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

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Message-ID: <48751B01.2010709@goop.org>
Date:	Wed, 09 Jul 2008 13:09:37 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jack Steiner <steiner@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Ingo Molnar wrote:
> * Christoph Lameter <cl@...ux-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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