linux-kernel - Re: [PATCH V3] x86/split_lock: Add sysctl to control the misery mode

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Date:   Thu, 10 Nov 2022 08:36:20 -0800
From:   Tony Luck <tony.luck@...el.com>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, luto@...nel.org, corbet@....net,
        linux-doc@...r.kernel.org, bagasdotme@...il.com,
        kernel-dev@...lia.com, kernel@...ccoli.net,
        Fenghua Yu <fenghua.yu@...el.com>,
        Joshua Ashton <joshua@...ggi.es>,
        Melissa Wen <mwen@...lia.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Pavel Machek <pavel@...x.de>,
        Pierre-Loup Griffais <pgriffais@...vesoftware.com>,
        Zebediah Figura <zfigura@...eweavers.com>,
        Andre Almeida <andrealmeid@...lia.com>
Subject: Re: [PATCH V3] x86/split_lock: Add sysctl to control the misery mode

On Mon, Oct 24, 2022 at 05:02:54PM -0300, Guilherme G. Piccoli wrote:
> Commit b041b525dab9 ("x86/split_lock: Make life miserable for split lockers")
> changed the way the split lock detector works when in "warn" mode;
> basically, not only it shows the warn message, but also intentionally
> introduces a slowdown (through sleeping plus serialization mechanism)
> on such task. Based on discussions in [0], seems the warning alone
> wasn't enough motivation for userspace developers to fix their
> applications.
> 
> Happens that originally the proposal in [0] was to add a new mode
> which would warns + slowdown the "split locking" task, keeping the
> old warn mode untouched. In the end, that idea was discarded and
> the regular/default "warn" mode now slowdowns the applications. This
> is quite aggressive with regards proprietary/legacy programs that
> basically are unable to properly run in kernel with this change.
> While it is understandable that a malicious application could DoS
> by split locking, it seems unacceptable to regress old/proprietary
> userspace programs through a default configuration that previously
> worked. An example of such breakage was reported in [1].
> 
> So let's add a sysctl to allow controlling the "misery mode" behavior,
> as per Thomas suggestion on [2]. This way, users running legacy and/or
> proprietary software are allowed to still execute them with a decent
> performance while still observe the warning messages on kernel log.

Reviewed-by: Tony Luck <tony.luck@...el.com>

-Tony