linux-kernel - Re: [this_cpu_xx V7 1/8] this_cpu

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Message-ID: <4B282F9E.7070902@kernel.org>
Date:	Wed, 16 Dec 2009 09:53:50 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, Mel Gorman <mel@....ul.ie>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [this_cpu_xx V7 1/8] this_cpu_ops: page allocator conversion

Hello, Christoph.

On 12/16/2009 12:04 AM, Christoph Lameter wrote:
> A static array would have to be dimensioned to NR_CPUS. That is one thing
> we are trying to avoid.
> 
> The assignment of the pageset "percpu" pointer does not mean that the pcp
> is usable. It must first be properly initialized through setup_pageset().
>
> setup_pageset() is run for each cpu. zone->pageset is the same for all
> cpus that is why it is in zone_pcp_init() and not in setup_pageset().
> 
> The boot pageset initialization was moved into __build_all_zonelists(). We
> could move the zone->pageset initialization there too?

Maybe that is a bit less scary. (at least for me :-) The reason why
I'm a bit worried is that different architectures handle percpu
pointers differently before setup_per_cpu_areas().  x86 sets up the
offsets and stuff such that cpu0 can access the original percpu
section in the kernel image.  ia64 sets up everything properly way
before setup_per_cpu_areas() and in some archs percpu pointers are
completely invalid before setup_per_cpu_areas().  So, percpu pointer
being handled in generic code which is being called before percpu
setup is a bit worrying.

Another thing is that there were attempts to simplify memory
initialization stages such that bootmem is removed and page / k*
allocators can be used earlier which kind of puts percpu allocator in
the dependency loop but I don't think it's something we need to worry
about at this point.

Thanks.

-- 
tejun
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