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Message-ID: <20090115122611.GJ22850@elte.hu>
Date: Thu, 15 Jan 2009 13:26:11 +0100
From: Ingo Molnar <mingo@...e.hu>
To: Tejun Heo <tj@...nel.org>
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
* Tejun Heo <tj@...nel.org> wrote:
> 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.
Sure - it would be commonly used as well. Perhaps we dont need
percpu_xor() at all? (or and and ops already give a complete algebra)
> 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.
hm, that removes much of its appeal - a preempt off+on sequence is quite
bloaty. Most percpu usage sites are already within critical sections.
I think they are more analogous to per_cpu(var, cpu), which does not
disable preemption either. There's no 'get/put' in them, which signals
that they dont auto-disable preemption.
Ingo
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