Re: [PATCH] x86: Drop KGDB support

[David Laight]

On Wed, 28 Oct 2026 14:58:12 -0500
Steve Wahl <steve.wahl@xxxxxxx> wrote:

> On Wed, Oct 28, 2026 at 08:10:54AM -0700, Dave Hansen wrote:
...
> 
> For an example, I recently used this method to discover that at
> least one version of Intel's MPI library likes to read up
> /proc/kallsyms from each of the processes it starts, and somewhere
> north of 1000's of processors the atomic increment on opening
> kallsyms, implemented with a compare-exchange, becomes a hot lock
> and initialization that takes little time on a smaller number of
> processors, suddenly balloons to half an hour.

I'm guessing that was pre-ARMv8.1 ??
The LL/SC retry loops for atomics are completely broken for contended
operations.
The kernel needs to patch them to use the LSE (single instruction
atomics) for ARMv8.1 which guarantee that some progress is made.
A CAS loop can still get starved on a single CPU, but some CPU
do proceed.
You really don't want to use LL/LC on a system with hraair CPU.

One CPU (not arm) that implements LL/SC locks the cache line for a
few clocks after LL to give a reasonable chance of the associated SC
actually succeeding before the cache line is 'stolen' by another CPU
doing an LL.

David