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From:   Baolin Wang <baolin.wang@linux.alibaba.com>
To:     akpm@linux-foundation.org
Cc:     mgorman@techsingularity.net, hughd@google.com, vbabka@suse.cz,
        ying.huang@intel.com, ziy@nvidia.com,
        baolin.wang@linux.alibaba.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH] mm: migrate: record the mlocked page status to remove unnecessary lru drain
Date:   Wed, 18 Oct 2023 21:04:32 +0800
Message-Id: <64899ad0bb78cde88b52abed1a5a5abbc9919998.1697632761.git.baolin.wang@linux.alibaba.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

When doing compaction, I found the lru_add_drain() is an obvious hotspot
when migrating pages. The distribution of this hotspot is as follows:
   - 18.75% compact_zone
      - 17.39% migrate_pages
         - 13.79% migrate_pages_batch
            - 11.66% migrate_folio_move
               - 7.02% lru_add_drain
                  + 7.02% lru_add_drain_cpu
               + 3.00% move_to_new_folio
                 1.23% rmap_walk
            + 1.92% migrate_folio_unmap
         + 3.20% migrate_pages_sync
      + 0.90% isolate_migratepages

The lru_add_drain() was added by commit c3096e6782b7 ("mm/migrate:
__unmap_and_move() push good newpage to LRU") to drain the newpage to LRU
immediately, to help to build up the correct newpage->mlock_count in
remove_migration_ptes() for mlocked pages. However, if there are no mlocked
pages are migrating, then we can avoid this lru drain operation, especailly
for the heavy concurrent scenarios.

So we can record the source pages' mlocked status in migrate_folio_unmap(),
and only drain the lru list when the mlocked status is set in migrate_folio_move().
In addition, the page was already isolated from lru when migrating, so we
check the mlocked status is stable by folio_test_mlocked() in migrate_folio_unmap().

After this patch, I can see the hotpot of the lru_add_drain() is gone:
   - 9.41% migrate_pages_batch
      - 6.15% migrate_folio_move
         - 3.64% move_to_new_folio
            + 1.80% migrate_folio_extra
            + 1.70% buffer_migrate_folio
         + 1.41% rmap_walk
         + 0.62% folio_add_lru
      + 3.07% migrate_folio_unmap

Meanwhile, the compaction latency shows some improvements when running
thpscale:
                            base                   patched
Amean     fault-both-1      1131.22 (   0.00%)     1112.55 *   1.65%*
Amean     fault-both-3      2489.75 (   0.00%)     2324.15 *   6.65%*
Amean     fault-both-5      3257.37 (   0.00%)     3183.18 *   2.28%*
Amean     fault-both-7      4257.99 (   0.00%)     4079.04 *   4.20%*
Amean     fault-both-12     6614.02 (   0.00%)     6075.60 *   8.14%*
Amean     fault-both-18    10607.78 (   0.00%)     8978.86 *  15.36%*
Amean     fault-both-24    14911.65 (   0.00%)    11619.55 *  22.08%*
Amean     fault-both-30    14954.67 (   0.00%)    14925.66 *   0.19%*
Amean     fault-both-32    16654.87 (   0.00%)    15580.31 *   6.45%*

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 mm/migrate.c | 50 ++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 38 insertions(+), 12 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 4caf405b6504..32c96f89710f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1027,22 +1027,32 @@ union migration_ptr {
 	struct anon_vma *anon_vma;
 	struct address_space *mapping;
 };
+
+enum {
+	PAGE_WAS_MAPPED = 1 << 0,
+	PAGE_WAS_MLOCKED = 1 << 1,
+};
+
 static void __migrate_folio_record(struct folio *dst,
-				   unsigned long page_was_mapped,
+				   unsigned long page_flags,
 				   struct anon_vma *anon_vma)
 {
 	union migration_ptr ptr = { .anon_vma = anon_vma };
 	dst->mapping = ptr.mapping;
-	dst->private = (void *)page_was_mapped;
+	dst->private = (void *)page_flags;
 }
 
