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SubjectRe: [RFC 00/15] x86_64: Optimize percpu accesses
Ingo Molnar wrote:
> * Christoph Lameter <cl@linux-foundation.org> 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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