Date:	Wed, 24 Mar 2010 10:19:27 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 10/11] Direct compact when a high-order allocation fails

On Tue, 23 Mar 2010 12:25:45 +0000
Mel Gorman <mel@....ul.ie> wrote:

> Ordinarily when a high-order allocation fails, direct reclaim is entered to
> free pages to satisfy the allocation.  With this patch, it is determined if
> an allocation failed due to external fragmentation instead of low memory
> and if so, the calling process will compact until a suitable page is
> freed. Compaction by moving pages in memory is considerably cheaper than
> paging out to disk and works where there are locked pages or no swap. If
> compaction fails to free a page of a suitable size, then reclaim will
> still occur.
> 
> Direct compaction returns as soon as possible. As each block is compacted,
> it is checked if a suitable page has been freed and if so, it returns.
> 
> Signed-off-by: Mel Gorman <mel@....ul.ie>
> Acked-by: Rik van Riel <riel@...hat.com>
> ---
>  include/linux/compaction.h |   16 +++++-
>  include/linux/vmstat.h     |    1 +
>  mm/compaction.c            |  118 ++++++++++++++++++++++++++++++++++++++++++++
>  mm/page_alloc.c            |   26 ++++++++++
>  mm/vmstat.c                |   15 +++++-
>  5 files changed, 172 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index c94890b..b851428 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -1,14 +1,26 @@
>  #ifndef _LINUX_COMPACTION_H
>  #define _LINUX_COMPACTION_H
>  
> -/* Return values for compact_zone() */
> +/* Return values for compact_zone() and try_to_compact_pages() */
>  #define COMPACT_INCOMPLETE	0
> -#define COMPACT_COMPLETE	1
> +#define COMPACT_PARTIAL		1
> +#define COMPACT_COMPLETE	2
>  
>  #ifdef CONFIG_COMPACTION
>  extern int sysctl_compact_memory;
>  extern int sysctl_compaction_handler(struct ctl_table *table, int write,
>  			void __user *buffer, size_t *length, loff_t *ppos);
> +
> +extern int fragmentation_index(struct zone *zone, unsigned int order);
> +extern unsigned long try_to_compact_pages(struct zonelist *zonelist,
> +			int order, gfp_t gfp_mask, nodemask_t *mask);
> +#else
> +static inline unsigned long try_to_compact_pages(struct zonelist *zonelist,
> +			int order, gfp_t gfp_mask, nodemask_t *nodemask)
> +{
> +	return COMPACT_INCOMPLETE;
> +}
> +
>  #endif /* CONFIG_COMPACTION */
>  
>  #if defined(CONFIG_COMPACTION) && defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index 56e4b44..b4b4d34 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -44,6 +44,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  		KSWAPD_SKIP_CONGESTION_WAIT,
>  		PAGEOUTRUN, ALLOCSTALL, PGROTATED,
>  		COMPACTBLOCKS, COMPACTPAGES, COMPACTPAGEFAILED,
> +		COMPACTSTALL, COMPACTFAIL, COMPACTSUCCESS,
>  #ifdef CONFIG_HUGETLB_PAGE
>  		HTLB_BUDDY_PGALLOC, HTLB_BUDDY_PGALLOC_FAIL,
>  #endif
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 8df6e3d..6688700 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -34,6 +34,8 @@ struct compact_control {
>  	unsigned long nr_anon;
>  	unsigned long nr_file;
>  
> +	unsigned int order;		/* order a direct compactor needs */
> +	int migratetype;		/* MOVABLE, RECLAIMABLE etc */
>  	struct zone *zone;
>  };
>  
> @@ -301,10 +303,31 @@ static void update_nr_listpages(struct compact_control *cc)
>  static inline int compact_finished(struct zone *zone,
>  						struct compact_control *cc)
>  {
> +	unsigned int order;
> +	unsigned long watermark = low_wmark_pages(zone) + (1 << cc->order);
> +
>  	/* Compaction run completes if the migrate and free scanner meet */
>  	if (cc->free_pfn <= cc->migrate_pfn)
>  		return COMPACT_COMPLETE;
>  
> +	/* Compaction run is not finished if the watermark is not met */
> +	if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
> +		return COMPACT_INCOMPLETE;
> +
> +	if (cc->order == -1)
