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

[Jeremy Fitzhardinge]

Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@xxxxxxxxx) wrote:
>   
> > 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 :
> > > Xeon 2.0GHz
> > > Summary
> > > 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 %
> > > 1 CPU, replace smp lock prefixes with DS segment selector prefixes
> > > 1 CPU, noreplace-smp
> > >       
> > For reference, could you also compare replace smp lock with NOPs?
> >
> > 	-hpa
> >     
>
> Sure, here are the updated tables. Basically, they show no significant
> difference between the NOP and the DS segment selector prefix
> approaches.
>   

BTW, are you changing the initial prefix to DS too?  Ie, are you doing a 
nop->lock->ds transition, or ds->lock->ds?

   J
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