 static void __migrate_folio_extract(struct folio *dst,
 				   int *page_was_mappedp,
+				   int *page_was_mlocked,
 				   struct anon_vma **anon_vmap)
 {
 	union migration_ptr ptr = { .mapping = dst->mapping };
+	unsigned long page_flags = (unsigned long)dst->private;
+
 	*anon_vmap = ptr.anon_vma;
-	*page_was_mappedp = (unsigned long)dst->private;
+	*page_was_mappedp = page_flags & PAGE_WAS_MAPPED ? 1 : 0;
+	*page_was_mlocked = page_flags & PAGE_WAS_MLOCKED ? 1 : 0;
 	dst->mapping = NULL;
 	dst->private = NULL;
 }
@@ -1103,7 +1113,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 {
 	struct folio *dst;
 	int rc = -EAGAIN;
-	int page_was_mapped = 0;
+	int page_was_mapped = 0, page_was_mlocked = 0;
 	struct anon_vma *anon_vma = NULL;
 	bool is_lru = !__folio_test_movable(src);
 	bool locked = false;
@@ -1157,6 +1167,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 		folio_lock(src);
 	}
 	locked = true;
+	page_was_mlocked = folio_test_mlocked(src);
 
 	if (folio_test_writeback(src)) {
 		/*
@@ -1206,7 +1217,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 	dst_locked = true;
 
 	if (unlikely(!is_lru)) {
-		__migrate_folio_record(dst, page_was_mapped, anon_vma);
+		__migrate_folio_record(dst, 0, anon_vma);
 		return MIGRATEPAGE_UNMAP;
 	}
 
@@ -1236,7 +1247,13 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 	}
 
 	if (!folio_mapped(src)) {
-		__migrate_folio_record(dst, page_was_mapped, anon_vma);
+		unsigned int page_flags = 0;
+
+		if (page_was_mapped)
+			page_flags |= PAGE_WAS_MAPPED;
+		if (page_was_mlocked)
+			page_flags |= PAGE_WAS_MLOCKED;
+		__migrate_folio_record(dst, page_flags, anon_vma);
 		return MIGRATEPAGE_UNMAP;
 	}
 
@@ -1261,12 +1278,13 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 			      struct list_head *ret)
 {
 	int rc;
-	int page_was_mapped = 0;
+	int page_was_mapped = 0, page_was_mlocked = 0;
 	struct anon_vma *anon_vma = NULL;
 	bool is_lru = !__folio_test_movable(src);
 	struct list_head *prev;
 
-	__migrate_folio_extract(dst, &page_was_mapped, &anon_vma);
+	__migrate_folio_extract(dst, &page_was_mapped,
+				&page_was_mlocked, &anon_vma);
 	prev = dst->lru.prev;
 	list_del(&dst->lru);
 
@@ -1287,7 +1305,7 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 	 * isolated from the unevictable LRU: but this case is the easiest.
 	 */
 	folio_add_lru(dst);
-	if (page_was_mapped)
+	if (page_was_mlocked)
 		lru_add_drain();
 
 	if (page_was_mapped)
@@ -1321,8 +1339,15 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 	 * right list unless we want to retry.
 	 */
 	if (rc == -EAGAIN) {
+		unsigned int page_flags = 0;
+
+		if (page_was_mapped)
+			page_flags |= PAGE_WAS_MAPPED;
+		if (page_was_mlocked)
+			page_flags |= PAGE_WAS_MLOCKED;
+
 		list_add(&dst->lru, prev);
-		__migrate_folio_record(dst, page_was_mapped, anon_vma);
+		__migrate_folio_record(dst, page_flags, anon_vma);
 		return rc;
 	}
 
@@ -1799,10 +1824,11 @@ static int migrate_pages_batch(struct list_head *from,
 	dst = list_first_entry(&dst_folios, struct folio, lru);
 	dst2 = list_next_entry(dst, lru);
 	list_for_each_entry_safe(folio, folio2, &unmap_folios, lru) {
-		int page_was_mapped = 0;
+		int page_was_mapped = 0, page_was_mlocked = 0;
 		struct anon_vma *anon_vma = NULL;
 
-		__migrate_folio_extract(dst, &page_was_mapped, &anon_vma);
+		__migrate_folio_extract(dst, &page_was_mapped,
+					&page_was_mlocked, &anon_vma);
 		migrate_folio_undo_src(folio, page_was_mapped, anon_vma,
 				       true, ret_folios);
 		list_del(&dst->lru);
-- 
2.39.3