linux-kernel - Re: [this_cpu_xx V4 13/20] this_cpu

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Message-ID: <alpine.DEB.1.10.0910021331020.26189@gentwo.org>
Date:	Fri, 2 Oct 2009 13:39:28 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
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 13/20] this_cpu_ops: page allocator conversion

On Fri, 2 Oct 2009, Mel Gorman wrote:

> On Thu, Oct 01, 2009 at 05:25:34PM -0400, cl@...ux-foundation.org wrote:
> > Use the per cpu allocator functionality to avoid per cpu arrays in struct zone.
> >
> > This drastically reduces the size of struct zone for systems with large
> > amounts of processors and allows placement of critical variables of struct
> > zone in one cacheline even on very large systems.
> >
>
> This seems reasonably accurate. The largest shrink is on !NUMA configured
> systems but the NUMA case deletes a lot of pointers.

True, the !NUMA case will then avoid allocating pagesets for unused
zones. But the NUMA case will have the most benefit since the large arrays
in struct zone are gone. Removing the pagesets from struct zone also
increases the cacheability of struct zone information. This is
particularly useful since the size of the pagesets grew with the addition
of the various types of allocation queues.

> > Another effect is that the pagesets of one processor are placed near one
> > another. If multiple pagesets from different zones fit into one cacheline
> > then additional cacheline fetches can be avoided on the hot paths when
> > allocating memory from multiple zones.
> >
>
> Out of curiousity, how common an occurance is it that a CPU allocate from
> multiple zones? I would have thought it was rare but I never checked
> either.

zone allocations are determined by their use. GFP_KERNEL allocs come from
ZONE_NORMAL whereas typical application pages may come from ZONE_HIGHMEM.
The mix depends on what the kernel and the application are doing.

> >  		pcp = &pset->pcp;
> > +	pcp = &this_cpu_ptr(zone->pageset)->pcp;
> >  	migratetype = get_pageblock_migratetype(page);
> >  	set_page_private(page, migratetype);
> >  	local_irq_save(flags);
> > @@ -1133,7 +1133,6 @@ static void free_hot_cold_page(struct pa
> >
> >  out:
> >  	local_irq_restore(flags);
> > -	put_cpu();
>
> Previously we get_cpu() to be preemption safe. We then disable
> interrupts and potentially take a spinlock later.

Right. WE need to move the local_irq_save() up two lines. Why disable
preempt and two instructions later disable interrupts? Isnt this bloating
the code?

> this_cpu_ptr() looks up PCP
> disable interrupts
> enable interrupts

Move disable interrupts before the this_cpu_ptr?

> > +/*
> > + * Allocate per cpu pagesets and initialize them.
> > + * Before this call only boot pagesets were available.
> > + * Boot pagesets will no longer be used after this call is complete.
>
> If they are no longer used, do we get the memory back?

No we need to keep them for onlining new processors.

> > -	 * as it comes online
> > +		for_each_possible_cpu(cpu) {
> > +			struct per_cpu_pageset *pcp = per_cpu_ptr(zone->pageset, cpu);
> > +
> > +			setup_pageset(pcp, zone_batchsize(zone));
> > +
> > +			if (percpu_pagelist_fraction)
> > +				setup_pagelist_highmark(pcp,
> > +					(zone->present_pages /
> > +						percpu_pagelist_fraction));
> > +		}
> > +	}
>
> This would have been easier to review if you left process_zones() where it
> was and converted it to the new API. I'm assuming this is just shuffling
> code around.

Yes I think this was the result of reducing #ifdefs.

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