[PATCH v2] mm: compaction: skip memory hole rapidly when isolating migratable pages

[Baolin Wang]

On some machines, the normal zone can have a large memory hole like
below memory layout, and we can see the range from 0x100000000 to
0x1800000000 is a hole. So when isolating some migratable pages, the
scanner can meet the hole and it will take more time to skip the large
hole. From my measurement, I can see the isolation scanner will take
80us ~ 100us to skip the large hole [0x100000000 - 0x1800000000].

So adding a new helper to fast search next online memory section
to skip the large hole can help to find next suitable pageblock
efficiently. With this patch, I can see the large hole scanning only
takes < 1us.

[    0.000000] Zone ranges:
[    0.000000]   DMA      [mem 0x0000000040000000-0x00000000ffffffff]
[    0.000000]   DMA32    empty
[    0.000000]   Normal   [mem 0x0000000100000000-0x0000001fa7ffffff]
[    0.000000] Movable zone start for each node
[    0.000000] Early memory node ranges
[    0.000000]   node   0: [mem 0x0000000040000000-0x0000000fffffffff]
[    0.000000]   node   0: [mem 0x0000001800000000-0x0000001fa3c7ffff]
[    0.000000]   node   0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff]
[    0.000000]   node   0: [mem 0x0000001fa4000000-0x0000001fa402ffff]
[    0.000000]   node   0: [mem 0x0000001fa4030000-0x0000001fa40effff]
[    0.000000]   node   0: [mem 0x0000001fa40f0000-0x0000001fa73cffff]
[    0.000000]   node   0: [mem 0x0000001fa73d0000-0x0000001fa745ffff]
[    0.000000]   node   0: [mem 0x0000001fa7460000-0x0000001fa746ffff]
[    0.000000]   node   0: [mem 0x0000001fa7470000-0x0000001fa758ffff]
[    0.000000]   node   0: [mem 0x0000001fa7590000-0x0000001fa7ffffff]

Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
Changes from v1:
 - Fix building errors if CONFIG_SPARSEMEM is not selected.
 - Use NR_MEM_SECTIONS instead of '-1' per Huang Ying.
---
 include/linux/mmzone.h | 10 ++++++++++
 mm/compaction.c        | 30 +++++++++++++++++++++++++++++-
 2 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 5a7ada0413da..5ff1fa2efe28 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -2000,6 +2000,16 @@ static inline unsigned long next_present_section_nr(unsigned long section_nr)
 	return -1;
 }
 
+static inline unsigned long next_online_section_nr(unsigned long section_nr)
+{
+	while (++section_nr <= __highest_present_section_nr) {
+		if (online_section_nr(section_nr))
+			return section_nr;
+	}
+
+	return NR_MEM_SECTIONS;
+}
+
 /*
  * These are _only_ used during initialisation, therefore they
  * can use __initdata ...  They could have names to indicate
diff --git a/mm/compaction.c b/mm/compaction.c
index 3398ef3a55fe..c31ff6123891 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -229,6 +229,28 @@ static void reset_cached_positions(struct zone *zone)
 				pageblock_start_pfn(zone_end_pfn(zone) - 1);
 }
 
+#ifdef CONFIG_SPARSEMEM
+static unsigned long skip_hole_pageblock(unsigned long start_pfn)
+{
+	unsigned long next_online_nr;
+	unsigned long start_nr = pfn_to_section_nr(start_pfn);
+
+	if (online_section_nr(start_nr))
+		return 0;
+
+	next_online_nr = next_online_section_nr(start_nr);
+	if (next_online_nr < NR_MEM_SECTIONS)
+		return section_nr_to_pfn(next_online_nr);
+
+	return 0;
+}
+#else
+static unsigned long skip_hole_pageblock(unsigned long start_pfn)
+{
+	return 0;
+}
+#endif
+
 /*
  * Compound pages of >= pageblock_order should consistently be skipped until
  * released. It is always pointless to compact pages of such order (if they are
@@ -1991,8 +2013,14 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
 
 		page = pageblock_pfn_to_page(block_start_pfn,
 						block_end_pfn, cc->zone);
-		if (!page)
+		if (!page) {
+			unsigned long next_pfn;
+
+			next_pfn = skip_hole_pageblock(block_start_pfn);
+			if (next_pfn != 0)
+				block_end_pfn = next_pfn;
 			continue;
+		}
 
 		/*
 		 * If isolation recently failed, do not retry. Only check the
-- 
2.27.0