linux-kernel - Re: [PATCH v2 1/9] mm/compaction: skip useless pfn when updating cached pfn

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Date:	Mon, 24 Aug 2015 11:07:12 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <js1304@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 1/9] mm/compaction: skip useless pfn when updating
 cached pfn

On 08/24/2015 04:19 AM, Joonsoo Kim wrote:
> Cached pfn is used to determine the start position of scanner
> at next compaction run. Current cached pfn points the skipped pageblock
> so we uselessly checks whether pageblock is valid for compaction and
> skip-bit is set or not. If we set scanner's cached pfn to next pfn of
> skipped pageblock, we don't need to do this check.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
>   mm/compaction.c | 13 ++++++-------
>   1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 6ef2fdf..c2d3d6a 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -261,10 +261,9 @@ void reset_isolation_suitable(pg_data_t *pgdat)
>    */
>   static void update_pageblock_skip(struct compact_control *cc,
>   			struct page *page, unsigned long nr_isolated,
> -			bool migrate_scanner)
> +			unsigned long pfn, bool migrate_scanner)
>   {
>   	struct zone *zone = cc->zone;
> -	unsigned long pfn;
>
>   	if (cc->ignore_skip_hint)
>   		return;
> @@ -277,8 +276,6 @@ static void update_pageblock_skip(struct compact_control *cc,
>
>   	set_pageblock_skip(page);
>
> -	pfn = page_to_pfn(page);
> -
>   	/* Update where async and sync compaction should restart */
>   	if (migrate_scanner) {
>   		if (pfn > zone->compact_cached_migrate_pfn[0])
> @@ -300,7 +297,7 @@ static inline bool isolation_suitable(struct compact_control *cc,
>
>   static void update_pageblock_skip(struct compact_control *cc,
>   			struct page *page, unsigned long nr_isolated,
> -			bool migrate_scanner)
> +			unsigned long pfn, bool migrate_scanner)
>   {
>   }
>   #endif /* CONFIG_COMPACTION */
> @@ -509,7 +506,8 @@ isolate_fail:
>
>   	/* Update the pageblock-skip if the whole pageblock was scanned */
>   	if (blockpfn == end_pfn)
> -		update_pageblock_skip(cc, valid_page, total_isolated, false);
> +		update_pageblock_skip(cc, valid_page, total_isolated,
> +					end_pfn, false);

In isolate_freepages_block() this means we actually go logically *back* 
one pageblock, as the direction is opposite? I know it's not an issue 
after the redesign patch so you wouldn't notice it when testing the 
whole series. But there's a non-zero chance that the smaller fixes are 
merged first and the redesign later...

>
>   	count_compact_events(COMPACTFREE_SCANNED, nr_scanned);
>   	if (total_isolated)
> @@ -811,7 +809,8 @@ isolate_success:
>   	 * if the whole pageblock was scanned without isolating any page.
>   	 */
>   	if (low_pfn == end_pfn)
> -		update_pageblock_skip(cc, valid_page, nr_isolated, true);
> +		update_pageblock_skip(cc, valid_page, nr_isolated,
> +					end_pfn, true);
>
>   	trace_mm_compaction_isolate_migratepages(start_pfn, low_pfn,
>   						nr_scanned, nr_isolated);
>

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