power cycles with its polling.
> >
> > But the polling would only happen if there's NMI traffic, so that's
> > fine. So as long as polling stops some time after the last PMI use,
> > it's a good solution.
>
> So you are thinking an NMI comes in, kicks off a delayed timer for
> say 10ms. The timer fires, rechecks the NMI for missed events and
> then stops? If another NMI happens before the timer fires, just kick
> the timer again?
>
> Something like that?

Yeah, exactly, using delayed IRQ work for that or so.

This would allow us to 'optimistic' processing of NMI events: the
first handler that manages to do any work causes a return. No need to
make a per handler distinction, etc.

It would generally be pretty robust and would possibly be a natural
workaround for 'stuck PMU' type of bugs as well.

[ As long as it does not result in spurious 'dazed and confused'
messages :-) ]

Thanks,

Ingo


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