linux-kernel - Re: [PATCH] x86, UV: Fix NMI handler for UV platforms

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Date:	Mon, 21 Mar 2011 13:22:35 -0500
From:	Jack Steiner <steiner@....com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>, Ingo Molnar <mingo@...e.hu>,
	tglx@...utronix.de, hpa@...or.com, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86, UV: Fix NMI handler for UV platforms

On Mon, Mar 21, 2011 at 01:51:10PM -0400, Don Zickus wrote:
> On Mon, Mar 21, 2011 at 07:26:51PM +0300, Cyrill Gorcunov wrote:
> > On 03/21/2011 07:14 PM, Ingo Molnar wrote:
> > > 
> > > * Jack Steiner <steiner@....com> wrote:
> > > 
> > >> This fixes a problem seen on UV systems handling NMIs from the node controller.
> > >> The original code used the DIE notifier as the hook to get to the UV NMI
> > >> handler. This does not work if performance counters are active - the hw_perf
> > >> code consumes the NMI and the UV handler is not called.
> 
> Well that is a bug in the perf code.  We have been dealing with 'perf'
> swallowing NMIs for a couple of releases now.  I think we got rid of most
> of the cases (p4 and acme's core2 quad are the only cases I know that are
> still an issue).
> 
> I would much prefer to investigate the reason why this is happening
> because the perf nmi handler is supposed to check the global interrupt bit
> to determine if the perf counters caused the nmi or not otherwise fall
> through to other handler like SGI's nmi button in this case.

The patch that I posted is based on a RHEL6.1 patch that I'm running internally.
Unless something has very recently changed in the RH sources, the perf
NMI handler unconditionally returns NOTIFY_STOP if it handles an NMI.
If no NMI was handled, it returns NOTIFY_DONE. This sometimes works
and allows the platform generated NMI to be processed but if both NMI
sources trigger at about he same time, the lower priority event
will be lost.

The root cause of the problem is that architecturally, x86 does not
have a way to identifies the source(s) that cause an NMI. If multiple
events occur at about the same time, there is no way that I can see that the
OS can detect it.

> 
> My first impression is the skip nmi logic in the perf handler is probably
> accidentally thinking the SGI external nmi is the perf's 'extra' nmi it is
> supposed to skip and thus swallows it.  At least that is the impression I

Agree


> get from the RedHat bugzilla which says SGI is running 'perf top', getting
> a hang, then pressing their nmi button to see the stack traces.
> 
> Jack,
> 
> I worked through a number of these issues upstream and I already talked to
> George and Russ over here at RedHat about working through the issue over
> here with them.  They can help me get access to your box to help debug.

Russ is right down the hall.


--- jack
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