linux-kernel - Re: [PATCH 1/2] x86/platform: Add a low priority low frequency NMI call chain

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Date:   Tue, 7 Mar 2017 10:22:06 -0500
From:   Don Zickus <dzickus@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Mike Travis <mike.travis@....com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Frank Ramsay <frank.ramsay@....com>,
        Russ Anderson <russ.anderson@....com>,
        Tony Ernst <tony.ernst@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/platform: Add a low priority low frequency NMI
 call chain

On Tue, Mar 07, 2017 at 08:42:10AM +0100, Ingo Molnar wrote:
> 
> * Mike Travis <mike.travis@....com> wrote:
> 
> > Add a new NMI call chain that is called last after all other NMI handlers
> > have been checked and did not "handle" the NMI.  This mimics the current
> > NMI_UNKNOWN call chain except it eliminates the WARNING message about
> > multiple NMI handlers registering on this call chain.
> > 
> > This call chain dramatically lowers the NMI call frequency when high
> > frequency NMI tools are in use, notably the perf tools.  It is required
> > for NMI handlers that cannot sustain a high NMI call rate without
> > ramifications to the system operability.
> 
> So how about we just turn off that warning instead? I don't remember the last time 
> it actually _helped_ us find any kernel or hardware bug - and it has caused tons 
> of problems...

Yeah, that is one way to solve it. :-)

Originally I put that in there because the unknown nmi handlers sometime do
not return, making it impossible for the second handler to run.

But you are right, it probably hasn't really helped find any problems.  I
wasn't aware of problems it was causing (not that I was looking through
emails to find them either).

Cheers,
Don

> 
> It's not like we warn about excess regular IRQs either - we either handle them or 
> at most increase a counter somewhere. We could do the same for NMIs: introduce a 
> counter somewhere that counts the number of seemingly unhandled NMIs.
> 
> But in any case, we should not spam the kernel log, neither with high, nor with 
> low frequency.
> 
> Thanks,
> 
> 	Ingo