Re: [PATCH] x86: Drop KGDB support

[Steve Wahl]

On Thu, Oct 29, 2026 at 10:51:21AM +0000, David Laight wrote:
> 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 ??

x86, not arm.  And it was on a distro kernel, I didn't check the
current community kernel because it was a situation where it was
enough to understand what's going on in the distro kernel, and avoid
the behavior in the application. 

> 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.

Thanks,

--> Steve

-- 
Steve Wahl, Hewlett Packard Enterprise