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x86/split_lock: Add sysctl to control the misery mode

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Commit b041b525da ("x86/split_lock: Make life miserable for split lockers")
changed the way the split lock detector works when in "warn" mode;
basically, it not only 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.

This slowdown is enough to totally break some proprietary (aka.
unfixable) userspace[1].

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 slows down 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].

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 observing 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/

[ dhansen: minor changelog tweaks, including clarifying the actual
  	   problem ]

Fixes: b041b525da ("x86/split_lock: Make life miserable for split lockers")
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Tested-by: Andre Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/all/20221024200254.635256-1-gpiccoli%40igalia.com
This commit is contained in:
Guilherme G. Piccoli
2022-10-24 17:02:54 -03:00
committed by Dave Hansen
parent f0c4d9fc9c
commit 727209376f
2 changed files with 76 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files
Documentation/admin-guide/sysctl/kernel.rst
+23
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@@ -1314,6 +1314,29 @@ watchdog work to be queued by the watchdog timer function, otherwise the NMI
watchdog — if enabled — can detect a hard lockup condition.
split_lock_mitigate (x86 only)
==============================
On x86, each "split lock" imposes a system-wide performance penalty. On larger
systems, large numbers of split locks from unprivileged users can result in
denials of service to well-behaved and potentially more important users.
The kernel mitigates these bad users by detecting split locks and imposing
penalties: forcing them to wait and only allowing one core to execute split
locks at a time.
These mitigations can make those bad applications unbearably slow. Setting
split_lock_mitigate=0 may restore some application performance, but will also
increase system exposure to denial of service attacks from split lock users.
= ===================================================================
0 Disable the mitigation mode - just warns the split lock on kernel log
and exposes the system to denials of service from the split lockers.
1 Enable the mitigation mode (this is the default) - penalizes the split
lockers with intentional performance degradation.
= ===================================================================
stack_erasing
=============