Re: Uhhuh. NMI received for unknown reason 2d on CPU 0.

[Cyrill Gorcunov <gorcunov@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 256 bytes --]

On Wed, Apr 13, 2011 at 8:34 AM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> There is a patch fromDon we are testing now, i will resend it in a couple of
> hours.
>

George, mind to give these patches a shot please? (Sorry for sending
it as attachment).

[-- Attachment #2: don-apic-nmi-unmask-fix.patch --]
[-- Type: application/octet-stream, Size: 1687 bytes --]

From: Don Zickus <dzickus@redhat.com>
Date: Tue, 12 Apr 2011 12:30:31 -0400
Subject: [PATCH] perf, x86: fix unknown NMIs on a Pentium4 box

When using perf on a Pentium4 box, lots of unknown NMIs would be generated.
This is the result of a P4 quirk that is subtle.  The P4 generates an NMI
when the counter overflow and unlike other arches where the NMI is a one time
event, the P4 continues to assert its NMI until clear by the OS.

As a side effect to this quirk, the NMI on the apic is masked off to prevent
a stream of NMIs until the overflow flag is cleared.  During the perf
re-design, this subtle-ness was overlooked and the apic was unmasked _before_
the overflow flag was cleared.  As a result, this generated an extra NMI on
the P4 mchines.

The fix is trivial, wait until the NMI is properly handled before un-masking
the apic.

Sadly, in the old nmi watchdog there was a note that explained this exact
behaviour.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Tested-by: Shaun Ruffell <sruffell@digium.com>
Tested-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
---
 arch/x86/kernel/cpu/perf_event.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index eed3673a..e108ef8 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1370,9 +1370,9 @@ perf_event_nmi_handler(struct notifier_block *self,
 		return NOTIFY_DONE;
 	}
 
-	apic_write(APIC_LVTPC, APIC_DM_NMI);
 
 	handled = x86_pmu.handle_irq(args->regs);
+	apic_write(APIC_LVTPC, APIC_DM_NMI);
 	if (!handled)
 		return NOTIFY_DONE;
 
-- 
1.7.4.2