linux-kernel - [v4 PATCH 2/2] mm: swap: use smp_mb__after

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Message-Id: <1584500541-46817-2-git-send-email-yang.shi@linux.alibaba.com>
Date:   Wed, 18 Mar 2020 11:02:21 +0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     shakeelb@...gle.com, vbabka@...e.cz, willy@...radead.org,
        akpm@...ux-foundation.org
Cc:     yang.shi@...ux.alibaba.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [v4 PATCH 2/2] mm: swap: use smp_mb__after_atomic() to order LRU bit set

Memory barrier is needed after setting LRU bit, but smp_mb() is too
strong.  Some architectures, i.e. x86, imply memory barrier with atomic
operations, so replacing it with smp_mb__after_atomic() sounds better,
which is nop on strong ordered machines, and full memory barriers on
others.  With this change the vm-scalability cases would perform better
on x86, I saw total 6% improvement with this patch and previous inline
fix.

The test data (lru-file-readtwice throughput) against v5.6-rc4:
	mainline	w/ inline fix	w/ both (adding this)
	150MB		154MB		159MB

Fixes: 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping pagevecs")
Acked-by: Vlastimil Babka <vbabka@...e.cz>
Reviewed-and-Tested-by: Shakeel Butt <shakeelb@...gle.com>
Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
---
v3: * Added Shakeel's review-and-test signature.
v2: * Solved the comment from Vlastimil and added his Ack.

 mm/swap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index cf39d24..74ea08a 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -931,7 +931,6 @@ static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
 
 	VM_BUG_ON_PAGE(PageLRU(page), page);
 
-	SetPageLRU(page);
 	/*
 	 * Page becomes evictable in two ways:
 	 * 1) Within LRU lock [munlock_vma_page() and __munlock_pagevec()].
@@ -958,7 +957,8 @@ static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
 	 * looking at the same page) and the evictable page will be stranded
 	 * in an unevictable LRU.
 	 */
-	smp_mb();
+	SetPageLRU(page);
+	smp_mb__after_atomic();
 
 	if (page_evictable(page)) {
 		lru = page_lru(page);
-- 
1.8.3.1