linux-kernel - Re: [PATCH] percpu: add optimized generic percpu accessors

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Date:	Fri, 16 Jan 2009 22:59:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	akpm@...ux-foundation.org, tj@...nel.org, hpa@...or.com,
	brgerst@...il.com, ebiederm@...ssion.com, cl@...ux-foundation.org,
	travis@....com, linux-kernel@...r.kernel.org, steiner@....com,
	hugh@...itas.com
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors


* Rusty Russell <rusty@...tcorp.com.au> wrote:

> On Friday 16 January 2009 10:42:00 Herbert Xu wrote:
> > Ingo Molnar <mingo@...e.hu> wrote:
> > >
> > >> Of course.  But do any architectures actually _need_ that for a single 
> > >> read?
> > > 
> > > not for a read i guess - but for the other ops like add/and/or/xor.
> > 
> > One of the things I'd like to see happen with this work is for
> > us to have a cheap per-cpu atomic counter that we can use for
> > SNMP stats.
> > 
> > If we can make the inc/add variants into a single instruction,
> > then it won't need to disable preemption or interrupts.
> > 
> > So if you could design the API such that we have a variant of
> > add/inc that automatically disables/enables preemption then we
> > can optimise that away on x86.
> 
> Yep, already on it.  It's called local_t; that's what it was originally 
> designed for.
> 
> Unfortunately, to use it efficiently, we need large per-cpu areas.

Do you mean constructs like:

        local_inc(&__get_cpu_var(var));

?

If yes then i think you are missing the point here.

Yes, local_t can be useful when something is in an object and we know only 
a local IRQ context can update it and we dont want to disable irqs or use 
heavy atomics.

But percpu_read()/write()/add()/sub() ops are about optimizing _percpu_ 
variables. local_t alone does not solve that problem - because to use 
local_t as a percpu variable you have to get to the address of that 
variable - and that alone is not well optimized.

Or do you propose some new API that would allow that?

	Ingo
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