linux-kernel - [PATCH] i386: Fix the K7 NMI watchdog checkbit

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Date:	Sat, 9 Jun 2007 04:27:10 +0200
From:	Björn Steinbrink <B.Steinbrink@....de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Udo A. Steinberg" <us15@...inf.tu-dresden.de>,
	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, ak@...e.de,
	dzickus@...hat.com
Subject: [PATCH] i386: Fix the K7 NMI watchdog checkbit

On 2007.06.08 22:43:25 +0200, Ingo Molnar wrote:
> 
> * Björn Steinbrink <B.Steinbrink@....de> wrote:
> 
> > Anyway, both are bugs and should be fixed. Maybe we're even lucky and 
> > it fixes your hang. *fingers crossed*
> 
> just to make it clear: the NMI watchdog was working perfectly fine on 
> that box (in v2.6.21 and in dozens of kernel releases before that, for 
> multiple years) before Andi's cleanup patch. So lets find that bug first 
> or revert the cleanups.

Might have been pure luck. ;-) The culprit seems to be commit
b7471c6da94d30d3deadc55986cc38d1ff57f9ca (from Sep 2006), which
introduced the check bit to figure out if a NMI was generated by the
watchdog timer. While the performance counter register on K7 is 64 bits
wide, the upper 16 bits are reserved and thus using bit 63 as the check
bit is wrong. A quick check using /dev/cpu/0/msr shows that
here, the upper 16 bits are zero all the time, chances are that this is
not deterministic and you got a 1 in bit 63 due to some random change.

Björn



The performance counters on K7 are only 48 bits wide, so using bit 63 to
check if the counter overflowed is wrong. Let's use bit 47 instead.

Signed-off-by: Björn Steinbrink <B.Steinbrink@....de>
Cc: Don Zickus <dzickus@...hat.com>
Cc: Andi Kleen <andi@...stfloor.org>
---
diff --git a/arch/i386/kernel/cpu/perfctr-watchdog.c b/arch/i386/kernel/cpu/perfctr-watchdog.c
index 2b04c8f..82c6967 100644
--- a/arch/i386/kernel/cpu/perfctr-watchdog.c
+++ b/arch/i386/kernel/cpu/perfctr-watchdog.c
@@ -294,7 +294,7 @@ static struct wd_ops k7_wd_ops = {
 	.stop = single_msr_stop_watchdog,
 	.perfctr = MSR_K7_PERFCTR0,
 	.evntsel = MSR_K7_EVNTSEL0,
-	.checkbit = 1ULL<<63,
+	.checkbit = 1ULL<<47,
 };
 
 /* Intel Model 6 (PPro+,P2,P3,P-M,Core1) */
-
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