The patch titled
Subject: mm, compaction: use free lists to quickly locate a migration source
has been added to the -mm tree. Its filename is
mm-compaction-use-free-lists-to-quickly-locate-a-migration-source.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-compaction-use-free-lists-to-quickly-locate-a-migration-source.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-compaction-use-free-lists-to-quickly-locate-a-migration-source.patch
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------------------------------------------------------
From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Subject: mm, compaction: use free lists to quickly locate a migration source
The migration scanner is a linear scan of a zone with a potentially large
search space. Furthermore, many pageblocks are unusable such as those
filled with reserved pages or partially filled with pages that cannot
migrate. These still get scanned in the common case of allocating a THP
and the cost accumulates.
The patch uses a partial search of the free lists to locate a migration
source candidate that is marked as MOVABLE when allocating a THP. It
prefers picking a block with a larger number of free pages already on the
basis that there are fewer pages to migrate to free the entire block. The
lowest PFN found during searches is tracked as the basis of the start for
the linear search after the first search of the free list fails. After
the search, the free list is shuffled so that the next search will not
encounter the same page. If the search fails then the subsequent searches
will be shorter and the linear scanner is used.
If this search fails, or if the request is for a small or
unmovable/reclaimable allocation then the linear scanner is still used.
It is somewhat pointless to use the list search in those cases. Small
free pages must be used for the search and there is no guarantee that
movable pages are located within that block that are contiguous.
4.20.0 4.20.0
failfast-v2r15 findmig-v2r15
Amean fault-both-1 0.00 ( 0.00%) 0.00 * 0.00%*
Amean fault-both-3 3833.72 ( 0.00%) 3505.69 ( 8.56%)
Amean fault-both-5 4967.15 ( 0.00%) 5794.13 * -16.65%*
Amean fault-both-7 7139.19 ( 0.00%) 7663.09 ( -7.34%)
Amean fault-both-12 11326.30 ( 0.00%) 10983.36 ( 3.03%)
Amean fault-both-18 16270.70 ( 0.00%) 13602.71 * 16.40%*
Amean fault-both-24 19839.65 ( 0.00%) 16145.77 * 18.62%*
Amean fault-both-30 21707.05 ( 0.00%) 19753.82 ( 9.00%)
Amean fault-both-32 21968.16 ( 0.00%) 20616.16 ( 6.15%)
4.20.0 4.20.0
failfast-v2r15 findmig-v2r15
Percentage huge-1 0.00 ( 0.00%) 0.00 ( 0.00%)
Percentage huge-3 84.62 ( 0.00%) 90.58 ( 7.05%)
Percentage huge-5 88.43 ( 0.00%) 91.34 ( 3.29%)
Percentage huge-7 88.33 ( 0.00%) 92.21 ( 4.39%)
Percentage huge-12 88.74 ( 0.00%) 92.48 ( 4.21%)
Percentage huge-18 86.52 ( 0.00%) 91.65 ( 5.93%)
Percentage huge-24 86.42 ( 0.00%) 90.23 ( 4.41%)
Percentage huge-30 86.67 ( 0.00%) 90.17 ( 4.04%)
Percentage huge-32 86.00 ( 0.00%) 89.72 ( 4.32%)
This shows an improvement in allocation latencies and a slight increase in
allocation success rates. While not presented, there was a 13% reduction
in migration scanning and a 10% reduction on system CPU usage. A 2-socket
machine showed similar benefits.
Link: http://lkml.kernel.org/r/20190104125011.16071-12-mgorman@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
--- a/mm/compaction.c~mm-compaction-use-free-lists-to-quickly-locate-a-migration-source
+++ a/mm/compaction.c
@@ -1050,6 +1050,12 @@ static bool suitable_migration_target(st
return false;
}
+static inline unsigned int
+freelist_scan_limit(struct compact_control *cc)
+{
+ return (COMPACT_CLUSTER_MAX >> cc->fast_search_fail) + 1;
+}
+
/*
* Test whether the free scanner has reached the same or lower pageblock than
* the migration scanner, and compaction should thus terminate.
@@ -1060,6 +1066,19 @@ static inline bool compact_scanners_met(
<= (cc->migrate_pfn >> pageblock_order);
}
+/* Reorder the free list to reduce repeated future searches */
+static void
+move_freelist_tail(struct list_head *freelist, struct page *freepage)
+{
+ LIST_HEAD(sublist);
+
+ if (!list_is_last(freelist, &freepage->lru)) {
+ list_cut_position(&sublist, freelist, &freepage->lru);
+ if (!list_empty(&sublist))
+ list_splice_tail(&sublist, freelist);
+ }
+}
+
/*
* Based on information in the current compact_control, find blocks
* suitable for isolating free pages from and then isolate them.
