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

[Zhenzhong Duan]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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