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

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Date:	Tue, 27 Jan 2009 15:08:57 -0500 (EST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
cc:	Rusty Russell <rusty@...tcorp.com.au>, Ingo Molnar <mingo@...e.hu>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	akpm@...ux-foundation.org, hpa@...or.com, brgerst@...il.com,
	ebiederm@...ssion.com, travis@....com,
	linux-kernel@...r.kernel.org, steiner@....com, hugh@...itas.com,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors

On Tue, 27 Jan 2009, Tejun Heo wrote:

> > later). That's because they use TLB tricks for a static 64k per-cpu
> > area, but this doesn't scale.  That might not be vital: abandoning
> > that trick will mean they can't optimise read_percpu/read_percpu_var
> > etc as much.

Why wont it scale? this is a separate TLB entry for each processor.

>
> Isn't something like the following possible?
>
> #define pcpu_read(ptr)						\
> ({								\
> 	if (__builtin_constant_p(ptr) &&			\
> 	    ptr >= PCPU_STATIC_START && ptr < PCPU_STATIC_END)	\
> 		do 64k TLB trick for static pcpu;		\
> 	else							\
> 		do generic stuff;				\
> })

The TLB trick is just to access the percpu data at a fixed base. I.e.
	value = SHIFT_PERCPU_PTR(percpu_var, FIXED_ADDRESS);

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