Re: [PATCH 05/10] mm, compaction: remember position within pageblock in free pages scanner

[Minchan Kim]

On Mon, Jun 09, 2014 at 11:26:17AM +0200, Vlastimil Babka wrote:
> Unlike the migration scanner, the free scanner remembers the beginning of the
> last scanned pageblock in cc->free_pfn. It might be therefore rescanning pages
> uselessly when called several times during single compaction. This might have
> been useful when pages were returned to the buddy allocator after a failed
> migration, but this is no longer the case.
> 
> This patch changes the meaning of cc->free_pfn so that if it points to a
> middle of a pageblock, that pageblock is scanned only from cc->free_pfn to the
> end. isolate_freepages_block() will record the pfn of the last page it looked
> at, which is then used to update cc->free_pfn.
> 
> In the mmtests stress-highalloc benchmark, this has resulted in lowering the
> ratio between pages scanned by both scanners, from 2.5 free pages per migrate
> page, to 2.25 free pages per migrate page, without affecting success rates.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx>

Below is a nitpick.

> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>  mm/compaction.c | 33 ++++++++++++++++++++++++++++-----
>  1 file changed, 28 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 83f72bd..58dfaaa 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -297,7 +297,7 @@ static bool suitable_migration_target(struct page *page)
>   * (even though it may still end up isolating some pages).
>   */
>  static unsigned long isolate_freepages_block(struct compact_control *cc,
> -				unsigned long blockpfn,
> +				unsigned long *start_pfn,
>  				unsigned long end_pfn,
>  				struct list_head *freelist,
>  				bool strict)
> @@ -306,6 +306,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  	struct page *cursor, *valid_page = NULL;
>  	unsigned long flags;
>  	bool locked = false;
> +	unsigned long blockpfn = *start_pfn;
>  
>  	cursor = pfn_to_page(blockpfn);
>  
> @@ -314,6 +315,9 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  		int isolated, i;
>  		struct page *page = cursor;
>  
> +		/* Record how far we have got within the block */
> +		*start_pfn = blockpfn;
> +

Couldn't we move this out of the loop for just one store?

>  		/*
>  		 * Periodically drop the lock (if held) regardless of its
>  		 * contention, to give chance to IRQs. Abort async compaction
> @@ -424,6 +428,9 @@ isolate_freepages_range(struct compact_control *cc,
>  	LIST_HEAD(freelist);
>  
>  	for (pfn = start_pfn; pfn < end_pfn; pfn += isolated) {
> +		/* Protect pfn from changing by isolate_freepages_block */
> +		unsigned long isolate_start_pfn = pfn;
> +
>  		if (!pfn_valid(pfn) || cc->zone != page_zone(pfn_to_page(pfn)))
>  			break;
>  
> @@ -434,8 +441,8 @@ isolate_freepages_range(struct compact_control *cc,
>  		block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
>  		block_end_pfn = min(block_end_pfn, end_pfn);
>  
> -		isolated = isolate_freepages_block(cc, pfn, block_end_pfn,
> -						   &freelist, true);
> +		isolated = isolate_freepages_block(cc, &isolate_start_pfn,
> +						block_end_pfn, &freelist, true);
>  
>  		/*
>  		 * In strict mode, isolate_freepages_block() returns 0 if
> @@ -774,6 +781,7 @@ static void isolate_freepages(struct zone *zone,
>  				block_end_pfn = block_start_pfn,
>  				block_start_pfn -= pageblock_nr_pages) {
>  		unsigned long isolated;
> +		unsigned long isolate_start_pfn;
>  
>  		/*
>  		 * This can iterate a massively long zone without finding any
> @@ -807,12 +815,27 @@ static void isolate_freepages(struct zone *zone,
>  			continue;
>  
>  		/* Found a block suitable for isolating free pages from */
> -		cc->free_pfn = block_start_pfn;
> -		isolated = isolate_freepages_block(cc, block_start_pfn,
> +		isolate_start_pfn = block_start_pfn;
> +
> +		/*
> +		 * If we are restarting the free scanner in this block, do not
> +		 * rescan the beginning of the block
> +		 */
> +		if (cc->free_pfn < block_end_pfn)
> +			isolate_start_pfn = cc->free_pfn;
> +
> +		isolated = isolate_freepages_block(cc, &isolate_start_pfn,
>  					block_end_pfn, freelist, false);
>  		nr_freepages += isolated;
>  
>  		/*
> +		 * Remember where the free scanner should restart next time.
> +		 * This will point to the last page of pageblock we just
> +		 * scanned, if we scanned it fully.
> +		 */
> +		cc->free_pfn = isolate_start_pfn;
> +
> +		/*
>  		 * Set a flag that we successfully isolated in this pageblock.
>  		 * In the next loop iteration, zone->compact_cached_free_pfn
>  		 * will not be updated and thus it will effectively contain the
> -- 
> 1.8.4.5
> 
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-- 
Kind regards,
Minchan Kim
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