[kvm-unit-tests PATCH v2 07/11] lib/alloc_page: Optimization to skip known empty freelists

[Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>]

Keep track of the largest block order available in each area, and do not
search past it when looking for free memory.

This will avoid needlessly scanning the freelists for the largest block
orders, which will be empty in most cases.

Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>
---
 lib/alloc_page.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/lib/alloc_page.c b/lib/alloc_page.c
index 7d1fa85..37f28ce 100644
--- a/lib/alloc_page.c
+++ b/lib/alloc_page.c
@@ -31,6 +31,8 @@ struct mem_area {
 	pfn_t top;
 	/* Per page metadata, each entry is a combination *_MASK and order */
 	u8 *page_states;
+	/* Highest block order available in this area */
+	u8 max_order;
 	/* One freelist for each possible block size, up to NLISTS */
 	struct linked_list freelists[NLISTS];
 };
@@ -104,6 +106,8 @@ static void split(struct mem_area *a, void *addr)
 		assert(a->page_states[idx + i] == order);
 		a->page_states[idx + i] = order - 1;
 	}
+	if ((order == a->max_order) && (is_list_empty(a->freelists + order)))
+		a->max_order--;
 	order--;
 	/* add the first half block to the appropriate free list */
 	list_add(a->freelists + order, addr);
@@ -127,13 +131,13 @@ static void *page_memalign_order(struct mem_area *a, u8 al, u8 sz)
 	order = sz > al ? sz : al;
 
 	/* search all free lists for some memory */
-	for ( ; order < NLISTS; order++) {
+	for ( ; order <= a->max_order; order++) {
 		p = a->freelists[order].next;
 		if (!is_list_empty(p))
 			break;
 	}
 	/* out of memory */
-	if (order >= NLISTS)
+	if (order > a->max_order)
 		return NULL;
 
 	/*
@@ -201,6 +205,8 @@ static bool coalesce(struct mem_area *a, u8 order, pfn_t pfn, pfn_t pfn2)
 	}
 	/* finally add the newly coalesced block to the appropriate freelist */
 	list_add(a->freelists + order + 1, pfn_to_virt(pfn));
+	if (order + 1 > a->max_order)
+		a->max_order = order + 1;
 	return true;
 }
 
@@ -438,6 +444,7 @@ static void _page_alloc_init_area(u8 n, pfn_t start_pfn, pfn_t top_pfn)
 	a->page_states = pfn_to_virt(start_pfn);
 	a->base = start_pfn + table_size;
 	a->top = top_pfn;
+	a->max_order = 0;
 	npages = top_pfn - a->base;
 	assert((a->base - start_pfn) * PAGE_SIZE >= npages);
 
@@ -472,6 +479,8 @@ static void _page_alloc_init_area(u8 n, pfn_t start_pfn, pfn_t top_pfn)
 		/* initialize the metadata and add to the freelist */
 		memset(a->page_states + (i - a->base), order, BIT(order));
 		list_add(a->freelists + order, pfn_to_virt(i));
+		if (order > a->max_order)
+			a->max_order = order;
 	}
 	/* finally mark the area as present */
 	areas_mask |= BIT(n);
-- 
2.26.2