> +		return COMPACT_INCOMPLETE;
> +
> +	/* Direct compactor: Is a suitable page free? */
> +	for (order = cc->order; order < MAX_ORDER; order++) {
> +		/* Job done if page is free of the right migratetype */
> +		if (!list_empty(&zone->free_area[order].free_list[cc->migratetype]))
> +			return COMPACT_PARTIAL;
> +
> +		/* Job done if allocation would set block type */
> +		if (order >= pageblock_order && zone->free_area[order].nr_free)
> +			return COMPACT_PARTIAL;
> +	}
> +
>  	return COMPACT_INCOMPLETE;
>  }
>  
> @@ -348,6 +371,101 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
>  	return ret;
>  }
>  
> +static inline unsigned long compact_zone_order(struct zone *zone,
> +						int order, gfp_t gfp_mask)
> +{
> +	struct compact_control cc = {
> +		.nr_freepages = 0,
> +		.nr_migratepages = 0,
> +		.order = order,
> +		.migratetype = allocflags_to_migratetype(gfp_mask),
> +		.zone = zone,
> +	};
> +	INIT_LIST_HEAD(&cc.freepages);
> +	INIT_LIST_HEAD(&cc.migratepages);
> +
> +	return compact_zone(zone, &cc);
> +}
> +
> +/**
> + * try_to_compact_pages - Direct compact to satisfy a high-order allocation
> + * @zonelist: The zonelist used for the current allocation
> + * @order: The order of the current allocation
> + * @gfp_mask: The GFP mask of the current allocation
> + * @nodemask: The allowed nodes to allocate from
> + *
> + * This is the main entry point for direct page compaction.
> + */
> +unsigned long try_to_compact_pages(struct zonelist *zonelist,
> +			int order, gfp_t gfp_mask, nodemask_t *nodemask)
> +{
> +	enum zone_type high_zoneidx = gfp_zone(gfp_mask);
> +	int may_enter_fs = gfp_mask & __GFP_FS;
> +	int may_perform_io = gfp_mask & __GFP_IO;
> +	unsigned long watermark;
> +	struct zoneref *z;
> +	struct zone *zone;
> +	int rc = COMPACT_INCOMPLETE;
> +
> +	/* Check whether it is worth even starting compaction */
> +	if (order == 0 || !may_enter_fs || !may_perform_io)
> +		return rc;
> +
> +	/*
> +	 * We will not stall if the necessary conditions are not met for
> +	 * migration but direct reclaim seems to account stalls similarly
> +	 */
> +	count_vm_event(COMPACTSTALL);
> +
> +	/* Compact each zone in the list */
> +	for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx,
> +								nodemask) {
> +		int fragindex;
> +		int status;
> +
> +		/*
> +		 * Watermarks for order-0 must be met for compaction. Note
> +		 * the 2UL. This is because during migration, copies of
> +		 * pages need to be allocated and for a short time, the
> +		 * footprint is higher
> +		 */
> +		watermark = low_wmark_pages(zone) + (2UL << order);
> +		if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
> +			continue;
> +
> +		/*
> +		 * fragmentation index determines if allocation failures are
> +		 * due to low memory or external fragmentation
> +		 *
> +		 * index of -1 implies allocations might succeed depending
> +		 * 	on watermarks
> +		 * index < 500 implies alloc failure is due to lack of memory
> +		 *
> +		 * XXX: The choice of 500 is arbitrary. Reinvestigate
> +		 *      appropriately to determine a sensible default.
> +		 *      and what it means when watermarks are also taken
> +		 *      into account. Consider making it a sysctl
> +		 */
> +		fragindex = fragmentation_index(zone, order);
> +		if (fragindex >= 0 && fragindex <= 500)
> +			continue;
> +
> +		if (fragindex == -1 && zone_watermark_ok(zone, order, watermark, 0, 0)) {
> +			rc = COMPACT_PARTIAL;
> +			break;
> +		}
> +
> +		status = compact_zone_order(zone, order, gfp_mask);
> +		rc = max(status, rc);

Hm...then, scanning over the whole zone until success of migration at
each failure ? Is it meaningful that multiple tasks run direct-compaction against
a zone (from zone->start_pfn to zone->end_pfn) in parallel ?
ex) running order=3 compaction while other thread runs order=5 compaction.

