linux-kernel - Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

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Date:	Wed, 11 Jun 2008 10:03:36 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

On Tuesday 10 June 2008 13:18:25 Christoph Lameter wrote:
> On Tue, 10 Jun 2008, Rusty Russell wrote:
> > > Right that is what the cpu alloc patches do. So you could implement
> > > cpu_local_inc on top of some of the cpu alloc patches.
> >
> > Or you could just implement it today as a standalone patch.
>
> You need at least the zero basing to enable the use of the segment
> register on x86_64.

Indeed.  Works for i386 as is, but 64 bit will need that patch.

> > > But then the whole point of local_t is gone. Why not use atomic_t in
> > > the first place?
> >
> > Because some archs can do better.
>
> The argument does not make any sense. First you want to use atomic_t then
> not?

You're being obtuse.  See previous mail about the three possible 
implementations of local_t, and the comment in asm-generic/local.h.

The paths forward are clear:
1) Improve x86 local_t (mostly orthogonal to the others, but useful).
2) Implement extensible per-cpu areas.
3) Generalize per-cpu accessors.
4) Extend or replace the module.c per-cpu allocator to alloc from the other
   areas.
5) Convert alloc_percpu et al. to use the new code.

Hope that clarifies,
Rusty.
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