Re: [PATCH 08/17] mm: page_alloc: Use word-based accesses for get/set pageblock bitmaps

[Mel Gorman]

On Fri, May 02, 2014 at 06:34:52PM -0400, Sasha Levin wrote:
> Hi Mel,
> 
> Vlastimil Babka suggested I should try this patch to work around a different
> issue I'm seeing, and noticed that it doesn't build because:
> 

Rebasing SNAFU. Can you try this instead?

---8<---
mm: page_alloc: Use word-based accesses for get/set pageblock bitmaps

The test_bit operations in get/set pageblock flags are expensive. This patch
reads the bitmap on a word basis and use shifts and masks to isolate the bits
of interest. Similarly masks are used to set a local copy of the bitmap and then
use cmpxchg to update the bitmap if there have been no other changes made in
parallel.

In a test running dd onto tmpfs the overhead of the pageblock-related
functions went from 1.27% in profiles to 0.5%.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index fac5509..c84703d 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -75,9 +75,14 @@ enum {
 
 extern int page_group_by_mobility_disabled;
 
+#define NR_MIGRATETYPE_BITS (PB_migrate_end - PB_migrate + 1)
+#define MIGRATETYPE_MASK ((1UL << NR_MIGRATETYPE_BITS) - 1)
+
 static inline int get_pageblock_migratetype(struct page *page)
 {
-	return get_pageblock_flags_group(page, PB_migrate, PB_migrate_end);
+	BUILD_BUG_ON(PB_migrate_end - PB_migrate != 2);
+	return get_pageblock_flags_mask(page, PB_migrate_end,
+					NR_MIGRATETYPE_BITS, MIGRATETYPE_MASK);
 }
 
 struct free_area {
diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
index 2ee8cd2..bc37036 100644
--- a/include/linux/pageblock-flags.h
+++ b/include/linux/pageblock-flags.h
@@ -30,9 +30,12 @@ enum pageblock_bits {
 	PB_migrate,
 	PB_migrate_end = PB_migrate + 3 - 1,
 			/* 3 bits required for migrate types */
-#ifdef CONFIG_COMPACTION
 	PB_migrate_skip,/* If set the block is skipped by compaction */
-#endif /* CONFIG_COMPACTION */
+
+	/*
+	 * Assume the bits will always align on a word. If this assumption
+	 * changes then get/set pageblock needs updating.
+	 */
 	NR_PAGEBLOCK_BITS
 };
 
@@ -62,11 +65,35 @@ extern int pageblock_order;
 /* Forward declaration */
 struct page;
 
+unsigned long get_pageblock_flags_mask(struct page *page,
+				unsigned long end_bitidx,
+				unsigned long nr_flag_bits,
+				unsigned long mask);
+void set_pageblock_flags_mask(struct page *page,
+				unsigned long flags,
+				unsigned long end_bitidx,
+				unsigned long nr_flag_bits,
+				unsigned long mask);
+
 /* Declarations for getting and setting flags. See mm/page_alloc.c */
-unsigned long get_pageblock_flags_group(struct page *page,
-					int start_bitidx, int end_bitidx);
-void set_pageblock_flags_group(struct page *page, unsigned long flags,
-					int start_bitidx, int end_bitidx);
+static inline unsigned long get_pageblock_flags_group(struct page *page,
+					int start_bitidx, int end_bitidx)
+{
+	unsigned long nr_flag_bits = end_bitidx - start_bitidx + 1;
+	unsigned long mask = (1 << nr_flag_bits) - 1;
+
+	return get_pageblock_flags_mask(page, end_bitidx, nr_flag_bits, mask);
+}
+
+static inline void set_pageblock_flags_group(struct page *page,
+					unsigned long flags,
+					int start_bitidx, int end_bitidx)
+{
+	unsigned long nr_flag_bits = end_bitidx - start_bitidx + 1;
+	unsigned long mask = (1 << nr_flag_bits) - 1;
+
+	set_pageblock_flags_mask(page, flags, end_bitidx, nr_flag_bits, mask);
+}
 
