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

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Date:	Mon, 18 May 2009 14:07:15 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	"H. Peter Anvin" <hpa@...or.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

On Mon, 2009-05-18 at 12:41 -0700, H. Peter Anvin wrote:
> 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.

This %gs:absolute type accesses are for static percpu data.

But what I was referring to is the dynamic percpu data(accessed through
per_cpu_ptr()). Instead of combining some part of the dynamic percpu
data into  the static percpu data(first percpu chunk), we can use
different chunks for dynamic percpu data and governed by a different
per_cpu_dynamic_offset[NR_CPUS] array.

Then we can use large-page kernel direct mappings for static percpu data
(or %gs:offset) and small-page vmalloc mappings for dynamic percpu data.

thanks,
suresh

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