Can't we find a clever way to find [start_pfn, end_pfn) for scanning rather than
[zone->start_pfn, zone->start_pfn + zone->spanned_pages) ?

I'm sorry if I miss something...

Thanks,
-Kame


> +
> +		if (zone_watermark_ok(zone, order, watermark, 0, 0))
> +			break;
> +	}
> +
> +	return rc;
> +}
> +
> +
>  /* Compact all zones within a node */
>  static int compact_node(int nid)
>  {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9708143..e301108 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -49,6 +49,7 @@
>  #include <linux/debugobjects.h>
>  #include <linux/kmemleak.h>
>  #include <linux/memory.h>
> +#include <linux/compaction.h>
>  #include <trace/events/kmem.h>
>  #include <linux/ftrace_event.h>
>  
> @@ -1765,6 +1766,31 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
>  
>  	cond_resched();
>  
> +	/* Try memory compaction for high-order allocations before reclaim */
> +	if (order) {
> +		*did_some_progress = try_to_compact_pages(zonelist,
> +						order, gfp_mask, nodemask);
> +		if (*did_some_progress != COMPACT_INCOMPLETE) {
> +			page = get_page_from_freelist(gfp_mask, nodemask,
> +					order, zonelist, high_zoneidx,
> +					alloc_flags, preferred_zone,
> +					migratetype);
> +			if (page) {
> +				__count_vm_event(COMPACTSUCCESS);
> +				return page;
> +			}
> +
> +			/*
> +			 * It's bad if compaction run occurs and fails.
> +			 * The most likely reason is that pages exist,
> +			 * but not enough to satisfy watermarks.
> +			 */
> +			count_vm_event(COMPACTFAIL);
> +
> +			cond_resched();
> +		}
> +	}
> +
>  	/* We now go into synchronous reclaim */
>  	cpuset_memory_pressure_bump();
>  	p->flags |= PF_MEMALLOC;
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index af88647..c88f285 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -560,7 +560,7 @@ static int unusable_show(struct seq_file *m, void *arg)
>   * The value can be used to determine if page reclaim or compaction
>   * should be used
>   */
> -int fragmentation_index(unsigned int order, struct contig_page_info *info)
> +int __fragmentation_index(unsigned int order, struct contig_page_info *info)
>  {
>  	unsigned long requested = 1UL << order;
>  
> @@ -580,6 +580,14 @@ int fragmentation_index(unsigned int order, struct contig_page_info *info)
>  	return 1000 - ( (1000+(info->free_pages * 1000 / requested)) / info->free_blocks_total);
>  }
>  
> +/* Same as __fragmentation index but allocs contig_page_info on stack */
> +int fragmentation_index(struct zone *zone, unsigned int order)
> +{
> +	struct contig_page_info info;
> +
> +	fill_contig_page_info(zone, order, &info);
> +	return __fragmentation_index(order, &info);
> +}
>  
>  static void extfrag_show_print(struct seq_file *m,
>  					pg_data_t *pgdat, struct zone *zone)
> @@ -595,7 +603,7 @@ static void extfrag_show_print(struct seq_file *m,
>  				zone->name);
>  	for (order = 0; order < MAX_ORDER; ++order) {
>  		fill_contig_page_info(zone, order, &info);
> -		index = fragmentation_index(order, &info);
> +		index = __fragmentation_index(order, &info);
>  		seq_printf(m, "%d.%03d ", index / 1000, index % 1000);
>  	}
>  
> @@ -895,6 +903,9 @@ static const char * const vmstat_text[] = {
>  	"compact_blocks_moved",
>  	"compact_pages_moved",
>  	"compact_pagemigrate_failed",
> +	"compact_stall",
> +	"compact_fail",
> +	"compact_success",
>  
>  #ifdef CONFIG_HUGETLB_PAGE
>  	"htlb_buddy_alloc_success",
> -- 
> 1.6.5
> 
> 

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