[PATCH v3 2/2] mm: drain per-cpu pages outside of isolate_migratepages_range

[Pavel Tatashin]

It is expensive to drain per-cpu page lists as a thread is started on each
CPU, and we are waiting for them to complete.

Currently, we drain on every block of pages that is isolated. Instead lets
drain once after pages are isolated.

For example, when 2G of memory is hot-removed drain is called 16 times,
with this change it will be done only one time on average.

Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 mm/memory_hotplug.c | 15 +--------------
 mm/page_alloc.c     |  2 ++
 mm/page_isolation.c | 40 ++++++++++++----------------------------
 3 files changed, 15 insertions(+), 42 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index b11a269e2356..5a2ed1a94555 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1536,6 +1536,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
 	}
 
 	do {
+		drain_all_pages(zone);
 		pfn = start_pfn;
 		do {
 			if (signal_pending(current)) {
@@ -1575,20 +1576,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
 		/* check again */
 		ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
 					    NULL, check_pages_isolated_cb);
-		/*
-		 * per-cpu pages are drained in start_isolate_page_range, but if
-		 * there are still pages that are not free, make sure that we
-		 * drain again, because when we isolated range we might
-		 * have raced with another thread that was adding pages to pcp
-		 * list.
-		 *
-		 * Forward progress should be still guaranteed because
-		 * pages on the pcp list can only belong to MOVABLE_ZONE
-		 * because has_unmovable_pages explicitly checks for
-		 * PageBuddy on freed pages on other zones.
-		 */
-		if (ret)
-			drain_all_pages(zone);
 	} while (ret);
 
 	/* Ok, all of our target is isolated.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fab5e97dc9ca..6d6a501a103e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8462,6 +8462,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 	if (ret < 0)
 		return ret;
 
+	drain_all_pages(cc.zone);
+
 	/*
 	 * In case of -EBUSY, we'd like to know which page causes problem.
 	 * So, just fall through. test_pages_isolated() has a tracepoint
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 63a3db10a8c0..8dfa6c6c668d 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -19,8 +19,8 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
 {
 	struct page *unmovable = NULL;
 	struct zone *zone;
-	unsigned long flags;
-	int ret = -EBUSY;
+	unsigned long flags, nr_pages;
+	int ret = -EBUSY, mt;
 
 	zone = page_zone(page);
 
@@ -39,24 +39,18 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
 	 * We just check MOVABLE pages.
 	 */
 	unmovable = has_unmovable_pages(zone, page, migratetype, isol_flags);
-	if (!unmovable) {
-		unsigned long nr_pages;
-		int mt = get_pageblock_migratetype(page);
-
-		set_pageblock_migratetype(page, MIGRATE_ISOLATE);
-		zone->nr_isolate_pageblock++;
-		nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE,
-									NULL);
-
-		__mod_zone_freepage_state(zone, -nr_pages, mt);
-		ret = 0;
-	}
+	if (unmovable)
+		goto out;
 
+	mt = get_pageblock_migratetype(page);
+	set_pageblock_migratetype(page, MIGRATE_ISOLATE);
+	zone->nr_isolate_pageblock++;
+	nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE, NULL);
+	__mod_zone_freepage_state(zone, -nr_pages, mt);
+	ret = 0;
 out:
 	spin_unlock_irqrestore(&zone->lock, flags);
-	if (!ret) {
-		drain_all_pages(zone);
-	} else {
+	if (ret) {
 		WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
 
 		if ((isol_flags & REPORT_FAILURE) && unmovable)
@@ -170,14 +164,6 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
  * pageblocks we may have modified and return -EBUSY to caller. This
  * prevents two threads from simultaneously working on overlapping ranges.
  *
- * Please note that there is no strong synchronization with the page allocator
- * either. Pages might be freed while their page blocks are marked ISOLATED.
- * In some cases pages might still end up on pcp lists and that would allow
- * for their allocation even when they are in fact isolated already. Depending
- * on how strong of a guarantee the caller needs drain_all_pages might be needed
- * (e.g. __offline_pages will need to call it after check for isolated range for
- * a next retry).
- *
  * Return: the number of isolated pageblocks on success and -EBUSY if any part
  * of range cannot be isolated.
  */
@@ -192,9 +178,7 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
 	BUG_ON(!IS_ALIGNED(start_pfn, pageblock_nr_pages));
 	BUG_ON(!IS_ALIGNED(end_pfn, pageblock_nr_pages));
 
-	for (pfn = start_pfn;
-	     pfn < end_pfn;
-	     pfn += pageblock_nr_pages) {
+	for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
 		page = __first_valid_page(pfn, pageblock_nr_pages);
 		if (page) {
 			if (set_migratetype_isolate(page, migratetype, flags)) {
-- 
2.25.1