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Message-ID: <496F0F50.6070200@kernel.org>
Date: Thu, 15 Jan 2009 19:26:24 +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.
Ingo Molnar wrote:
> Tejun, could you please also add the patch below to your lineup too?
Sure thing.
> It is an optimization and a cleanup, and adds the following new generic
> percpu methods:
>
> percpu_read()
> percpu_write()
> percpu_add()
> percpu_sub()
> percpu_or()
> percpu_xor()
>
> and implements support for them on x86. (other architectures will fall
> back to a default implementation)
>
> The advantage is that for example to read a local percpu variable, instead
> of this sequence:
>
> return __get_cpu_var(var);
>
> ffffffff8102ca2b: 48 8b 14 fd 80 09 74 mov -0x7e8bf680(,%rdi,8),%rdx
> ffffffff8102ca32: 81
> ffffffff8102ca33: 48 c7 c0 d8 59 00 00 mov $0x59d8,%rax
> ffffffff8102ca3a: 48 8b 04 10 mov (%rax,%rdx,1),%rax
>
> We can get a single instruction by using the optimized variants:
>
> return percpu_read(var);
>
> ffffffff8102ca3f: 65 48 8b 05 91 8f fd mov %gs:0x7efd8f91(%rip),%rax
>
> I also cleaned up the x86-specific APIs and made the x86 code use these
> new generic percpu primitives.
>
> It looks quite hard to convince the compiler to generate the optimized
> single-instruction sequence for us out of __get_cpu_var(var) - or can you
> perhaps see a way to do it?
Yeah, I thought about that too but couldn't think of a way to persuade
the compiler because the compiler doesn't know how to access the
address. I'll play with it a bit more but the clumsy percpu_*()
accessors probably might be the only way. :-(
Thanks.
--
tejun
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