linux-kernel - Re: [PATCH -mm 02/25] Use an indexed array for LRU variables

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Date:	Sat, 7 Jun 2008 14:42:45 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, lee.schermerhorn@...com,
	kosaki.motohiro@...fujitsu.com, clameter@....com
Subject: Re: [PATCH -mm 02/25] Use an indexed array for LRU variables

On Fri, 6 Jun 2008 18:04:26 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> I would have spat the dummy at pointless churn and code uglification
> but I see that we end up with five LRU lsits so ho hum.

> >  	/* Fields commonly accessed by the page reclaim scanner */
> >  	spinlock_t		lru_lock;	
> > -	struct list_head	active_list;
> > -	struct list_head	inactive_list;
> > -	unsigned long		nr_scan_active;
> > -	unsigned long		nr_scan_inactive;
> > +	struct list_head	list[NR_LRU_LISTS];
> > +	unsigned long		nr_scan[NR_LRU_LISTS];
> 
> It'd be a little cache-friendlier to lay this out as
> 
> 	struct {
> 		struct list_head list;
> 		unsigned long nr_scan;
> 	} lru_stuff[NR_LRU_LISTS];

OK, done.  As zone.lru though for brevity.

> > +++ linux-2.6.26-rc2-mm1/include/linux/mm_inline.h	2008-05-23 14:21:33.000000000 -0400
> > @@ -1,40 +1,51 @@
> >  static inline void
> > +add_page_to_lru_list(struct zone *zone, struct page *page, enum lru_list l)
> > +{
> > +	list_add(&page->lru, &zone->list[l]);
> > +	__inc_zone_state(zone, NR_INACTIVE + l);
> 
>                                ^ that's a bug, isn't it?
> 
> oh, no it isn't.
> 
> Can we rename NR_INACTIVE?  Maybe VMSCAN_BASE or something?

I turned it into NR_LRU_BASE, since everything in zone_stat_item
starts with NR_

enum zone_stat_item {
        /* First 128 byte cacheline (assuming 64 bit words) */
        NR_FREE_PAGES,
        NR_LRU_BASE,
        NR_INACTIVE = NR_LRU_BASE, /* must match order of LRU_[IN]ACTIVE */
        NR_ACTIVE,      /*  "     "     "   "       "         */

> > -			if (PageActive(page))
> > -				add_page_to_active_list(zone, page);
> > -			else
> > -				add_page_to_inactive_list(zone, page);
> > +			add_page_to_lru_list(zone, page, PageActive(page));
> 
> urgh.  the third arg to add_page_to_lru_list() is an `enum lru_list'
> and here we are secretly coercing PageActive()'s boolean return into a
> just-happens-to-be-right `enum lru_list'.
> 
> That's pretty nasty?

I've pulled page_lru() back from patch 03/25 into 02/25 and
will use that.
 
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