One of the main advantages of this design of memory regions is that page
allocations can potentially be extremely fast - almost with no extra
overhead from memory regions.
To exploit that, introduce an optimized version of del_from_freelist(), which
utilizes the fact that we always delete items from the head of the list
during page allocation.
Basically, we want to keep a note of the region from which we are allocating
in a given freelist, to avoid having to compute the page-to-zone-region for
every page allocation. So introduce a 'next_region' pointer in every freelist
to achieve that, and use it to keep the fastpath of page allocation almost as
fast as it would have been without memory regions.
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 14 +++++++++++
include/linux/mmzone.h | 6 +++++
mm/page_alloc.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++-
3 files changed, 81 insertions(+), 1 deletion(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 307f375..4286a75 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -740,6 +740,20 @@ static inline int page_zone_region_id(const struct page *page)
return pgdat->node_regions[node_region_idx].zone_region_idx[z_num];
}
+static inline void set_next_region_in_freelist(struct free_list *free_list)
+{
+ struct page *page;
+ int region_id;
+
+ if (unlikely(list_empty(&free_list->list))) {
+ free_list->next_region = NULL;
+ } else {
+ page = list_entry(free_list->list.next, struct page, lru);
+ region_id = page_zone_region_id(page);
+ free_list->next_region = &free_list->mr_list[region_id];
+ }
+}
+
#ifdef SECTION_IN_PAGE_FLAGS
static inline void set_page_section(struct page *page, unsigned long section)
{
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 2ac8025..4721a22 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -92,6 +92,12 @@ struct free_list {
struct list_head list;
/*
+ * Pointer to the region from which the next allocation will be
+ * satisfied. (Same as the freelist's first pageblock's region.)
+ */
+ struct mem_region_list *next_region; /* for fast page allocation */
+
+ /*
* Demarcates pageblocks belonging to different regions within
* this freelist.
*/
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c40715c..fe812e0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -551,6 +551,15 @@ static void add_to_freelist(struct page *page, struct free_list *free_list)
/* This is the first region, so add to the head of the list */
prev_region_list = &free_list->list;
+#ifdef CONFIG_DEBUG_PAGEALLOC
+ WARN((list_empty(&free_list->list) && free_list->next_region != NULL),
+ "%s: next_region not NULL\n", __func__);
+#endif
+ /*
+ * Set 'next_region' to this region, since this is the first region now
+ */
+ free_list->next_region = region;
+
out:
list_add(lru, prev_region_list);
@@ -558,6 +567,47 @@ out:
region->page_block = lru;
}
+/**
+ * __rmqueue_smallest() *always* deletes elements from the head of the
+ * list. Use this knowledge to keep page allocation fast, despite being
+ * region-aware.
+ *
+ * Do *NOT* call this function if you are deleting from somewhere deep
+ * inside the freelist.
+ */
+static void rmqueue_del_from_freelist(struct page *page,
+ struct free_list *free_list)
+{
+ struct list_head *lru = &page->lru;
+
+#ifdef CONFIG_DEBUG_PAGEALLOC
+ WARN((free_list->list.next != lru),
+ "%s: page not at head of list", __func__);
+#endif
+
+ list_del(lru);
+
+ /* Fastpath */
+ if (--(free_list->next_region->nr_free)) {
+
+#ifdef CONFIG_DEBUG_PAGEALLOC
+ WARN(free_list->next_region->nr_free < 0,
+ "%s: nr_free is negative\n", __func__);
+#endif
+ return;
+ }
+
+ /*
+ * Slowpath, when this is the last pageblock of this region
+ * in this freelist.
+ */
+ free_list->next_region->page_block = NULL;
+
+ /* Set 'next_region' to the new first region in the freelist. */
+ set_next_region_in_freelist(free_list);
+}
+
+/* Generic delete function for region-aware buddy allocator. */
static void del_from_freelist(struct page *page, struct free_list *free_list)
{
struct list_head *prev_page_lru, *lru, *p;
@@ -565,6 +615,11 @@ static void del_from_freelist(struct page *page, struct free_list *free_list)
int region_id;
lru = &page->lru;
+
+ /* Try to fastpath, if deleting from the head of the list */
+ if (lru == free_list->list.next)
+ return rmqueue_del_from_freelist(page, free_list);
+
region_id = page_zone_region_id(page);
region = &free_list->mr_list[region_id];
region->nr_free--;
@@ -600,6 +655,11 @@ page_found:
prev_page_lru = lru->prev;
list_del(lru);
+ /*
+ * Since we are not deleting from the head of the freelist, the
+ * 'next_region' pointer doesn't have to change.
+ */
+
if (region->nr_free == 0) {
region->page_block = NULL;
} else {
@@ -1025,7 +1085,7 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
page = list_entry(area->free_list[migratetype].list.next,
struct page, lru);
- del_from_freelist(page, &area->free_list[migratetype]);
+ rmqueue_del_from_freelist(page, &area->free_list[migratetype]);
rmv_page_order(page);
area->nr_free--;
expand(zone, page, order, current_order, area, migratetype);
--
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