Re: [PATCH v2 2/5] mm, compaction: simplify handling restart position in free pages scanner

[Michal Nazarewicz]

On Fri, Jul 31 2015, Vlastimil Babka wrote:
> Handling the position where compaction free scanner should restart (stored in
> cc->free_pfn) got more complex with commit e14c720efdd7 ("mm, compaction:
> remember position within pageblock in free pages scanner"). Currently the
> position is updated in each loop iteration of isolate_freepages(), although it
> should be enough to update it only when breaking from the loop. There's also
> an extra check outside the loop updates the position in case we have met the
> migration scanner.
>
> This can be simplified if we move the test for having isolated enough from the
> for-loop header next to the test for contention, and determining the restart
> position only in these cases. We can reuse the isolate_start_pfn variable for
> this instead of setting cc->free_pfn directly. Outside the loop, we can simply
> set cc->free_pfn to current value of isolate_start_pfn without any extra check.
>
> Also add a VM_BUG_ON to catch possible mistake in the future, in case we later
> add a new condition that terminates isolate_freepages_block() prematurely
> without also considering the condition in isolate_freepages().
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Acked-by: Mel Gorman <mgorman@xxxxxxx>
> Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>

Acked-by: 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 | 35 ++++++++++++++++++++---------------
>  1 file changed, 20 insertions(+), 15 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index d46aaeb..7e0a814 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -947,8 +947,7 @@ static void isolate_freepages(struct compact_control *cc)
>  	 * pages on cc->migratepages. We stop searching if the migrate
>  	 * and free page scanners meet or enough free pages are isolated.
>  	 */
> -	for (; block_start_pfn >= low_pfn &&
> -			cc->nr_migratepages > cc->nr_freepages;
> +	for (; block_start_pfn >= low_pfn;
>  				block_end_pfn = block_start_pfn,
>  				block_start_pfn -= pageblock_nr_pages,
>  				isolate_start_pfn = block_start_pfn) {
> @@ -980,6 +979,8 @@ static void isolate_freepages(struct compact_control *cc)
>  					block_end_pfn, freelist, false);
>  
>  		/*
> +		 * If we isolated enough freepages, or aborted due to async
> +		 * compaction being contended, terminate the loop.
>  		 * Remember where the free scanner should restart next time,
>  		 * which is where isolate_freepages_block() left off.
>  		 * But if it scanned the whole pageblock, isolate_start_pfn
> @@ -988,27 +989,31 @@ static void isolate_freepages(struct compact_control *cc)
>  		 * In that case we will however want to restart at the start
>  		 * of the previous pageblock.
>  		 */
> -		cc->free_pfn = (isolate_start_pfn < block_end_pfn) ?
> -				isolate_start_pfn :
> -				block_start_pfn - pageblock_nr_pages;
> -
> -		/*
> -		 * isolate_freepages_block() might have aborted due to async
> -		 * compaction being contended
> -		 */
> -		if (cc->contended)
> +		if ((cc->nr_freepages >= cc->nr_migratepages)
> +							|| cc->contended) {
> +			if (isolate_start_pfn >= block_end_pfn)
> +				isolate_start_pfn =
> +					block_start_pfn - pageblock_nr_pages;
>  			break;
> +		} else {
> +			/*
> +			 * isolate_freepages_block() should not terminate
> +			 * prematurely unless contended, or isolated enough
> +			 */
> +			VM_BUG_ON(isolate_start_pfn < block_end_pfn);
> +		}
>  	}
>  
>  	/* split_free_page does not map the pages */
>  	map_pages(freelist);
>  
>  	/*
> -	 * If we crossed the migrate scanner, we want to keep it that way
> -	 * so that compact_finished() may detect this
> +	 * Record where the free scanner will restart next time. Either we
> +	 * broke from the loop and set isolate_start_pfn based on the last
> +	 * call to isolate_freepages_block(), or we met the migration scanner
> +	 * and the loop terminated due to isolate_start_pfn < low_pfn
>  	 */
> -	if (block_start_pfn < low_pfn)
> -		cc->free_pfn = cc->migrate_pfn;
> +	cc->free_pfn = isolate_start_pfn;
>  }
>  
>  /*
> -- 
> 2.4.6
>

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  MichaÅ âmina86â Nazarewicz    (o o)
ooo +--<mpn@xxxxxxxxxx>--<xmpp:mina86@xxxxxxxxxx>--ooO--(_)--Ooo--
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