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

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Message-ID: <CAD=FV=VuDYiu5nL5ZeZcY2b+YXOzZtSu2E4qBBHz9fWTW8gPhg@mail.gmail.com>
Date: Tue, 7 Oct 2025 15:58:06 -0700
From: Doug Anderson <dianders@...omium.org>
To: Ian Rogers <irogers@...gle.com>
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Subject: Re: [RFC PATCH V1] watchdog: Add boot-time selection for hard lockup detector

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.

-Doug