 #ifdef CONFIG_COMPACTION
 #define get_pageblock_skip(page) \
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index dc123ff..f393b0e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6032,53 +6032,64 @@ static inline int pfn_to_bitidx(struct zone *zone, unsigned long pfn)
  * @end_bitidx: The last bit of interest
  * returns pageblock_bits flags
  */
-unsigned long get_pageblock_flags_group(struct page *page,
-					int start_bitidx, int end_bitidx)
+unsigned long get_pageblock_flags_mask(struct page *page,
+					unsigned long end_bitidx,
+					unsigned long nr_flag_bits,
+					unsigned long mask)
 {
 	struct zone *zone;
 	unsigned long *bitmap;
-	unsigned long pfn, bitidx;
-	unsigned long flags = 0;
-	unsigned long value = 1;
+	unsigned long pfn, bitidx, word_bitidx;
+	unsigned long word;
 
 	zone = page_zone(page);
 	pfn = page_to_pfn(page);
 	bitmap = get_pageblock_bitmap(zone, pfn);
 	bitidx = pfn_to_bitidx(zone, pfn);
+	word_bitidx = bitidx / BITS_PER_LONG;
+	bitidx &= (BITS_PER_LONG-1);
 
-	for (; start_bitidx <= end_bitidx; start_bitidx++, value <<= 1)
-		if (test_bit(bitidx + start_bitidx, bitmap))
-			flags |= value;
-
-	return flags;
+	word = bitmap[word_bitidx];
+	bitidx += end_bitidx;
+	return (word >> (BITS_PER_LONG - bitidx - 1)) & mask;
 }
 
 /**
- * set_pageblock_flags_group - Set the requested group of flags for a pageblock_nr_pages block of pages
+ * set_pageblock_flags_mask - Set the requested group of flags for a pageblock_nr_pages block of pages
  * @page: The page within the block of interest
  * @start_bitidx: The first bit of interest
  * @end_bitidx: The last bit of interest
  * @flags: The flags to set
  */
-void set_pageblock_flags_group(struct page *page, unsigned long flags,
-					int start_bitidx, int end_bitidx)
+void set_pageblock_flags_mask(struct page *page, unsigned long flags,
+					unsigned long end_bitidx,
+					unsigned long nr_flag_bits,
+					unsigned long mask)
 {
 	struct zone *zone;
 	unsigned long *bitmap;
-	unsigned long pfn, bitidx;
-	unsigned long value = 1;
+	unsigned long pfn, bitidx, word_bitidx;
+	unsigned long old_word, new_word;
+
+	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
 
 	zone = page_zone(page);
 	pfn = page_to_pfn(page);
 	bitmap = get_pageblock_bitmap(zone, pfn);
 	bitidx = pfn_to_bitidx(zone, pfn);
+	word_bitidx = bitidx / BITS_PER_LONG;
+	bitidx &= (BITS_PER_LONG-1);
+
 	VM_BUG_ON_PAGE(!zone_spans_pfn(zone, pfn), page);
 
-	for (; start_bitidx <= end_bitidx; start_bitidx++, value <<= 1)
-		if (flags & value)
-			__set_bit(bitidx + start_bitidx, bitmap);
-		else
-			__clear_bit(bitidx + start_bitidx, bitmap);
+	bitidx += end_bitidx;
+	mask <<= (BITS_PER_LONG - bitidx - 1);
+	flags <<= (BITS_PER_LONG - bitidx - 1);
+
+	do {
+		old_word = ACCESS_ONCE(bitmap[word_bitidx]);
+		new_word = (old_word & ~mask) | flags;
+	} while (cmpxchg(&bitmap[word_bitidx], old_word, new_word) != old_word);
 }
 
 /*
--
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