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

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On 6/12/2023 2:39 PM, Huang, Ying wrote:
Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:

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>
---
  include/linux/mmzone.h | 10 ++++++++++
  mm/compaction.c        | 23 ++++++++++++++++++++++-
  2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 5a7ada0413da..87e6c535d895 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 -1UL;
+}
+
  /*
   * 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..3a55fdd20c49 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -229,6 +229,21 @@ static void reset_cached_positions(struct zone *zone)
  				pageblock_start_pfn(zone_end_pfn(zone) - 1);
  }
+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 -1UL;

Define a macro for the maigic "-1UL"?  Which is used for multiple times
in the patch.

I am struggling to find a readable macro for these '-1UL', since the '-1UL' in next_online_section_nr() indicates that it can not find an online section. However the '-1' in skip_hole_pageblock() indicates that it can not find an online pfn.

So after more thinking, I will change to return 'NR_MEM_SECTIONS' if can not find next online section in next_online_section_nr(). And in skip_hole_pageblock(), I will change to return 0 if can not find next online pfn. What do you think?

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;
}

+
+	next_online_nr = next_online_section_nr(start_nr);
+	if (next_online_nr != -1UL)
+		return section_nr_to_pfn(next_online_nr);
+
+	return -1UL;
+}
+
  /*
   * 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 +2006,14 @@ 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 != -1UL)
+				block_end_pfn = next_pfn;
  			continue;
+		}
/*
  		 * If isolation recently failed, do not retry. Only check the

Do we need to do similar change in isolate_freepages()?

Yes, it's in my todo list with some measurement data.

Thanks for your comments.




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