Jan Kiszka wrote:
> The root of all this: When Nick Piggin posted his first suggestion for
> ticket spinlocks on LKML, I immediately liked the idea. For details
> check LWN , in a nutshell: This algorithm enforces strict FIFO order
> for the admission to contended spinlocks, thus it improves the
> determinism on SMP systems with more than 2 CPUs.
> Meanwhile, ticket spinlocks are mainline (2.6.25). But that version has
> to drawbacks for us: it doesn't support nesting like xnlock does, and it
> is x86-only so far.
> So I designed a version for Xenomai which is both nestable and
> arch-independent. It is certainly not as optimal as mainline's version,
> but our code path stresses the locking code differently anyway.
> This thing here /seems/ to work, but I'm lacking CPUs at home to test.
> You can't truly stress ticket locks with only a single dual-core :-/.
> QEMU runs into a live-lock with -smp 2, this patch applied and two
> moderate latency loops, but that might be an artifact of its
> single-threaded VCPU scheduling. And kvm currently locks up under SMP
> even without any change, but kvm and SMP is a story of its own. There is
> hope: 16-way is waiting at work... :)
Xenomai uses a Big Kernel Lock, an approach known to not scale very
well. So, if we want to scale correctly on machines with many cpus, we
should change our locking strategy first.
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