Re: A question of perf NMI handler

Cyrill Gorcunov
2010-08-04T15:19:06Z

On Wed, Aug 04, 2010 at 05:02:41PM +0200, Peter Zijlstra wrote:
> On Wed, 2010-08-04 at 10:52 -0400, Don Zickus wrote:
> > > Right so I looked up your thing and while that limits the damage in that
> > > at some point it will let NMIs pass, it will still consume too many.
> > > Meaning that Yinghai will have to potentially press his NMI button
> > > several times before it registers.
> >
> > Ok. Thanks for reviewing. How does it consume to many? I probably don't
> > understand how perf is being used in the non-simple scenarios.
>
> Suppose you have 4 counters (AMD, intel-nhm+), when more than 2 overflow
> the first will raise the PMI, if the other 2+ overflow before we disable
> the PMU it will try to raise 2+ more PMIs, but because hardware only has
> a single interrupt pending bit it will at most cause a single extra
> interrupt after we finish servicing the first one.
>
> So then the first interrupt will see 3+ overflows, return 3+, and will
> thus eat 2+ NMIs, only one of which will be the pending interrupt,
> leaving 1+ NMIs from other sources to consume unhandled.
>
> In which case Yinghai will have to press his NMI button 2+ times before
> it registers.
>
> That said, that might be a better situation than always consuming
> unknown NMIs..
>

Well, first I guess having Yinghai CC'ed is a bonus ;)
The second thing is that I don't get why perf handler can't be _last_
call in default_do_nmi, if there were any nmi with reason (serr or parity)
I think they should be calling first which of course don't eliminate
the former issue but somewhat make it weaken.

-- Cyrill