From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Lameter <cl@linux-foundation.org>
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors
Date: Tue, 27 Jan 2009 15:08:57 -0500 (EST)
Message-ID: <alpine.DEB.1.10.0901271506580.3114@qirst.com>
References: <20090115183942.GA6325@elte.hu> <200901201328.24605.rusty@rustcorp.com.au> <49756E6E.2060409@kernel.org> <200901271213.18605.rusty@rustcorp.com.au> <497E705B.5000302@kernel.org>
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Cc: Rusty Russell <rusty@rustcorp.com.au>, Ingo Molnar <mingo@elte.hu>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	akpm@linux-foundation.org, hpa@zytor.com, brgerst@gmail.com,
	ebiederm@xmission.com, travis@sgi.com,
	linux-kernel@vger.kernel.org, steiner@sgi.com, hugh@veritas.com,
	"David S. Miller" <davem@davemloft.net>, netdev@vger.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
To: Tejun Heo <tj@kernel.org>
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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);