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

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Date:	Tue, 10 Jun 2008 10:42:15 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Rusty Russell <rusty@...tcorp.com.au>
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 Mon, 2 Jun 2008, Rusty Russell wrote:

> > Believe me I have tried to use local_t repeatedly for vm statistics etc.
> > It always fails on that issue.
> 
> Frankly, I am finding it increasingly easy to believe that you failed.  But 
> you are blaming the wrong thing.
> 
> There are three implementations of local_t which are obvious.  The best is for 
> architectures which can locate and increment a per-cpu var in one instruction 
> (eg. x86).  Otherwise, using atomic_t/atomic64_t for local_t provides a 
> general solution.  The other general solution would involve 
> local_irq_disable()/increment/local_irq_enable().
> 
> My (fading) hope is that this idiocy is an abberation,

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.

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.

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.

4. The modeling of local_t on atomic_t limits it to 32bit! There is no
   way to use this with pointers or 64 bit entities. Adding that would 
   duplicate the API for each type added.
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