linux-kernel - Re: Linux 6.10-rc2 - massive performance regression

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Date: Sat, 8 Jun 2024 18:28:17 -0400
From: Woody Suwalski <terraluna977@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 David Laight <David.Laight@...lab.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 6.10-rc2 - massive performance regression

Linus Torvalds wrote:
> On Sat, 8 Jun 2024 at 14:36, David Laight <David.Laight@...lab.com> wrote:
>> I'll try to remember how to bisect through the merge :-)
> git bisect should just do all the work for you. All you need to do is
> give a know good and bad point, and keep testing what git bisect asks
> you to do.
>
>> I've done some tests.
>> I'm seeing a three-fold slow down on:
>> $ i=0; while [ $i -lt 1000000 ]; do i=$((i + 1)); done
>> which goes from 1 second to 3.
>>
>> I can run that with ftrace monitoring scheduler events (and a few
>> other things) and can't spot anywhere the process isn't running
>> for a significant time.
> Sounds like cpu frequency. Almost certainly hw-specific. I went
> through that on my Threadripper in the 6.9 timeframe, but I'm not
> seeing any issues in this current release.
>
> If you bisect it, we have somebody to blame and point fingers at...
>
>                  Linus
>
I have run a quick test on my laptop doing

time (i=0; while [ $i -lt 1000000 ]; do i=$((i+1)); done)

and 6.8.12, 6.9.3 and 6.10-rc2 have identical results...

Woody