Re: [RFC 00/15] x86_64: Optimize percpu accesses

[Jeremy Fitzhardinge]

Ingo Molnar wrote:
> * Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>   
> > Ingo Molnar wrote:
> >
> >     
> > > that makes sense. Does everyone agree on #1-#2-#3 and then gradual 
> > > elimination of most pda members (without going through an 
> > > intermediate renaming of pda members) being the way to go?
> > >       
> > I think we all agree on 1-2-3.
> >
> > The rest is TBD. Hope Jeremy can add his wisdom there to get the pda.X 
> > replaced by the proper percpu names for 32 bit.
> >
> > With Jeremy's approach we would be doing two steps at once (getting 
> > rid of pda ops plus unifying the variable names between 32 and 64 
> > bit). Maybe more difficult to verify correctness. The removal of the 
> > pda ops is a pretty straighforward conversion.
> >     
>
> Yes, but there's nothing magic about pda variables versus percpu 
> variables. We should be able to do the pda -> unified step just as much 
> as we can do a percpu -> unified step. We can think of pda as a funky, 
> pre-percpu-era relic.
>
> The only thing that percpu really offers over pda is its familarity. 
> read_pda() has the per-cpu-ness embedded in it, which is nasty with 
> regard to tracking preemption properties, etc.
>
> So converting to percpu would bring us CONFIG_PREEMPT_DEBUG=y checking 
> to those ex-pda variables. Today if a read_pda() (or anything but 
> pcurrent) is done in a non-preempt region that's likely a bug - but 
> nothing warns about it.
>
> So in that light 4-15 might make some sense in standardizing all these 
> accesses and making sure it all fits into an existing, familar API 
> world, with no register level assumptions and assembly (and ABI) ties, 
> which is instrumented as well, with explicit smp_processor_id() 
> dependencies, etc.
>   

Yeah, but doing

#define read_pda(x)      x86_read_percpu(x)

gives you all that anyway.   Though because x86_X_percpu and X_pda are 
guaranteed to be atomic with respect to preemption, it's actually 
reasonable to use them with preemption enabled.

   J
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