Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with preemptible kernel and CPU hotplug

[Jeremy Fitzhardinge <jeremy@goop.org>]

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