Running will-it-scale/page_fault1 process mode workload on a 2 sockets
Intel Skylake server showed severe lock contention of zone->lock, as
high as about 80%(42% on allocation path and 35% on free path) CPU
cycles are burnt spinning. With perf, the most time consuming part inside
that lock on free path is cache missing on page structures, mostly on
the to-be-freed page's buddy due to merging.
One way to avoid this overhead is not do any merging at all for order-0
pages. With this approach, the lock contention for zone->lock on free
path dropped to 1.1% but allocation side still has as high as 42% lock
contention. In the meantime, the dropped lock contention on free side
doesn't translate to performance increase, instead, it's consumed by
increased lock contention of the per node lru_lock(rose from 5% to 37%)
and the final performance slightly dropped about 1%.
Though performance dropped a little, it almost eliminated zone lock
contention on free path and it is the foundation for the next patch
that eliminates zone lock contention for allocation path.
A new document file called "struct_page_filed" is added to explain
the newly reused field in "struct page".
Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
---
Documentation/vm/struct_page_field | 5 +++
include/linux/mm_types.h | 1 +
mm/compaction.c | 13 +++++-
mm/internal.h | 27 ++++++++++++
mm/page_alloc.c | 89 +++++++++++++++++++++++++++++++++-----
5 files changed, 122 insertions(+), 13 deletions(-)
create mode 100644 Documentation/vm/struct_page_field
diff --git a/Documentation/vm/struct_page_field b/Documentation/vm/struct_page_field
new file mode 100644
index 000000000000..1ab6c19ccc7a
--- /dev/null
+++ b/Documentation/vm/struct_page_field
@@ -0,0 +1,5 @@
+buddy_merge_skipped:
+Used to indicate this page skipped merging when added to buddy. This
+field only makes sense if the page is in Buddy and is order zero.
+It's a bug if any higher order pages in Buddy has this field set.
+Shares space with index.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index fd1af6b9591d..7edc4e102a8e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -91,6 +91,7 @@ struct page {
pgoff_t index; /* Our offset within mapping. */
void *freelist; /* sl[aou]b first free object */
/* page_deferred_list().prev -- second tail page */
+ bool buddy_merge_skipped; /* skipped merging when added to buddy */
};
union {
diff --git a/mm/compaction.c b/mm/compaction.c
index 2c8999d027ab..fb9031fdca41 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -776,8 +776,19 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
* potential isolation targets.
*/
if (PageBuddy(page)) {
- unsigned long freepage_order = page_order_unsafe(page);
+ unsigned long freepage_order;
+ /*
+ * If this is a merge_skipped page, do merge now
+ * since high-order pages are needed. zone lock
+ * isn't taken for the merge_skipped check so the
+ * check could be wrong but the worst case is we
+ * lose a merge opportunity.
+ */
+ if (page_merge_was_skipped(page))
+ try_to_merge_page(page);
+
+ freepage_order = page_order_unsafe(page);
/*
* Without lock, we cannot be sure that what we got is
* a valid page order. Consider only values in the
diff --git a/mm/internal.h b/mm/internal.h
index e6bd35182dae..2bfbaae2d835 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -538,4 +538,31 @@ static inline bool is_migrate_highatomic_page(struct page *page)
}
void setup_zone_pageset(struct zone *zone);
+
+static inline bool page_merge_was_skipped(struct page *page)
+{
+ return page->buddy_merge_skipped;
+}
+
+void try_to_merge_page(struct page *page);
+
+#ifdef CONFIG_COMPACTION
+static inline bool can_skip_merge(struct zone *zone, int order)
+{
+ /* Compaction has failed in this zone, we shouldn't skip merging */
+ if (zone->compact_considered)
+ return false;
+
+ /* Only consider no_merge for order 0 pages */
+ if (order)
+ return false;
+
+ return true;
+}
+#else /* CONFIG_COMPACTION */
+static inline bool can_skip_merge(struct zone *zone, int order)
+{
+ return false;
+}
+#endif /* CONFIG_COMPACTION */
#endif /* __MM_INTERNAL_H */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3cdf1e10d412..eb78014dfbde 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -730,6 +730,16 @@ static inline void clear_page_guard(struct zone *zone, struct page *page,
unsigned int order, int migratetype) {}
#endif
+static inline void set_page_merge_skipped(struct page *page)
+{
+ page->buddy_merge_skipped = true;
+}
+
+static inline void clear_page_merge_skipped(struct page *page)
+{
+ page->buddy_merge_skipped = false;
+}
+
static inline void set_page_order(struct page *page, unsigned int order)
{
set_page_private(page, order);
@@ -739,6 +749,13 @@ static inline void set_page_order(struct page *page, unsigned int order)
static inline void add_to_buddy_common(struct page *page, struct zone *zone,
unsigned int order, int mt)
{
+ /*
+ * Always clear buddy_merge_skipped when added to buddy because
+ * buddy_merge_skipped shares space with index and index could
+ * be used as migratetype for PCP pages.
