Commit ee414bd9 authored by Johannes Weiner's avatar Johannes Weiner Committed by Andrew Morton
Browse files

mm: page_alloc: tighten up find_suitable_fallback() 

find_suitable_fallback() is not as efficient as it could be, and somewhat
difficult to follow.

1. should_try_claim_block() is a loop invariant. There is no point in
   checking fallback areas if the caller is interested in claimable
   blocks but the order and the migratetype don't allow for that.

2. __rmqueue_steal() doesn't care about claimability, so it shouldn't
   have to run those tests.

Different callers want different things from this helper:

1. __compact_finished() scans orders up until it finds a claimable block
2. __rmqueue_claim() scans orders down as long as blocks are claimable
3. __rmqueue_steal() doesn't care about claimability at all

Move should_try_claim_block() out of the loop. Only test it for the
two callers who care in the first place. Distinguish "no blocks" from
"order + mt are not claimable" in the return value; __rmqueue_claim()
can stop once order becomes unclaimable, __compact_finished() can keep
advancing until order becomes claimable.

Before:

 Performance counter stats for './run case-lru-file-mmap-read' (5 runs):

	 85,294.85 msec task-clock                       #    5.644 CPUs utilized               ( +-  0.32% )
	    15,968      context-switches                 #  187.209 /sec                        ( +-  3.81% )
	       153      cpu-migrations                   #    1.794 /sec                        ( +-  3.29% )
	   801,808      page-faults                      #    9.400 K/sec                       ( +-  0.10% )
   733,358,331,786      instructions                     #    1.87  insn per cycle              ( +-  0.20% )  (64.94%)
   392,622,904,199      cycles                           #    4.603 GHz                         ( +-  0.31% )  (64.84%)
   148,563,488,531      branches                         #    1.742 G/sec                       ( +-  0.18% )  (63.86%)
       152,143,228      branch-misses                    #    0.10% of all branches             ( +-  1.19% )  (62.82%)

	   15.1128 +- 0.0637 seconds time elapsed  ( +-  0.42% )

After:

 Performance counter stats for './run case-lru-file-mmap-read' (5 runs):

         84,380.21 msec task-clock                       #    5.664 CPUs utilized               ( +-  0.21% )
            16,656      context-switches                 #  197.392 /sec                        ( +-  3.27% )
               151      cpu-migrations                   #    1.790 /sec                        ( +-  3.28% )
           801,703      page-faults                      #    9.501 K/sec                       ( +-  0.09% )
   731,914,183,060      instructions                     #    1.88  insn per cycle              ( +-  0.38% )  (64.90%)
   388,673,535,116      cycles                           #    4.606 GHz                         ( +-  0.24% )  (65.06%)
   148,251,482,143      branches                         #    1.757 G/sec                       ( +-  0.37% )  (63.92%)
       149,766,550      branch-misses                    #    0.10% of all branches             ( +-  1.22% )  (62.88%)

           14.8968 +- 0.0486 seconds time elapsed  ( +-  0.33% )

Link: https://lkml.kernel.org/r/20250407180154.63348-2-hannes@cmpxchg.org


Signed-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Reviewed-by: default avatarBrendan Jackman <jackmanb@google.com>
Tested-by: default avatarShivank Garg <shivankg@amd.com>
Reviewed-by: default avatarVlastimil Babka <vbabka@suse.cz>
Cc: Carlos Song <carlos.song@nxp.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 6e3092d7

mm/compaction.c

+1 −3
Original line number Diff line number Diff line
@@ -2344,7 +2344,6 @@ static enum compact_result __compact_finished(struct compact_control *cc) 
	ret = COMPACT_NO_SUITABLE_PAGE;

	for (order = cc->order; order < NR_PAGE_ORDERS; order++) {

		struct free_area *area = &cc->zone->free_area[order];

		bool claim_block;



		/* Job done if page is free of the right migratetype */

		if (!free_area_empty(area, migratetype))
@@ -2360,8 +2359,7 @@ static enum compact_result __compact_finished(struct compact_control *cc) 
		 * Job done if allocation would steal freepages from

		 * other migratetype buddy lists.

		 */

		if (find_suitable_fallback(area, order, migratetype,

						true, &claim_block) != -1)

		if (find_suitable_fallback(area, order, migratetype, true) >= 0)

			/*

			 * Movable pages are OK in any pageblock. If we are

			 * stealing for a non-movable allocation, make sure

mm/internal.h

+1 −1
Original line number Diff line number Diff line
@@ -910,7 +910,7 @@ static inline void init_cma_pageblock(struct page *page) 




int find_suitable_fallback(struct free_area *area, unsigned int order,

			int migratetype, bool claim_only, bool *claim_block);

			   int migratetype, bool claimable);



static inline bool free_area_empty(struct free_area *area, int migratetype)

{

mm/page_alloc.c

+13 −18
Original line number Diff line number Diff line
@@ -2068,31 +2068,25 @@ static bool should_try_claim_block(unsigned int order, int start_mt) 


/*

 * Check whether there is a suitable fallback freepage with requested order.

 * Sets *claim_block to instruct the caller whether it should convert a whole

 * pageblock to the returned migratetype.

 * If only_claim is true, this function returns fallback_mt only if

 * If claimable is true, this function returns fallback_mt only if

 * we would do this whole-block claiming. This would help to reduce

 * fragmentation due to mixed migratetype pages in one pageblock.

 */

int find_suitable_fallback(struct free_area *area, unsigned int order,

			int migratetype, bool only_claim, bool *claim_block)

			   int migratetype, bool claimable)

{

	int i;

	int fallback_mt;



	if (claimable && !should_try_claim_block(order, migratetype))

		return -2;



	if (area->nr_free == 0)

		return -1;



	*claim_block = false;

	for (i = 0; i < MIGRATE_PCPTYPES - 1 ; i++) {

		fallback_mt = fallbacks[migratetype][i];

		if (free_area_empty(area, fallback_mt))

			continue;

		int fallback_mt = fallbacks[migratetype][i];



		if (should_try_claim_block(order, migratetype))

			*claim_block = true;



		if (*claim_block || !only_claim)

		if (!free_area_empty(area, fallback_mt))

			return fallback_mt;

	}
@@ -2189,7 +2183,6 @@ __rmqueue_claim(struct zone *zone, int order, int start_migratetype, 
	int min_order = order;

	struct page *page;

	int fallback_mt;

	bool claim_block;



	/*

	 * Do not steal pages from freelists belonging to other pageblocks
@@ -2208,11 +2201,14 @@ __rmqueue_claim(struct zone *zone, int order, int start_migratetype, 
				--current_order) {

		area = &(zone->free_area[current_order]);

		fallback_mt = find_suitable_fallback(area, current_order,

				start_migratetype, false, &claim_block);

						     start_migratetype, true);



		/* No block in that order */

		if (fallback_mt == -1)

			continue;



		if (!claim_block)

		/* Advanced into orders too low to claim, abort */

		if (fallback_mt == -2)

			break;



		page = get_page_from_free_area(area, fallback_mt);
@@ -2240,12 +2236,11 @@ __rmqueue_steal(struct zone *zone, int order, int start_migratetype) 
	int current_order;

	struct page *page;

	int fallback_mt;

	bool claim_block;



	for (current_order = order; current_order < NR_PAGE_ORDERS; current_order++) {

		area = &(zone->free_area[current_order]);

		fallback_mt = find_suitable_fallback(area, current_order,

				start_migratetype, false, &claim_block);

						     start_migratetype, false);

		if (fallback_mt == -1)

			continue;