linux-kernel - Re: [PATCH 6.0.y / 6.1.y] x86/split_lock: Add sysctl to control the misery mode

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Date:   Mon, 19 Dec 2022 09:48:18 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     Peter Zijlstra <peterz@...radead.org>, Greg KH <greg@...ah.com>
Cc:     stable@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, corbet@....net,
        kernel-dev@...lia.com, kernel@...ccoli.net, fenghua.yu@...el.com,
        joshua@...ggi.es, pgofman@...eweavers.com, pavel@...x.de,
        pgriffais@...vesoftware.com, zfigura@...eweavers.com,
        cristian.ciocaltea@...labora.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andre Almeida <andrealmeid@...lia.com>
Subject: Re: [PATCH 6.0.y / 6.1.y] x86/split_lock: Add sysctl to control the
 misery mode

On 19/12/2022 07:59, Peter Zijlstra wrote:
> On Mon, Dec 19, 2022 at 11:17:39AM +0100, Greg KH wrote:
> 
>> What specific programs have this problem and what are the exact results
>> of it?
> 
> IIRC it was God of War (2018) that triggered this initially. But it's
> possible more games were found to tickle this specific thing. Since it's
> binary only gunk that is unlikely to ever get fixed we need something to
> allow for it.
> 
> (slow motion Kratos yelling B...o...y...)
> 
>> Also, this is really a new feature and not really a "fix", but one could
>> argue a lot that this is a "resolve a performance problem" if you want
>> to and have the numbers to back it up  {hint}
> 
> Right, there were some, they should indeed have been included.

Thanks Peter, that's exactly it - the current report is linked on commit
message.

About performance numbers, the only "numbers" I have is: game is
unplayable, according to the report "When I launch God of War through
Steam or Lutris I get around 25 fps, on lowest settings and at 10%
resolution scaling", FPS for for games is double of that usually.

I understand this is not a regular fix in which something is completely
broken, but it does fix a behavior introduced on kernel that prevent
some userspace binaries to properly run, in practice. Ofc some will
argue that we already have the kernel parameter, but it's different -
requires reboot and bootloader understanding.

If 6.0.y is too much, I'd ask we have it at least for 6.1, which is
long-term, that will help a lot of people for sure.
Thanks,

Guilherme