linux-kernel - Re: [PATCH 2/2] doc: kernel-parameters.txt: fix documentation of nmi

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Date:   Sun, 12 May 2019 22:31:50 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        paulmck@...ux.ibm.com, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        corbet@....net, tglx@...utronix.de, mingo@...nel.org,
        gregkh@...uxfoundation.org, keescook@...omium.org,
        srinivas.eeda@...cle.com
Subject: Re: [PATCH 2/2] doc: kernel-parameters.txt: fix documentation of
 nmi_watchdog parameter

On Fri, May 10, 2019 at 02:47:49PM -0400, Steven Rostedt wrote:
> 
> As nobody else commented, I will ;)
> 
> Hi Zhenzhong!
> 
> On Sun, 14 Apr 2019 11:11:04 +0800
> Zhenzhong Duan <zhenzhong.duan@...cle.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@...cle.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.

Steven, your update makes it more clear to me. If patch is being written with
such update, here is my Reviewed-by:

Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks!

 - Joel

> 
> -- Steve
> 
> 
> >  			please see 'nowatchdog'.
> >  			This is useful when you use a panic=... timeout and
>