linux-kernel - Re: [PATCH 18/22] mm: page allocator: Split magazine lock in two to reduce contention

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Date:	Thu, 09 May 2013 08:21:13 -0700
From:	Dave Hansen <dave@...1.net>
To:	Mel Gorman <mgorman@...e.de>
CC:	Linux-MM <linux-mm@...ck.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Lameter <cl@...ux.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 18/22] mm: page allocator: Split magazine lock in two
 to reduce contention

On 05/08/2013 09:03 AM, Mel Gorman wrote:
> @@ -368,10 +375,9 @@ struct zone {
>  
>  	/*
>  	 * Keep some order-0 pages on a separate free list
> -	 * protected by an irq-unsafe lock
> +	 * protected by an irq-unsafe lock.
>  	 */
> -	spinlock_t			_magazine_lock;
> -	struct free_area_magazine	_noirq_magazine;
> +	struct free_magazine	noirq_magazine[NR_MAGAZINES];

Looks like pretty cool stuff!

The old per-cpu-pages stuff was all hung off alloc_percpu(), which
surely wasted lots of memory with many NUMA nodes.  It's nice to see
this decoupled a bit from the online cpu count.

That said, the alloc_percpu() stuff is nice in how much it hides from
you when doing cpu hotplug.  We'll _probably_ need this to be
dynamically-sized at some point, right?

> -static inline struct free_area_magazine *find_lock_magazine(struct zone *zone)
> +static inline struct free_magazine *lock_magazine(struct zone *zone)
>  {
> -	struct free_area_magazine *area = &zone->_noirq_magazine;
> -	spin_lock(&zone->_magazine_lock);
> -	return area;
> +	int i = (raw_smp_processor_id() >> 1) & (NR_MAGAZINES-1);
> +	spin_lock(&zone->noirq_magazine[i].lock);
> +	return &zone->noirq_magazine[i];
>  }

I bet this logic will be fun to play with once we have more magazines
around.  For instance, on my system processors 0/80 are HT twins, so
they'd always be going after the same magazine.  I guess that's a good
thing.
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