Re: [PATCH v2 2/4] mm/compaction: stop the isolation when we isolate enough freepage

[Vlastimil Babka <vbabka@suse.cz>]

On 01/30/2015 01:34 PM, Joonsoo Kim wrote:
> From: Joonsoo <iamjoonsoo.kim@lge.com>
> 
> Currently, freepage isolation in one pageblock doesn't consider how many
> freepages we isolate. When I traced flow of compaction, compaction
> sometimes isolates more than 256 freepages to migrate just 32 pages.
> 
> In this patch, freepage isolation is stopped at the point that we
> have more isolated freepage than isolated page for migration. This
> results in slowing down free page scanner and make compaction success
> rate higher.
> 
> stress-highalloc test in mmtests with non movable order 7 allocation shows
> increase of compaction success rate.
> 
> Compaction success rate (Compaction success * 100 / Compaction stalls, %)
> 27.13 : 31.82
> 
> pfn where both scanners meets on compaction complete
> (separate test due to enormous tracepoint buffer)
> (zone_start=4096, zone_end=1048576)
> 586034 : 654378

Now I that I know that scanners meeting further in zone is better for success
rate, the better success rate makes sense. Still not sure why they meet further :)

> In fact, I didn't fully understand why this patch results in such good
> result. There was a guess that not used freepages are released to pcp list
> and on next compaction trial we won't isolate them again so compaction
> success rate would decrease. To prevent this effect, I tested with adding
> pcp drain code on release_freepages(), but, it has no good effect.
> 
> Anyway, this patch reduces waste time to isolate unneeded freepages so
> seems reasonable.

I briefly tried it on top of the pivot-changing series and with order-9
allocations it reduced free page scanned counter by almost 10%. No effect on
success rates (maybe because pivot changing already took care of the scanners
meeting problem) but the scanning reduction is good on its own.

It also explains why e14c720efdd7 ("mm, compaction: remember position within
pageblock in free pages scanner") had less than expected improvements. It would
only actually stop within pageblock in case of async compaction detecting
contention. I guess that's also why the infinite loop problem fixed by
1d5bfe1ffb5b affected so relatively few people.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks!

> ---
>  mm/compaction.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 4954e19..782772d 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -490,6 +490,13 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  
>  		/* If a page was split, advance to the end of it */
>  		if (isolated) {
> +			cc->nr_freepages += isolated;
> +			if (!strict &&
> +				cc->nr_migratepages <= cc->nr_freepages) {
> +				blockpfn += isolated;
> +				break;
> +			}
> +
>  			blockpfn += isolated - 1;
>  			cursor += isolated - 1;
>  			continue;
> @@ -899,7 +906,6 @@ static void isolate_freepages(struct compact_control *cc)
>  	unsigned long isolate_start_pfn; /* exact pfn we start at */
>  	unsigned long block_end_pfn;	/* end of current pageblock */
>  	unsigned long low_pfn;	     /* lowest pfn scanner is able to scan */
> -	int nr_freepages = cc->nr_freepages;
>  	struct list_head *freelist = &cc->freepages;
>  
>  	/*
> @@ -924,11 +930,11 @@ 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 > nr_freepages;
> +	for (; block_start_pfn >= low_pfn &&
> +			cc->nr_migratepages > cc->nr_freepages;
>  				block_end_pfn = block_start_pfn,
>  				block_start_pfn -= pageblock_nr_pages,
>  				isolate_start_pfn = block_start_pfn) {
> -		unsigned long isolated;
>  
>  		/*
>  		 * This can iterate a massively long zone without finding any
> @@ -953,9 +959,8 @@ static void isolate_freepages(struct compact_control *cc)
>  			continue;
>  
>  		/* Found a block suitable for isolating free pages from. */
> -		isolated = isolate_freepages_block(cc, &isolate_start_pfn,
> +		isolate_freepages_block(cc, &isolate_start_pfn,
>  					block_end_pfn, freelist, false);
> -		nr_freepages += isolated;
>  
>  		/*
>  		 * Remember where the free scanner should restart next time,
> @@ -987,8 +992,6 @@ static void isolate_freepages(struct compact_control *cc)
>  	 */
>  	if (block_start_pfn < low_pfn)
>  		cc->free_pfn = cc->migrate_pfn;
> -
> -	cc->nr_freepages = nr_freepages;
>  }
>  
>  /*
>