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Message-ID: <20250407180154.63348-2-hannes@cmpxchg.org>
Date: Mon, 7 Apr 2025 14:01:54 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Vlastimil Babka <vbabka@...e.cz>,
Brendan Jackman <jackmanb@...gle.com>,
Mel Gorman <mgorman@...hsingularity.net>,
Carlos Song <carlos.song@....com>,
linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] 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% )
Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
mm/compaction.c | 4 +---
mm/internal.h | 2 +-
mm/page_alloc.c | 31 +++++++++++++------------------
3 files changed, 15 insertions(+), 22 deletions(-)
diff --git a/mm/compaction.c b/mm/compaction.c
index 139f00c0308a..7462a02802a5 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2348,7 +2348,6 @@ 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))
@@ -2364,8 +2363,7 @@ 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
diff --git a/mm/internal.h b/mm/internal.h
index 50c2f590b2d0..55384b9971c3 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -915,7 +915,7 @@ 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)
{
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 03b0d45ed45a..1522e3a29b16 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2077,31 +2077,25 @@ 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;
}
@@ -2206,7 +2200,6 @@ __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
@@ -2225,11 +2218,14 @@ __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);
@@ -2254,12 +2250,11 @@ __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;
--
2.49.0
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