@@ -1217,6 +1236,160 @@ typedef enum {
*/
int sysctl_compact_unevictable_allowed __read_mostly = 1;
+static inline void
+update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)
+{
+ if (cc->fast_start_pfn == ULONG_MAX)
+ return;
+
+ if (!cc->fast_start_pfn)
+ cc->fast_start_pfn = pfn;
+
+ cc->fast_start_pfn = min(cc->fast_start_pfn, pfn);
+}
+
+static inline void
+reinit_migrate_pfn(struct compact_control *cc)
+{
+ if (!cc->fast_start_pfn || cc->fast_start_pfn == ULONG_MAX)
+ return;
+
+ cc->migrate_pfn = cc->fast_start_pfn;
+ cc->fast_start_pfn = ULONG_MAX;
+}
+
+/*
+ * Briefly search the free lists for a migration source that already has
+ * some free pages to reduce the number of pages that need migration
+ * before a pageblock is free.
+ */
+static unsigned long fast_find_migrateblock(struct compact_control *cc)
+{
+ unsigned int limit = freelist_scan_limit(cc);
+ unsigned int nr_scanned = 0;
+ unsigned long distance;
+ unsigned long pfn = cc->migrate_pfn;
+ unsigned long high_pfn;
+ int order;
+
+ /* Skip hints are relied on to avoid repeats on the fast search */
+ if (cc->ignore_skip_hint)
+ return pfn;
+
+ /*
+ * If the migrate_pfn is not at the start of a zone or the start
+ * of a pageblock then assume this is a continuation of a previous
+ * scan restarted due to COMPACT_CLUSTER_MAX.
+ */
+ if (pfn != cc->zone->zone_start_pfn && pfn != pageblock_start_pfn(pfn))
+ return pfn;
+
+ /*
+ * For smaller orders, just linearly scan as the number of pages
+ * to migrate should be relatively small and does not necessarily
+ * justify freeing up a large block for a small allocation.
+ */
+ if (cc->order <= PAGE_ALLOC_COSTLY_ORDER)
+ return pfn;
+
+ /*
+ * Only allow kcompactd and direct requests for movable pages to
+ * quickly clear out a MOVABLE pageblock for allocation. This
+ * reduces the risk that a large movable pageblock is freed for
+ * an unmovable/reclaimable small allocation.
+ */
+ if (cc->direct_compaction && cc->migratetype != MIGRATE_MOVABLE)
+ return pfn;
+
+ /*
+ * When starting the migration scanner, pick any pageblock within the
+ * first half of the search space. Otherwise try and pick a pageblock
+ * within the first eighth to reduce the chances that a migration
+ * target later becomes a source.
+ */
+ distance = (cc->free_pfn - cc->migrate_pfn) >> 1;
+ if (cc->migrate_pfn != cc->zone->zone_start_pfn)
+ distance >>= 2;
+ high_pfn = pageblock_start_pfn(cc->migrate_pfn + distance);
+
+ for (order = cc->order - 1;
+ order >= PAGE_ALLOC_COSTLY_ORDER && pfn == cc->migrate_pfn && nr_scanned < limit;
+ order--) {
+ struct free_area *area = &cc->zone->free_area[order];
+ struct list_head *freelist;
+ unsigned long nr_skipped = 0;
+ unsigned long flags;
+ struct page *freepage;
+
+ if (!area->nr_free)
+ continue;
+
+ spin_lock_irqsave(&cc->zone->lock, flags);
+ freelist = &area->free_list[MIGRATE_MOVABLE];
+ list_for_each_entry(freepage, freelist, lru) {
+ unsigned long free_pfn;
+
+ nr_scanned++;
+ free_pfn = page_to_pfn(freepage);
+ if (free_pfn < high_pfn) {
+ update_fast_start_pfn(cc, free_pfn);
+
+ /*
+ * Avoid if skipped recently. Move to the tail
+ * of the list so it will not be found again
+ * soon
+ */
+ if (get_pageblock_skip(freepage)) {
+
+ if (list_is_last(freelist, &freepage->lru))
+ break;
+
+ nr_skipped++;
+ list_del(&freepage->lru);
+ list_add_tail(&freepage->lru, freelist);
+ if (nr_skipped > 2)
+ break;
+ continue;
+ }
+
+ /* Reorder to so a future search skips recent pages */
+ move_freelist_tail(freelist, freepage);
+
+ pfn = pageblock_start_pfn(free_pfn);
+ cc->fast_search_fail = 0;
+ set_pageblock_skip(freepage);
+ break;
+ }
+
+ /*
+ * If low PFNs are being found and discarded then
+ * limit the scan as fast searching is finding
+ * poor candidates.
