linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Message-ID: <m1ej61y2da.fsf@frodo.ebiederm.org>
Date:	Thu, 10 Jul 2008 12:18:25 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Mike Travis <travis@....com>, "H. Peter Anvin" <hpa@...or.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> Eric W. Biederman wrote:
>> I think we can get away with just simply realloc'ing the percpu area
>> on each cpu.  No fancy table manipulations required.  Just update
>> the base pointer in %gs and in someplace global.
>>
>
> It's perfectly legitimate to take the address of a percpu variable and store it
> somewhere.  We can't move them around.

Really?  I guess there are cases where that makes sense.  It is a pretty
rare case though.  Especially when you are not talking about doing it temporarily
with preemption disabled.  There are few enough users of the API I think we can
certainly explore the cost of forbidding in the general case of storing the
address of a percpu variable.

Eric

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