linux-kernel - Re: [PATCH 1/5] x86, nmi: Add new nmi type 'external'

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Date:	Wed, 7 May 2014 17:38:54 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Don Zickus <dzickus@...hat.com>
Cc:	x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	ak@...ux.intel.com, gong.chen@...ux.intel.com,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/5] x86, nmi:  Add new nmi type 'external'


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

> I noticed when debugging a perf problem on a machine with GHES enabled,
> perf seemed slow.  I then realized that the GHES NMI routine was taking
> a global lock all the time to inspect the hardware.  This contended
> with all the local perf counters which did not need a lock.  So each cpu
> accidentally was synchronizing with itself when using perf.
> 
> This is because the way the nmi handler works.  It executes all the handlers
> registered to a particular subtype (to deal with nmi sharing).  As a result
> the GHES handler was executed on every PMI.
> 
> Fix this by creating a new nmi type called NMI_EXT, which is used by
> handlers that need to probe external hardware and require a global lock
> to do so.
> 
> Now the main NMI handler can check the internal NMI handlers first and
> then the external ones if nothing is found.
> 
> This makes perf a little faster again on those machines with GHES enabled.

So what happens if GHES asserts an NMI at the same time a PMI 
triggers?

If the perf PMI executes and indicates that it has handled something, 
we don't execute the GHES handler, right? Will the GHES re-trigger the 
NMI after we return?

Thanks,

	Ingo
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