linux-kernel - Re: PROBLEM: Only one CPU active on Ultra 60 since ~4.8 (regression)

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Message-ID: <5f022f30-e1f2-46e3-b75b-0cb4b157448e@gaisler.com>
Date: Fri, 5 Apr 2024 17:05:49 +0200
From: Andreas Larsson <andreas@...sler.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 Linux regressions mailing list <regressions@...ts.linux.dev>
Cc: Nick Bowler <nbowler@...conx.ca>, linux-kernel@...r.kernel.org,
 "David S. Miller" <davem@...emloft.net>, sparclinux@...r.kernel.org,
 Sam Ravnborg <sam@...nborg.org>
Subject: Re: PROBLEM: Only one CPU active on Ultra 60 since ~4.8 (regression)

On 2024-03-28 21:09, Linus Torvalds wrote:
> On Thu, 28 Mar 2024 at 12:36, Linux regression tracking (Thorsten
> Leemhuis) <regressions@...mhuis.info> wrote:
>>
>> [CCing Linus, in case I say something to his disliking]
>>
>> On 22.03.24 05:57, Nick Bowler wrote:
>>>
>>> Just a friendly reminder that this issue still happens on Linux 6.8 and
>>> reverting commit 9b2f753ec237 as indicated below is still sufficient to
>>> resolve the problem.
>>
>> FWIW, that commit 9b2f753ec23710 ("sparc64: Fix cpu_possible_mask if
>> nr_cpus is set") is from v4.8. Reverting it after all that time might
>> easily lead to even bigger trouble.
> 
> I'm definitely not reverting a patch from almost a decade ago as a regression.
> 
> If it took that long to find, it can't be that critical of a regression.
> 
> So yes, let's treat it as a regular bug. And let's bring in Andreas to
> the discussion too (although presumably he has seen it on the
> sparclinux mailing list).
Yes, I am aware and I agree we should treat it as a regular bug.

Reverting it as a regression fix would lead to followup issues like
canceling the effect of commit ebb99a4c12e4 ("sparc64: Fix irq stack
bootmem allocation.") but with misleading comments left in place. 

Sam's fix looks like a good solution for me to pick up to my
for-next branch.

Thanks,
Andreas