Subject: [PATCH v12 2/2] mm: page_alloc: reduce unnecessary binary search in memblock_next_valid_pfn

From: Hanjun Guo <[email-masked]>
Date: Tue, 23 Jul 2019 13:51:13 +0800

From: Jia He <[email-masked]>

After skipping some invalid pfns in memmap_init_zone(), there is still
some room for improvement.

E.g. if pfn and pfn+1 are in the same memblock region, we can simply pfn++
instead of doing the binary search in memblock_next_valid_pfn.

Furthermore, if the pfn is in a gap of two memory region, skip to next
region directly to speedup the binary search.

Signed-off-by: Jia He <[email-masked]>
Signed-off-by: Hanjun Guo <[email-masked]>
---
 mm/memblock.c | 37 +++++++++++++++++++++++++++++++------
 1 file changed, 31 insertions(+), 6 deletions(-)

diff --git a/mm/memblock.c b/mm/memblock.c
index d57ba51bb9cd..95d5916716a0 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -1256,28 +1256,53 @@ int __init_memblock memblock_set_node(phys_addr_t base, phys_addr_t size,
 unsigned long __init_memblock memblock_next_valid_pfn(unsigned long pfn)
 {
 	struct memblock_type *type = &memblock.memory;
+	struct memblock_region *regions = type->regions;
 	unsigned int right = type->cnt;
 	unsigned int mid, left = 0;
+	unsigned long start_pfn, end_pfn, next_start_pfn;
 	phys_addr_t addr = PFN_PHYS(++pfn);
+	static int early_region_idx __initdata_memblock = -1;
 
+	/* fast path, return pfn+1 if next pfn is in the same region */
+	if (early_region_idx != -1) {
+		start_pfn = PFN_DOWN(regions[early_region_idx].base);
+		end_pfn = PFN_DOWN(regions[early_region_idx].base +
+				regions[early_region_idx].size);
+
+		if (pfn >= start_pfn && pfn < end_pfn)
+			return pfn;
+
+		/* try slow path */
+		if (++early_region_idx == type->cnt)
+			goto slow_path;
+
+		next_start_pfn = PFN_DOWN(regions[early_region_idx].base);
+
+		if (pfn >= end_pfn && pfn <= next_start_pfn)
+			return next_start_pfn;
+	}
+
+slow_path:
+	/* slow path, do the binary searching */
 	do {
 		mid = (right + left) / 2;
 
-		if (addr < type->regions[mid].base)
+		if (addr < regions[mid].base)
 			right = mid;
-		else if (addr >= (type->regions[mid].base +
-				  type->regions[mid].size))
+		else if (addr >= (regions[mid].base + regions[mid].size))
 			left = mid + 1;
 		else {
-			/* addr is within the region, so pfn is valid */
+			early_region_idx = mid;
 			return pfn;
 		}
 	} while (left < right);
 
 	if (right == type->cnt)
 		return -1UL;
-	else
-		return PHYS_PFN(type->regions[right].base);
+
+	early_region_idx = right;
+
+	return PHYS_PFN(regions[early_region_idx].base);
 }
 EXPORT_SYMBOL(memblock_next_valid_pfn);
 #endif /* CONFIG_HAVE_MEMBLOCK_PFN_VALID */
-- 
2.19.1

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