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Date: Thu, 14 Apr 2011 19:43:27 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Lin Ming <ming.m.lin@intel.com>, Don Zickus <dzickus@redhat.com>,
        Shaun Ruffell <sruffell@digium.com>,
        Maciej Rutecki <maciej.rutecki@gmail.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Stephane Eranian <eranian@google.com>,
        Robert Richter <robert.richter@amd.com>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH -tip] perf, x86: fix unknown NMIs on a Pentium4 box
Message-ID: <20110414174327.GA8863@elte.hu>
References: <4DA70950.3060102@openvz.org>
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* Cyrill Gorcunov <gorcunov@openvz.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