[PATCH v3] 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]

Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Acked-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
---
Changes from v2:
 - Simplify the function suggested by David.
 - Collect acked tag from David and Ying. Thanks.
Changes from v1:
 - Fix building errors if CONFIG_SPARSEMEM is not selected.
 - Use NR_MEM_SECTIONS instead of '-1' per Huang Ying.
---
 mm/compaction.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 3398ef3a55fe..0b2680b075fd 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -229,6 +229,33 @@ static void reset_cached_positions(struct zone *zone)
 				pageblock_start_pfn(zone_end_pfn(zone) - 1);
 }
 
+#ifdef CONFIG_SPARSEMEM
+/*
+ * If the PFN falls into an offline section, return the start PFN of the
+ * next online section. If the PFN falls into an online section or if
+ * there is no next online section, return 0.
+ */
+static unsigned long skip_offline_sections(unsigned long start_pfn)
+{
+	unsigned long start_nr = pfn_to_section_nr(start_pfn);
+
+	if (online_section_nr(start_nr))
+		return 0;
+
+	while (++start_nr <= __highest_present_section_nr) {
+		if (online_section_nr(start_nr))
+			return section_nr_to_pfn(start_nr);
+	}
+
+	return 0;
+}
+#else
+static unsigned long skip_offline_sections(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 +2018,14 @@ 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_offline_sections(block_start_pfn);
+			if (next_pfn)
+				block_end_pfn = next_pfn;
 			continue;
+		}
 
 		/*
 		 * If isolation recently failed, do not retry. Only check the
-- 
2.27.0