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Subject: Re: [PATCH 2/2] doc: kernel-parameters.txt: fix documentation of
 nmi_watchdog parameter
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        paulmck@linux.ibm.com, josh@joshtriplett.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        joel@joelfernandes.org, corbet@lwn.net, tglx@linutronix.de,
        mingo@kernel.org, gregkh@linuxfoundation.org,
        keescook@chromium.org, srinivas.eeda@oracle.com
References: <1555211464-28652-1-git-send-email-zhenzhong.duan@oracle.com>
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 <20190510144749.592f4249@gandalf.local.home>
From:   Zhenzhong Duan <zhenzhong.duan@oracle.com>
Organization: Oracle Corporation
Message-ID: <88c81205-8b9d-5dae-95ba-ad115878ece9@oracle.com>
Date:   Mon, 13 May 2019 10:43:44 +0800
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Hi Steven,

On 2019/5/11 2:47, Steven Rostedt wrote:
> As nobody else commented, I will ;)
>
> Hi Zhenzhong!
>
> On Sun, 14 Apr 2019 11:11:04 +0800
> Zhenzhong Duan <zhenzhong.duan@oracle.com> wrote:
>
>> As stated in "Documentation/lockup-watchdogs.txt:line 22", the default
>> behaivor after 'hardlockup' is to stay locked up rather than panic.
> That actually says:
>
>   A 'hardlockup' is defined as a bug that causes the CPU to loop in
>   kernel mode for more than 10 seconds (see "Implementation" below for
>   details), without letting other interrupts have a chance to run.
>   Similarly to the softlockup case, the current stack trace is displayed
>   upon detection and the system will stay locked up unless the default
>   behavior is changed, which can be done through a sysctl,
>   'hardlockup_panic', a compile time knob, "BOOTPARAM_HARDLOCKUP_PANIC",
>   and a kernel parameter, "nmi_watchdog"
>
> If your config has:
>
>   CONFIG_BOOTPARAM_HARDLOCKUP_PANIC=y
>
> The kernel will panic on hard lockup by default unless you add nopanic.
>
> If your config has:
>
>   # CONFIG_BOOTPARAM_HARDLOCKUP_PANIC is not set
>
> Then the default will be not to panic unless you add "panic" to the
> kernel command line.
>
>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
>> ---
>>   Documentation/admin-guide/kernel-parameters.txt | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index 2b8ee90..fcc9579 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -2769,7 +2769,7 @@
>>   			0 - turn hardlockup detector in nmi_watchdog off
>>   			1 - turn hardlockup detector in nmi_watchdog on
>>   			When panic is specified, panic when an NMI watchdog
>> -			timeout occurs (or 'nopanic' to override the opposite
>> +			timeout occurs (or 'nopanic' which is the opposite
>>   			default). To disable both hard and soft lockup detectors,
> Honestly, I think the original text states what it does better than
> your update. Because the nopanic is added to override the "opposite
> default" which is if the config was set to do so.
>
> That said, this all still can be explained better. What about:
>
>          nmi_watchdog=   [KNL,BUGS=X86] Debugging features for SMP kernels
>                          Format: [panic,][nopanic,][num]
>                          Valid num: 0 or 1
>                          0 - turn hardlockup detector in nmi_watchdog off
>                          1 - turn hardlockup detector in nmi_watchdog on
>                          When panic is specified, panic when an NMI watchdog
>                          timeout occurs (or 'nopanic' to not panic on an NMI
> 			watchdog, if CONFIG_BOOTPARAM_HARDLOCKUP_PANIC is set)
>                          To disable both hard and soft lockup detectors,
>                          please see 'nowatchdog'.
>                          This is useful when you use a panic=... timeout and
>                          need the box quickly up again.

Thanks Seeve for your comments, so the default behavior of hardlockup 
depends on the setting of CONFIG_BOOTPARAM_HARDLOCKUP_PANIC.

It did confused me previously. You suggested fix is better, I'll fix it 
later.


Zhenzhong