linux-kernel - Re: local_add

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Date:	Wed, 24 Dec 2008 22:12:55 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	David Miller <davem@...emloft.net>, rostedt@...dmis.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	paulus@...ba.org, benh@...nel.crashing.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	Christoph Lameter <christoph@...eter.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Martin Bligh <mbligh@...gle.com>
Subject: Re: local_add_return

On Tuesday 23 December 2008 05:13:28 Mathieu Desnoyers wrote:
> > I can be convinced, but I'll need more than speculation.  Assuming
> > local_long_atomic_t, can you produce a patch which uses it somewhere else?
> 
> I had this patch applying over Christoph Lameter's vm tree last
> February. It did accelerate the slub fastpath allocator by using
> cmpxchg_local rather than disabling interrupts. cmpxchg_local is not
> using the local_t type, but behaves similarly to local_cmpxchg.
> 
> http://lkml.org/lkml/2008/2/28/568

OK, I'll buy that.  So we split local_t into a counter and an atomic type.

> I know that
> local_counter_long_t and local_atomic_long_t are painful to write, but
> that would follow the current atomic_t vs atomic_long_t semantics. Hm ?

OK, I've looked at how they're used, to try to figure out whether long
is the right thing.  Counters generally want to be long, but I was in doubt
about atomics; yet grep shows that atomic_long_t is quite popular.  Then
I hit struct nfs_iostats which would want a u64 and a long.  I don't think
we want local_counter_u64 etc.

Just thinking out loud, perhaps a new *type* is the wrong direction?  How
about a set of macros which take a fundamental type, such as:

	DECLARE_LOCAL_COUNTER(type, name);
	local_counter_inc(type, addr);
	...
	DECLARE_LOCAL_ATOMIC(type, name);
	local_atomic_add_return(type, addr);

This allows pointers, u32, u64, long, etc.  If a 32-bit arch can't do 64-bit
local_counter_inc easily, at least the hairy 64-bit code can be eliminated at
compile time.

Or maybe that's overdesign?
Rusty.
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