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Subject: A question of perf NMI handler
From: Lin Ming <ming.m.lin@intel.com>
To: Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <peterz@infradead.org>,
        Don Zickus <dzickus@redhat.com>, Cyrill Gorcunov <gorcunov@gmail.com>,
        robert.richter@amd.com, fweisbec@gmail.com
Cc: linux-kernel@vger.kernel.org, "Huang, Ying" <ying.huang@intel.com>
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Date: Wed, 04 Aug 2010 17:21:10 +0800
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With nmi_watchdog enabled, perf_event_nmi_handler always return
NOTIFY_STOP(active_events > 0), and the notifier call chain will not
call further.

If it was not perf NMI, does the perf nmi handler may stop the real NMI
handler get called because NOTIFY_STOP is returned??

static int __kprobes
perf_event_nmi_handler(struct notifier_block *self,
			 unsigned long cmd, void *__args)
{
	struct die_args *args = __args;
	struct pt_regs *regs;

	if (!atomic_read(&active_events)) ===> With nmi_watchdog enabled, active_events > 0
		return NOTIFY_DONE;

	switch (cmd) {
	case DIE_NMI:
	case DIE_NMI_IPI:
		break;

	default:
		return NOTIFY_DONE;
	}

	regs = args->regs;

	apic_write(APIC_LVTPC, APIC_DM_NMI);
	/*
	 * Can't rely on the handled return value to say it was our NMI, two
	 * events could trigger 'simultaneously' raising two back-to-back NMIs.
	 *
	 * If the first NMI handles both, the latter will be empty and daze
	 * the CPU.
	 */
	x86_pmu.handle_irq(regs);

	return NOTIFY_STOP;
}

Thanks,
Lin Ming