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

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Message-ID: <8e02de22-aaa0-413e-91fe-534d4a74c4e7@vt.edu>
Date: Tue, 29 Apr 2025 16:39:22 -0500
From: Carlos Bilbao <bilbao@...edu>
To: Andrew Morton <akpm@...ux-foundation.org>
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

Hey Andrew,

On 4/29/25 17:53, Andrew Morton wrote:
> 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.

It’s difficult for me to imagine how someone might be negatively impacted,
but I understand that it could happen.

> 
>> 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.

I think you'd then prefer what I originally proposed:

https://lore.kernel.org/lkml/20250326151204.67898-1-carlos.bilbao@kernel.org/T/

IMHO it's true that this feature might not be necessary ATM, but as Sean
pointed out, it could be useful in the future. I don't have strong
preferences either way. Would you be happier with the current v3 approach
if we add comments to the code explaining the purpose of the priority
feature?

> 
>> 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.

Thanks,
Carlos