linux-kernel - Re: [PATCH] x86/split_lock: Restore warn mode (and add a new one) to avoid userspace regression

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Date:   Wed, 28 Sep 2022 17:56:56 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     "Luck, Tony" <tony.luck@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Cc:     "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "kernel-dev@...lia.com" <kernel-dev@...lia.com>,
        "kernel@...ccoli.net" <kernel@...ccoli.net>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Joshua Ashton <joshua@...ggi.es>,
        Paul Gofman <pgofman@...eweavers.com>,
        Pavel Machek <pavel@...x.de>,
        Pierre-Loup Griffais <pgriffais@...vesoftware.com>,
        Melissa Wen <mwen@...lia.com>
Subject: Re: [PATCH] x86/split_lock: Restore warn mode (and add a new one) to
 avoid userspace regression

On 28/09/2022 17:24, Luck, Tony wrote:
> [...] 
> Why not just use the workaround suggested in that bug report:
> 
>    "so manual switching from default setting to split_lock_detect=off helps as workaround here"
> 
> If you add this extra mode, I'm going to argue that the kernel default
> should be "seq" rather than "warn". So these game players will need
> to add a split_lock_detect=off (or warn) option.
> 

Hi Tony, thanks for your response. The workaround is the way to
circumvent the issue for now, but not all users want (or know how) to
deal with the kernel parameters. If a distro wants to default to show a
warning only, but don't break performance so hard, this would be
currently impossible.

I understand that maybe the kernel default could be seq, I could
re-submit - not a deal breaker, at least distros are given a way to
collect split lock reports without breaking!

The main/big issues here are two: defaulting to the disruptive behavior
(with no way of building a kernel not defaulting to that without
patching), and not having a way to warn about split locking without
breaking the performance, hence the new mode "seq".

> Has a bug report been filed against the God Of War game? Probably worth fixing,
> the performance penalty for split lock is only going to get worse as numbers of
> cores keeps increasing.
> 

Definitely worth fixing! And the plan is always to report it -
meanwhile, we'd like to have all users of old/proprietary software/games
to continue using the kernel the way it was before :-)

Regarding God of War specifically, I'm not sure but I can verify - the
devels should be made aware, for sure.

Cheers,

Guilherme