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SubjectRe: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with preemptible kernel and CPU hotplug
Mathieu Desnoyers wrote:
> I can't argue about the benefit of using VM CPU pinning to manage
> resources because I don't use it myself, but I ran some tests out of
> curiosity to find if uncontended locks were that cheap, and it turns out
> they aren't. Here are the results :
>

OK, let me clarify my point a bit. If you've got a kernel which is
switching between UP and SMP on a regular basis, you're going to be
taking the hit for the locked instructions whenever you're in the SMP
state anyway. It's only going to be a workload where you're mostly UP
with occasional excursions into being SMP that patching out the lock
prefixes is actually going to make a difference.

And that just doesn't seem like a very likely use-case to me. Certainly
I don't think it would ever happen on a physical machine. And it
doesn't seem all that likely on a virtual machine either. Certainly
resources are more dynamic in a virtual environment, but I think there's
a fairly good chance that the domain knows from the outset whether it's
going to be UP or SMP, or does the UP->SMP transition once.

(That said, the XenServer product does precisely what I say is unusual:
the dom0 kernel hotplugs all the cpus so it can do ucode updates, etc,
and then unplugs all but one...)

> Xeon 2.0GHz
>
>
> Summary
>
> no lock prefix (s) with lock prefix (s) Speedup
> make -j1 kernel/ 33.94 +/- 0.07 34.91 +/- 0.27 2.8 %
> hackbench 50 2.99 +/- 0.01 3.74 +/- 0.01 25.1 %
>

Yeah, that's more severe than I would have expected. Perhaps I have AMD
numbers in my head.

J


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