linux-kernel - [PATCH] documentation: nmi_watchdog=2 works on x86_64 (was: 2.6.25-rc: complete lockup on boot/start of X (bisected))

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Message-ID: <20080323200625.GB6028@joi>
Date:	Sun, 23 Mar 2008 21:06:30 +0100
From:	Marcin Slusarz <marcin.slusarz@...il.com>
To:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: [PATCH] documentation: nmi_watchdog=2 works on x86_64 (was:
	2.6.25-rc: complete lockup on boot/start of X (bisected))

On Sun, Mar 23, 2008 at 08:06:08PM +0100, Peter Zijlstra wrote:
> On Sun, 2008-03-23 at 19:46 +0100, Marcin Slusarz wrote:
> 
> > 4 different lockups:
> > http://alan.umcs.lublin.pl/~mslusarz/kernel/2008.03.23-lockup/ 
> 
> Brilliant, these are extremely helpful.
> 
> What seems to happen is a classic xtime deadlock, doing a xtime
> seq_readlock while holding a seq_writelock.
> 
> Only need to figure out _why_ that is happening....
> 
> > Are there any downsides of using nmi_watchdog=2 all the time?
> 
> Some machines are said to not boot due to NMIs, hence the default is
> off, but afaik, _if_ it works there are no downsides aside from the
> slight NMI handling overhead.
> 
> > ps: Documentation/nmi_watchdog.txt says: "Currently, local APIC mode
> > (nmi_watchdog=2) does not work on x86-64.". It's not true, so maybe someone
> > should update this file?
> 
> Care to send a patch to the x86 maintainers? :-)
> 

---
documentation: nmi_watchdog=2 works on x86_64

Signed-off-by: Marcin Slusarz <marcin.slusarz@...il.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
 Documentation/nmi_watchdog.txt |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/Documentation/nmi_watchdog.txt b/Documentation/nmi_watchdog.txt
index c025a45..757c729 100644
--- a/Documentation/nmi_watchdog.txt
+++ b/Documentation/nmi_watchdog.txt
@@ -23,8 +23,7 @@ kernel debugging options, such as Kernel Stack Meter or Kernel Tracer,
 may implicitly disable the NMI watchdog.]
 
 For x86-64, the needed APIC is always compiled in, and the NMI watchdog is
-always enabled with I/O-APIC mode (nmi_watchdog=1). Currently, local APIC
-mode (nmi_watchdog=2) does not work on x86-64.
+always enabled with I/O-APIC mode (nmi_watchdog=1).
 
 Using local APIC (nmi_watchdog=2) needs the first performance register, so
 you can't use it for other purposes (such as high precision performance
-- 
1.5.3.7

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