linux-kernel - Re: [RFC PATCH V1] watchdog: Add boot-time selection for hard lockup detector

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Message-ID: <CAP-5=fX4=fV70N3GCdXgV6o-YoJynnSppxJp0MwdRrtsyDrs0w@mail.gmail.com>
Date: Tue, 7 Oct 2025 17:11:52 -0700
From: Ian Rogers <irogers@...gle.com>
To: Doug Anderson <dianders@...omium.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jinchao Wang <wangjinchao600@...il.com>, 
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Subject: Re: [RFC PATCH V1] watchdog: Add boot-time selection for hard lockup detector

On Tue, Oct 7, 2025 at 3:58 PM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Tue, Oct 7, 2025 at 3:45 PM Ian Rogers <irogers@...gle.com> wrote:
> >
> > On Tue, Oct 7, 2025 at 2:43 PM Doug Anderson <dianders@...omium.org> wrote:
> > ...
> > > The buddy watchdog was pretty much following the conventions that were
> > > already in the code: that the hardlockup detector (whether backed by
> > > perf or not) was essentially called the "nmi watchdog". There were a
> > > number of people that were involved in reviews and I don't believe
> > > suggesting creating a whole different mechanism for enabling /
> > > disabling the buddy watchdog was never suggested.
> >
> > I suspect they lacked the context that 1 in the nmi_watchdog is taken
> > to mean there's a perf event in use by the kernel with implications on
> > how group events behave. This behavior has been user
> > visible/advertised for 9 years. I don't doubt that there were good
> > intentions by PowerPC's watchdog and in the buddy watchdog patches in
> > using the file, that use will lead to spurious warnings and behaviors
> > by perf.
> >
> > My points remain:
> > 1) using multiple files regresses perf's performance;
> > 2) the file name by its meaning is wrong;
> > 3) old perf tools on new kernels won't behave as expected wrt warnings
> > and metrics because the meaning of the file has changed.
> > Using a separate file for each watchdog resolves this. It seems that
> > there wasn't enough critical mass for getting this right to have
> > mattered before, but that doesn't mean we shouldn't get it right now.
>
> Presumably your next steps then are to find someone to submit a patch
> and try to convince others on the list that this is a good idea. The
> issue with perf has been known for a while now and I haven't seen any
> patches. As I've said, I won't stand in the way if everyone else
> agrees, but given that I'm still not convinced I'm not going to author
> any patches for this myself.

Writing >1 of:
```
static struct ctl_table watchdog_hardlockup_sysctl[] = {
{
.procname       = "nmi_watchdog",
.data = &watchdog_hardlockup_user_enabled,
.maxlen = sizeof(int),
.mode = 0444,
.proc_handler   = proc_nmi_watchdog,
.extra1 = SYSCTL_ZERO,
.extra2 = SYSCTL_ONE,
},
};
```
is an exercise of copy-and-paste, if you need me to do the copy and
pasting then it is okay.

Thanks,
Ian


> -Doug
>