linux-kernel - Re: [patch] add optimized generic percpu accessors

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Message-ID: <496F2032.5080502@kernel.org>
Date:	Thu, 15 Jan 2009 20:38:26 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	"H. Peter Anvin" <hpa@...or.com>, Brian Gerst <brgerst@...il.com>,
	ebiederm@...ssion.com, cl@...ux-foundation.org,
	rusty@...tcorp.com.au, travis@....com,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	steiner@....com, hugh@...itas.com
Subject: Re: [patch] add optimized generic percpu accessors

Hello,

Ingo Molnar wrote:
> The new ops are a pretty nice and clean solution i think.
> 
> Firstly, accessing the current CPU is the only safe shortcut anyway (there 
> is where we can do %fs/%gs / rip-relative addressing modes), and the 
> generic per_cpu() APIs dont really provide that guarantee for us. We might 
> be able to hook into __get_cpu_var() but those both require to be an 
> lvalue and are also relatively rarely used.
> 
> So introducing the new, rather straightforward APIs and using them 
> wherever they matter for performance is good. Your patchset already shaved 
> off an instruction from ordinary per_cpu() accesses, so it's all moving 
> rather close to the most-optimal situation already.

Yeah, I don't think we can do much better than those ops.  I have two
issues tho.

1. percpu_and() is missing.  I added it for completeness's sake.

2. The generic percpu_op() should be local to the cpu, so it should
   expand to...

   do { get_cpu_var(var) OP (val); put_cpu_var(var) } while (0)

   as the original x86_OP_percpu() did.  Right?

Thanks.

-- 
tejun
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