Re: [PATCH V2] x86/split_lock: Add sysctl to control the misery mode

Guilherme G. Piccoli
2022-10-21T16:57:26Z

On 14/10/2022 15:05, 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 is understandable that a malicious application could try a 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.
>
> [0] https://lore.kernel.org/lkml/20220217012721.9694-1-tony.luck@intel.com/
>
> [1] https://github.com/doitsujin/dxvk/issues/2938
>
> [2] https://lore.kernel.org/lkml/87pmf4bter.ffs@tglx/
>
> Fixes: b041b525dab9 ("x86/split_lock: Make life miserable for split lockers")
> Cc: Andre Almeida <andrealmeid@igalia.com>
> Cc: Fenghua Yu <fenghua.yu@intel.com>
> Cc: Joshua Ashton <joshua@froggi.es>
> Cc: Melissa Wen <mwen@igalia.com>
> Cc: Paul Gofman <pgofman@codeweavers.com>
> Cc: Pavel Machek <pavel@denx.de>
> Cc: Pierre-Loup Griffais <pgriffais@valvesoftware.com>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Zebediah Figura <zfigura@codeweavers.com>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> ---

Hi Dave / Thomas, do you think this version is good enough?

If so, would be possible to pick it still in the v6.1-rc cycle, since it
is a fix?

What about the documentation improvements from Bagas, should I re-send
(V3) with that, or would you pick them when merging?

Thanks in advance,

Guilherme