linux-kernel - Re: A question of perf NMI handler

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Message-ID: <20100804192634.GG5130@lenovo>
Date:	Wed, 4 Aug 2010 23:26:34 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Robert Richter <robert.richter@....com>
Cc:	Don Zickus <dzickus@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Lin Ming <ming.m.lin@...el.com>, Ingo Molnar <mingo@...e.hu>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Huang, Ying" <ying.huang@...el.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: A question of perf NMI handler

On Wed, Aug 04, 2010 at 08:48:06PM +0200, Robert Richter wrote:
> (cc'ing Andi)
> 
> On 04.08.10 12:39:30, Cyrill Gorcunov wrote:
> > On Wed, Aug 04, 2010 at 12:20:26PM -0400, Don Zickus wrote:
> 
> > > there.  The problem is the bits in register 0x61 are not always set
> > > correctly in the case of SERRs (well at least in all the cases I have
> > > dealt with).  So you can easily can a flood of unknown nmis from an SERR
> > > and register 0x61 would have the PERR/SERR bits set to 0.  Fun, huh?
> > 
> > if there is nothing in nmi_sc the code flows into another branch. And
> > it hits the problem of perf events eating all nmi giving no chance the
> > others. So we take if (!(reason & 0xc0)) case and hit DIE_NMI_IPI
> > (/me scratching the head why it's not under CONFIG_X86_LOCAL_APIC) and
> > drop all code, unpleasant.
> 
> Only the upper 2 bits in io_61h indicate the nmi reason, so in case of
> (!(reason & 0xc0)) the source simply can not be determined and all nmi
> handlers in the chain must be called (DIE_NMI/DIE_NMI_IPI). The
> perfctr handler then stops it.

yes, that is what I meant by nmi_sc register. I think we need to restucturize
current default_do_nmi handler but how to be with perfs I don't know at moment
if perf register gets overflowed (ie already has pedning nmi) but we handle
it in early nmi cycle this would lead to strange results. Need to think.

> 
> So you can decide to either get an unrecovered nmi panic triggered by
> a perfctr or losing unknown nmis from other sources. Maybe this can be
> fixed by implementing handlers for those sources.
> 
> -Robert
> 
> -- 
> Advanced Micro Devices, Inc.
> Operating System Research Center
> 
	-- Cyrill
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