linux-kernel - Re: [GIT PATCH] x86,percpu: fix pageattr handling with remap allocator

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Date:	Mon, 18 May 2009 12:41:27 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	suresh.b.siddha@...el.com
CC:	Tejun Heo <tj@...nel.org>,
	"JBeulich@...ell.com" <JBeulich@...ell.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel-owner@...r.kernel.org" 
	<linux-kernel-owner@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PATCH] x86,percpu: fix pageattr handling with remap allocator

Suresh Siddha wrote:
> 
> Can we don't use PERCPU_DYNAMIC_RESERVE in the first chunk and for
> dynamic per_cpu_ptr's we can use some other offset such as
> per_cpu_ptr_offset() or some such thing?
> 
> Then we can separate the static and dynamic chunks. And use large page
> kernel-direct mappings for fast access for critical common things and
> use small page accesses for dynamic and not so common accesses.
> 
> Just checking to see if we can reduce the complexity of setting up the
> percpu areas (different versions for embed, non-embed etc) and handling
> all these aliases with simple code, rather than making it complex.
> 

I'm confused what you're suggesting here.  The whole point of the percpu
unification work is that we can use %gs:absolute type references to hit
a variable right away.  Although in theory we could use both %fs and %gs
for pointers, setting up both would greatly increase the cost of
entering the kernel, especially on 64 bits.

This means all percpu data has to have a constant (virtual) offset from
the beginning of the static percpu area.

	-hpa

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