mm, compaction: reduce premature advancement of the migration target scanner

gormanm · torvalds · commit d097a6f63522 · 2019-03-05T21:07:17.000-08:00
The fast isolation of free pages allows the cached PFN of the free scanner to advance faster than necessary depending on the contents of the free list. The key is that fast_isolate_freepages() can update zone->compact_cached_free_pfn via isolate_freepages_block(). When the fast search fails, the linear scan can start from a point that has skipped valid migration targets, particularly pageblocks with just low-order free pages. This can cause the migration source/target scanners to meet prematurely causing a reset. This patch starts by avoiding an update of the pageblock skip information and cached PFN from isolate_freepages_block() and puts the responsibility of updating that information in the callers. The fast scanner will update the cached PFN if and only if it finds a block that is higher than the existing cached PFN and sets the skip if the pageblock is full or nearly full. The linear scanner will update skipped information and the cached PFN only when a block is completely scanned. The total impact is that the free scanner advances more slowly as it is primarily driven by the linear scanner instead of the fast search. 5.0.0-rc1 5.0.0-rc1 noresched-v3r17 slowfree-v3r17 Amean fault-both-3 2965.68 ( 0.00%) 3036.75 ( -2.40%) Amean fault-both-5 3995.90 ( 0.00%) 4522.24 * -13.17%* Amean fault-both-7 5842.12 ( 0.00%) 6365.35 ( -8.96%) Amean fault-both-12 9550.87 ( 0.00%) 10340.93 ( -8.27%) Amean fault-both-18 13304.72 ( 0.00%) 14732.46 ( -10.73%) Amean fault-both-24 14618.59 ( 0.00%) 16288.96 ( -11.43%) Amean fault-both-30 16650.96 ( 0.00%) 16346.21 ( 1.83%) Amean fault-both-32 17145.15 ( 0.00%) 19317.49 ( -12.67%) The impact to latency is higher than the last version but it appears to be due to a slight increase in the free scan rates which is a potential side-effect of the patch. However, this is necessary for later patches that are more careful about how pageblocks are treated as earlier iterations of those patches hit corner cases where the restarts were punishing and very visible. Link: http://lkml.kernel.org/r/20190118175136.31341-19-mgorman@techsingularity.net Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Acked-by: Vlastimil Babka <vbabka@suse.cz> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Dan Carpenter <dan.carpenter@oracle.com> Cc: David Rientjes <rientjes@google.com> Cc: YueHaibing <yuehaibing@huawei.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/compaction.c b/mm/compaction.c
@@ -330,24 +330,18 @@ static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
  * future. The information is later cleared by __reset_isolation_suitable().
  */
 static void update_pageblock_skip(struct compact_control *cc,
-			struct page *page, unsigned long nr_isolated)
+			struct page *page, unsigned long pfn)
 {
 	struct zone *zone = cc->zone;
-	unsigned long pfn;
 
 	if (cc->no_set_skip_hint)
 		return;
 
 	if (!page)
 		return;
 
-	if (nr_isolated)
-		return;
-
 	set_pageblock_skip(page);
 
-	pfn = page_to_pfn(page);
-
 	/* Update where async and sync compaction should restart */
 	if (pfn < zone->compact_cached_free_pfn)
 		zone->compact_cached_free_pfn = pfn;
@@ -365,7 +359,7 @@ static inline bool pageblock_skip_persistent(struct page *page)
 }
 
 static inline void update_pageblock_skip(struct compact_control *cc,
-			struct page *page, unsigned long nr_isolated)
+			struct page *page, unsigned long pfn)
 {
 }
 
@@ -449,7 +443,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 				bool strict)
 {
 	int nr_scanned = 0, total_isolated = 0;
-	struct page *cursor, *valid_page = NULL;
+	struct page *cursor;
 	unsigned long flags = 0;
 	bool locked = false;
 	unsigned long blockpfn = *start_pfn;
@@ -476,9 +470,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 		if (!pfn_valid_within(blockpfn))
 			goto isolate_fail;
 
-		if (!valid_page)
-			valid_page = page;
-
 		/*
 		 * For compound pages such as THP and hugetlbfs, we can save
 		 * potentially a lot of iterations if we skip them at once.
@@ -566,10 +557,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 	if (strict && blockpfn < end_pfn)
 		total_isolated = 0;
 
-	/* Update the pageblock-skip if the whole pageblock was scanned */
-	if (blockpfn == end_pfn)
-		update_pageblock_skip(cc, valid_page, total_isolated);
-
 	cc->total_free_scanned += nr_scanned;
 	if (total_isolated)
 		count_compact_events(COMPACTISOLATED, total_isolated);
@@ -1293,8 +1280,10 @@ fast_isolate_freepages(struct compact_control *cc)
 		}
 	}
 
-	if (highest && highest > cc->zone->compact_cached_free_pfn)
+	if (highest && highest >= cc->zone->compact_cached_free_pfn) {
+		highest -= pageblock_nr_pages;
 		cc->zone->compact_cached_free_pfn = highest;
+	}
 
 	cc->total_free_scanned += nr_scanned;
 	if (!page)
@@ -1374,6 +1363,10 @@ static void isolate_freepages(struct compact_control *cc)
 		isolate_freepages_block(cc, &isolate_start_pfn, block_end_pfn,
 					freelist, false);
 
+		/* Update the skip hint if the full pageblock was scanned */
+		if (isolate_start_pfn == block_end_pfn)
+			update_pageblock_skip(cc, page, block_start_pfn);
+
 		/* Are enough freepages isolated? */
 		if (cc->nr_freepages >= cc->nr_migratepages) {
 			if (isolate_start_pfn >= block_end_pfn) {
