linux-kernel - [PATCH v2] mm/mmzone.c: use try_cmpxchg() in page_cpupid_xchg

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Message-Id: <20220120011200.1322836-1-pcc@google.com>
Date:   Wed, 19 Jan 2022 17:11:59 -0800
From:   Peter Collingbourne <pcc@...gle.com>
To:     Andrey Konovalov <andreyknvl@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Peter Collingbourne <pcc@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>
Subject: [PATCH v2] mm/mmzone.c: use try_cmpxchg() in page_cpupid_xchg_last()

This will let us avoid an additional read from page->flags when
retrying the compare-exchange on some architectures.

Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Peter Collingbourne <pcc@...gle.com>
Link: https://linux-review.googlesource.com/id/I2e1f5b5b080ac9c4e0eb7f98768dba6fd7821693
---
 mm/mmzone.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/mmzone.c b/mm/mmzone.c
index eb89d6e018e2..d8a9b0e1b526 100644
--- a/mm/mmzone.c
+++ b/mm/mmzone.c
@@ -89,13 +89,14 @@ int page_cpupid_xchg_last(struct page *page, int cpupid)
 	unsigned long old_flags, flags;
 	int last_cpupid;
 
+	old_flags = READ_ONCE(page->flags);
 	do {
-		old_flags = flags = page->flags;
-		last_cpupid = page_cpupid_last(page);
+		flags = old_flags;
+		last_cpupid = (flags >> LAST_CPUPID_PGSHIFT) & LAST_CPUPID_MASK;
 
 		flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
 		flags |= (cpupid & LAST_CPUPID_MASK) << LAST_CPUPID_PGSHIFT;
-	} while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags));
+	} while (unlikely(!try_cmpxchg(&page->flags, &old_flags, flags)));
 
 	return last_cpupid;
 }
-- 
2.34.1.703.g22d0c6ccf7-goog