+ */
+ if (free_pfn < cc->migrate_pfn)
+ limit >>= 1;
+
+ if (nr_scanned >= limit) {
+ cc->fast_search_fail++;
+ move_freelist_tail(freelist, freepage);
+ break;
+ }
+ }
+ spin_unlock_irqrestore(&cc->zone->lock, flags);
+ }
+
+ cc->total_migrate_scanned += nr_scanned;
+
+ /*
+ * If fast scanning failed then use a cached entry for a page block
+ * that had free pages as the basis for starting a linear scan.
+ */
+ if (pfn == cc->migrate_pfn)
+ reinit_migrate_pfn(cc);
+
+ return pfn;
+}
+
/*
* Isolate all pages that can be migrated from the first suitable block,
* starting at the block pointed to by the migrate scanner pfn within
@@ -1235,9 +1408,10 @@ static isolate_migrate_t isolate_migrate
/*
* Start at where we last stopped, or beginning of the zone as
- * initialized by compact_zone()
+ * initialized by compact_zone(). The first failure will use
+ * the lowest PFN as the starting point for linear scanning.
*/
- low_pfn = cc->migrate_pfn;
+ low_pfn = fast_find_migrateblock(cc);
block_start_pfn = pageblock_start_pfn(low_pfn);
if (block_start_pfn < zone->zone_start_pfn)
block_start_pfn = zone->zone_start_pfn;
@@ -1560,6 +1734,7 @@ static enum compact_result compact_zone(
* want to compact the whole zone), but check that it is initialised
* by ensuring the values are within zone boundaries.
*/
+ cc->fast_start_pfn = 0;
if (cc->whole_zone) {
cc->migrate_pfn = start_pfn;
cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
--- a/mm/internal.h~mm-compaction-use-free-lists-to-quickly-locate-a-migration-source
+++ a/mm/internal.h
@@ -188,9 +188,11 @@ struct compact_control {
unsigned int nr_migratepages; /* Number of pages to migrate */
unsigned long free_pfn; /* isolate_freepages search base */
unsigned long migrate_pfn; /* isolate_migratepages search base */
+ unsigned long fast_start_pfn; /* a pfn to start linear scan from */
struct zone *zone;
unsigned long total_migrate_scanned;
unsigned long total_free_scanned;
+ unsigned int fast_search_fail; /* failures to use free list searches */
const gfp_t gfp_mask; /* gfp mask of a direct compactor */
int order; /* order a direct compactor needs */
int migratetype; /* migratetype of direct compactor */
_
Patches currently in -mm which might be from mgorman@xxxxxxxxxxxxxxxxxxx are
mm-page_alloc-do-not-wake-kswapd-with-zone-lock-held.patch
mm-compaction-shrink-compact_control.patch
mm-compaction-rearrange-compact_control.patch
mm-compaction-remove-last_migrated_pfn-from-compact_control.patch
mm-compaction-remove-unnecessary-zone-parameter-in-some-instances.patch
mm-compaction-rename-map_pages-to-split_map_pages.patch
mm-compaction-skip-pageblocks-with-reserved-pages.patch
mm-migrate-immediately-fail-migration-of-a-page-with-no-migration-handler.patch
mm-compaction-always-finish-scanning-of-a-full-pageblock.patch
mm-compaction-use-the-page-allocator-bulk-free-helper-for-lists-of-pages.patch
mm-compaction-ignore-the-fragmentation-avoidance-boost-for-isolation-and-compaction.patch
mm-compaction-use-free-lists-to-quickly-locate-a-migration-source.patch
mm-compaction-keep-migration-source-private-to-a-single-compaction-instance.patch
mm-compaction-use-free-lists-to-quickly-locate-a-migration-target.patch
mm-compaction-avoid-rescanning-the-same-pageblock-multiple-times.patch
mm-compaction-finish-pageblock-scanning-on-contention.patch
mm-compaction-check-early-for-huge-pages-encountered-by-the-migration-scanner.patch
mm-compaction-keep-cached-migration-pfns-synced-for-unusable-pageblocks.patch
mm-compaction-rework-compact_should_abort-as-compact_check_resched.patch
mm-compaction-do-not-consider-a-need-to-reschedule-as-contention.patch
mm-compaction-reduce-unnecessary-skipping-of-migration-target-scanner.patch
mm-compaction-round-robin-the-order-while-searching-the-free-lists-for-a-target.patch
mm-compaction-sample-pageblocks-for-free-pages.patch
mm-compaction-be-selective-about-what-pageblocks-to-clear-skip-hints.patch
mm-compaction-capture-a-page-under-direct-compaction.patch
mm-compaction-do-not-direct-compact-remote-memory.patch
