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

[Ingo Molnar]

* Cyrill Gorcunov <gorcunov@xxxxxxxxxx> 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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