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 20:24:23 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.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

> So let's revamp the idea of having another option/mode for the split
> lock detector, which is hereby called "seq" (based on the original
> "sequential" naming in [0]). Also introduces a Kconfig option to give
> the option of Linux vendors have a choice what mode should be their
> default. While at it, fix/improve the documentation about bus locking.
>
> [1] https://github.com/doitsujin/dxvk/issues/2938

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.

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.

-Tony