+ */
+ clear_page_merge_skipped(page);
+
set_page_order(page, order);
zone->free_area[order].nr_free++;
}
@@ -769,6 +786,7 @@ static inline void remove_from_buddy(struct page *page, struct zone *zone,
list_del(&page->lru);
zone->free_area[order].nr_free--;
rmv_page_order(page);
+ clear_page_merge_skipped(page);
}
/*
@@ -839,7 +857,7 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
* -- nyc
*/
-static inline void __free_one_page(struct page *page,
+static inline void do_merge(struct page *page,
unsigned long pfn,
struct zone *zone, unsigned int order,
int migratetype)
@@ -851,16 +869,6 @@ static inline void __free_one_page(struct page *page,
max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
- VM_BUG_ON(!zone_is_initialized(zone));
- VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
-
- VM_BUG_ON(migratetype == -1);
- if (likely(!is_migrate_isolate(migratetype)))
- __mod_zone_freepage_state(zone, 1 << order, migratetype);
-
- VM_BUG_ON_PAGE(pfn & ((1 << order) - 1), page);
- VM_BUG_ON_PAGE(bad_range(zone, page), page);
-
continue_merging:
while (order < max_order - 1) {
buddy_pfn = __find_buddy_pfn(pfn, order);
@@ -933,6 +941,61 @@ static inline void __free_one_page(struct page *page,
add_to_buddy_head(page, zone, order, migratetype);
}
+void try_to_merge_page(struct page *page)
+{
+ unsigned long pfn, buddy_pfn, flags;
+ struct page *buddy;
+ struct zone *zone;
+
+ /*
+ * No need to do merging if buddy is not free.
+ * zone lock isn't taken so this could be wrong but worst case
+ * is we lose a merge opportunity.
+ */
+ pfn = page_to_pfn(page);
+ buddy_pfn = __find_buddy_pfn(pfn, 0);
+ buddy = page + (buddy_pfn - pfn);
+ if (!PageBuddy(buddy))
+ return;
+
+ zone = page_zone(page);
+ spin_lock_irqsave(&zone->lock, flags);
+ /* Verify again after taking the lock */
+ if (likely(PageBuddy(page) && page_merge_was_skipped(page) &&
+ PageBuddy(buddy))) {
+ int mt = get_pageblock_migratetype(page);
+
+ remove_from_buddy(page, zone, 0);
+ do_merge(page, pfn, zone, 0, mt);
+ }
+ spin_unlock_irqrestore(&zone->lock, flags);
+}
+
+static inline void __free_one_page(struct page *page,
+ unsigned long pfn,
+ struct zone *zone, unsigned int order,
+ int migratetype)
+{
+ VM_BUG_ON(!zone_is_initialized(zone));
+ VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
+
+ VM_BUG_ON(migratetype == -1);
+ if (likely(!is_migrate_isolate(migratetype)))
+ __mod_zone_freepage_state(zone, 1 << order, migratetype);
+
+ VM_BUG_ON_PAGE(pfn & ((1 << order) - 1), page);
+ VM_BUG_ON_PAGE(bad_range(zone, page), page);
+
+ if (can_skip_merge(zone, order)) {
+ add_to_buddy_head(page, zone, 0, migratetype);
+ set_page_merge_skipped(page);
+ return;
+ }
+
+ do_merge(page, pfn, zone, order, migratetype);
+}
+
+
/*
* A bad page could be due to a number of fields. Instead of multiple branches,
* try and check multiple fields with one check. The caller must do a detailed
@@ -1183,8 +1246,10 @@ static void free_pcppages_bulk(struct zone *zone, int count,
* can be offset by reduced memory latency later. To
* avoid excessive prefetching due to large count, only
* prefetch buddy for the last pcp->batch nr of pages.
+ *
+ * If merge can be skipped, no need to prefetch buddy.
*/
- if (count > pcp->batch)
+ if (can_skip_merge(zone, 0) || count > pcp->batch)
continue;
pfn = page_to_pfn(page);
buddy_pfn = __find_buddy_pfn(pfn, 0);
--
2.14.3
