linux-kernel - Re: [PATCH v3 0/2] Reduce CPU consumption after panic

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Message-Id: <20250429155329.73bd3f5835e8d6a2864873f9@linux-foundation.org>
Date: Tue, 29 Apr 2025 15:53:29 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Carlos Bilbao <bilbao@...edu>
Cc: carlos.bilbao@...nel.org, tglx@...utronix.de, seanjc@...gle.com,
 jan.glauber@...il.com, pmladek@...e.com, jani.nikula@...el.com,
 linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
 takakura@...inux.co.jp, john.ogness@...utronix.de, x86@...nel.org
Subject: Re: [PATCH v3 0/2] Reduce CPU consumption after panic

On Tue, 29 Apr 2025 15:17:33 -0500 Carlos Bilbao <bilbao@...edu> wrote:

> Hey Andrew,
> 
> On 4/29/25 15:39, Andrew Morton wrote:
> > (cc more x86 people)
> > 
> > On Tue, 29 Apr 2025 10:06:36 -0500 carlos.bilbao@...nel.org wrote:
> > 
> >> From: Carlos Bilbao <carlos.bilbao@...nel.org>
> >>
> >> Provide a priority-based mechanism to set the behavior of the kernel at
> >> the post-panic stage -- the current default is a waste of CPU except for
> >> cases with console that generate insightful output.
> >>
> >> In v1 cover letter [1], I illustrated the potential to reduce unnecessary
> >> CPU resources with an experiment with VMs, reducing more than 70% of CPU
> >> usage. The main delta of v2 [2] was that, instead of a weak function that
> >> archs can overwrite, we provided a flexible priority-based mechanism
> >> (following suggestions by Sean Christopherson), panic_set_handling().
> >>
> > 
> > An effect of this is that the blinky light will never again occur on
> > any x86, I think?  I don't know what might the effects of changing such
> > longstanding behavior.
> 
> Yep, someone pointed this out before. I don't think it's super relevant? 

Why not?  It's an alteration in very longstanding behavior - nobody
knows who will be affected by this and how they will be affected.

> Also, in the second patch, I added a check to see that there's no console
> output left to be flushed.

It's unclear how this affects such considerations.  Please fully
changelog all these things.

> 
> > 
> > Also, why was the `priority' feature added?  It has no effect in this
> > patchset.
> > 
> 
> This was done to allow for flexibility, for example, if panic devices
> wish to override the default panic behavior.

There are no such callers.  We can add this feature later, if a need is
demonstrated.

> Other benefits of such
> flexibility (as opposed to, for example, a weak function that archs can
> override) were outlined by Sean here:
> 
> https://lore.kernel.org/lkml/20250326151204.67898-1-carlos.bilbao@kernel.org/T/#m93704ff5cb32ade8b8187764aab56403bbd2b331

Again, please fully describe these matters in changelogging and code
comments.