linux-kernel - Re: [PATCH -tip] perf, x86: fix unknown NMIs on a Pentium4 box

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Date:	Thu, 14 Apr 2011 19:43:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	Lin Ming <ming.m.lin@...el.com>, Don Zickus <dzickus@...hat.com>,
	Shaun Ruffell <sruffell@...ium.com>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>,
	Robert Richter <robert.richter@....com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -tip] perf, x86: fix unknown NMIs on a Pentium4 box


* Cyrill Gorcunov <gorcunov@...nvz.org> wrote:

> --- linux-2.6.git.orig/arch/x86/kernel/cpu/perf_event.c
> +++ linux-2.6.git/arch/x86/kernel/cpu/perf_event.c
> @@ -1370,9 +1370,16 @@ perf_event_nmi_handler(struct notifier_b
>  		return NOTIFY_DONE;
>  	}
> 
> -	apic_write(APIC_LVTPC, APIC_DM_NMI);
> 
>  	handled = x86_pmu.handle_irq(args->regs);
> +
> +	/*
> +	 * Note the unmasking of LVTPC entry must be
> +	 * done *after* counter oveflow flag is cleared
> +	 * otherwise it might lead to double NMIs generation.
> +	 */
> +	apic_write(APIC_LVTPC, APIC_DM_NMI);
> +
>  	if (!handled)
>  		return NOTIFY_DONE;
> 

This breaks 'perf top' on Intel Nehalem and probably other CPUs. The NMI gets 
stuck fast on all CPUs:

  NMI: 16 6 3 3 3 3 3 3 3 3 3 3 3 3 4 5 Non-maskable interrupts

Thanks,

	Ingo
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