Re: [PATCH 20/25] mm, compaction: Reduce unnecessary skipping of migration target scanner

[Vlastimil Babka]

On 1/4/19 1:50 PM, Mel Gorman wrote:
> The fast isolation of pages can move the scanner faster than is necessary
> depending on the contents of the free list. This patch will only allow
> the fast isolation to initialise the scanner and advance it slowly. The
> primary means of moving the scanner forward is via the linear scanner
> to reduce the likelihood the migration source/target scanners meet
> prematurely triggering a rescan.

Maybe I've seen enough code today and need to stop, but AFAICS the description
here doesn't match the actual code changes? What I see are some cleanups, and a
change in free scanner that will set pageblock skip bit after a pageblock has
been scanned, even if there were pages isolated, while previously it would set
the skip bit only if nothing was isolated.

>                                         4.20.0                 4.20.0
>                                noresched-v2r15         slowfree-v2r15
> Amean     fault-both-1         0.00 (   0.00%)        0.00 *   0.00%*
> Amean     fault-both-3      2736.50 (   0.00%)     2512.53 (   8.18%)
> Amean     fault-both-5      4133.70 (   0.00%)     4159.43 (  -0.62%)
> Amean     fault-both-7      5738.61 (   0.00%)     5950.15 (  -3.69%)
> Amean     fault-both-12     9392.82 (   0.00%)     8674.38 (   7.65%)
> Amean     fault-both-18    13257.15 (   0.00%)    12850.79 (   3.07%)
> Amean     fault-both-24    16859.44 (   0.00%)    17242.86 (  -2.27%)
> Amean     fault-both-30    16249.30 (   0.00%)    19404.18 * -19.42%*
> Amean     fault-both-32    14904.71 (   0.00%)    16200.79 (  -8.70%)
> 
> The impact to latency, success rates and scan rates is marginal but
> avoiding unnecessary restarts is important. It helps later patches that
> are more careful about how pageblocks are treated as earlier iterations
> of those patches hit corner cases where the restarts were punishing and
> very visible.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>  mm/compaction.c | 27 ++++++++++-----------------
>  1 file changed, 10 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 75eb0d40d4d7..6c5552c6d8f9 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -324,10 +324,9 @@ static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
>   * future. The information is later cleared by __reset_isolation_suitable().
>   */
>  static void update_pageblock_skip(struct compact_control *cc,
> -			struct page *page, unsigned long nr_isolated)
> +			struct page *page, unsigned long pfn)
>  {
>  	struct zone *zone = cc->zone;
> -	unsigned long pfn;
>  
>  	if (cc->no_set_skip_hint)
>  		return;
> @@ -335,13 +334,8 @@ static void update_pageblock_skip(struct compact_control *cc,
>  	if (!page)
>  		return;
>  
> -	if (nr_isolated)
> -		return;
> -
>  	set_pageblock_skip(page);
>  
> -	pfn = page_to_pfn(page);
> -
>  	/* Update where async and sync compaction should restart */
>  	if (pfn < zone->compact_cached_free_pfn)
>  		zone->compact_cached_free_pfn = pfn;
> @@ -359,7 +353,7 @@ static inline bool pageblock_skip_persistent(struct page *page)
>  }
>  
>  static inline void update_pageblock_skip(struct compact_control *cc,
> -			struct page *page, unsigned long nr_isolated)
> +			struct page *page, unsigned long pfn)
>  {
>  }
>  
> @@ -450,7 +444,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  				bool strict)
>  {
>  	int nr_scanned = 0, total_isolated = 0;
> -	struct page *cursor, *valid_page = NULL;
> +	struct page *cursor;
>  	unsigned long flags = 0;
>  	bool locked = false;
>  	unsigned long blockpfn = *start_pfn;
> @@ -477,9 +471,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  		if (!pfn_valid_within(blockpfn))
>  			goto isolate_fail;
>  
> -		if (!valid_page)
> -			valid_page = page;
> -
>  		/*
>  		 * For compound pages such as THP and hugetlbfs, we can save
>  		 * potentially a lot of iterations if we skip them at once.
> @@ -576,10 +567,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  	if (strict && blockpfn < end_pfn)
>  		total_isolated = 0;
>  
> -	/* Update the pageblock-skip if the whole pageblock was scanned */
> -	if (blockpfn == end_pfn)
> -		update_pageblock_skip(cc, valid_page, total_isolated);
> -
>  	cc->total_free_scanned += nr_scanned;
>  	if (total_isolated)
>  		count_compact_events(COMPACTISOLATED, total_isolated);
> @@ -1295,8 +1282,10 @@ fast_isolate_freepages(struct compact_control *cc)
>  		}
>  	}
>  
> -	if (highest && highest > cc->zone->compact_cached_free_pfn)
> +	if (highest && highest >= cc->zone->compact_cached_free_pfn) {
> +		highest -= pageblock_nr_pages;
>  		cc->zone->compact_cached_free_pfn = highest;
> +	}
>  
>  	cc->total_free_scanned += nr_scanned;
>  	if (!page)
> @@ -1376,6 +1365,10 @@ static void isolate_freepages(struct compact_control *cc)
>  		isolate_freepages_block(cc, &isolate_start_pfn, block_end_pfn,
>  					freelist, false);
>  
> +		/* Update the skip hint if the full pageblock was scanned */
> +		if (isolate_start_pfn == block_end_pfn)
> +			update_pageblock_skip(cc, page, block_start_pfn);
> +
>  		/* Are enough freepages isolated? */
>  		if (cc->nr_freepages >= cc->nr_migratepages) {
>  			if (isolate_start_pfn >= block_end_pfn) {
>