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

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Message-ID: <943c6d11-e214-43c8-8813-8e1aba6be15c@draconx.ca>
Date: Thu, 28 Mar 2024 17:08:50 -0400
From: Nick Bowler <nbowler@...conx.ca>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 sparclinux@...r.kernel.org, Andreas Larsson <andreas@...sler.com>,
 Linux regressions mailing list <regressions@...ts.linux.dev>
Subject: Re: PROBLEM: Only one CPU active on Ultra 60 since ~4.8 (regression)

On 2024-03-28 16:09, 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.

FWIW I'm not the first person to notice this problem.  Searching the sparclinux
archive for "ultra 60" which turns up this very similar report[1] from two years
prior to mine which also went nowhere (sadly, this reporter did not perform a
bisection to find the problematic commit -- perhaps because nobody asked).

[1] https://lore.kernel.org/sparclinux/20201009161924.c8f031c079dd852941307870@gmx.de/

Cheers,
  Nick