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 21:10:56 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	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>,
	Mike Travis <travis@....com>
Subject: Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

On Wednesday 11 June 2008 03:42:15 Christoph Lameter wrote:
> 1. The x86 implementation does not exist because the segment register has
>    so far not been available on x86_64. So you could not do the solution.
>    You need the zero basing. Then you can use per_xxx_add in cpu_inc.

Yes: for 64 bit x86, getting rid of the PDA or zero-basing is required.

> 2. The general solution created overhead that is often not needed. If we
>    would have done vm event counters with local_t then we would have
>    atomic overhead for each increment on f.e. IA64. That was not
>    acceptable. cpu_alloc never falls back to atomic operations.

You can implement it either way.  I've said that three times now.  The current 
generic one uses atomics, but preempt disable/enable is possible.

> 3. local_t is based on the atomic logic. But percpu handling is
>    fundamentally different in that accesses without the special macros
>    are okay provided you are in a non preemptible or irq context!
>    A local_t declaration makes such accesses impossible.

Again, untrue.  The interface is already there.  So feel free to implement 
__cpu_local_inc et al in terms of preempt enable and disable so it doesn't 
need to use atomics.  

> 4. The modeling of local_t on atomic_t limits it to 32bit!

Again wrong.  And adding an exclamation mark doesn't make it true.

Rusty.
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