linux-kernel - Re: [this_cpu_xx V4 12/20] Move early initialization of pagesets out of zone_wait_table

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Message-ID: <20091005093534.GA12681@csn.ul.ie>
Date:	Mon, 5 Oct 2009 10:35:35 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, mingo@...e.hu,
	rusty@...tcorp.com.au, Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [this_cpu_xx V4 12/20] Move early initialization of pagesets
	out of zone_wait_table_init()

On Fri, Oct 02, 2009 at 01:30:58PM -0400, Christoph Lameter wrote:
> On Fri, 2 Oct 2009, Mel Gorman wrote:
> 
> > On Thu, Oct 01, 2009 at 05:25:33PM -0400, cl@...ux-foundation.org wrote:
> > > Explicitly initialize the pagesets after the per cpu areas have been
> > > initialized. This is necessary in order to be able to use per cpu
> > > operations in later patches.
> > >
> >
> > Can you be more explicit about this? I think the reasoning is as follows
> >
> > A later patch will use DEFINE_PER_CPU which allocates memory later in
> > the boot-cycle after zones have already been initialised. Without this
> > patch, use of DEFINE_PER_CPU would result in invalid memory accesses
> > during pageset initialisation.
> 
> Nope. Pagesets are not statically allocated per cpu data. They are
> allocated with the per cpu allocator.
> 

I don't think I said they were statically allocated.

> The per cpu allocator is not initialized that early in boot. We cannot
> allocate the pagesets then. Therefore we use a fake single item pageset
> (like used now for NUMA boot) to take its place until the slab and percpu
> allocators are up. Then we allocate the real pagesets.
> 

Ok, that explanation matches my expectations. Thanks.

> > > -static __meminit void zone_pcp_init(struct zone *zone)
> > > +/*
> > > + * Early setup of pagesets.
> > > + *
> > > + * In the NUMA case the pageset setup simply results in all zones pcp
> > > + * pointer being directed at a per cpu pageset with zero batchsize.
> > > + *
> >
> > The batchsize becomes 1, not 0 if you look at setup_pageset() but that aside,
> > it's unclear from the comment *why* the batchsize is 1 in the NUMA case.
> > Maybe something like the following?
> >
> > =====
> > In the NUMA case, the boot_pageset is used until the slab allocator is
> > available to allocate per-zone pagesets as each CPU is brought up. At
> > this point, the batchsize is set to 1 to prevent pages "leaking" onto the
> > boot_pageset freelists.
> > =====
> >
> > Otherwise, nothing in the patch jumped out at me other than to double
> > check CPU-up events actually result in process_zones() being called and
> > that boot_pageset is not being accidentally used in the long term.
> 
> This is already explained in a commment where boot_pageset is defined.
> Should we add some more elaborate comments to zone_pcp_init()?
> 

I suppose not. Point them to the comment in boot_pageset so there is a
chance the comment stays up to date.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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