linux-kernel - [PATCH 1/2] sbitmap: optimise sbitmap_deferred

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Date:   Sat, 21 Nov 2020 00:01:15 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Omar Sandoval <osandov@...ndov.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] sbitmap: optimise sbitmap_deferred_clear()

Because of spinlocks and atomics sbitmap_deferred_clear() have to reload
&sb->map[index] on each access even though the map address won't change.
Hint it by explicitly caching it in a variable.

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 lib/sbitmap.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 267aa7709416..49afb34e8340 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -14,30 +14,31 @@
  */
 static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
 {
+	struct sbitmap_word *map = &sb->map[index];
 	unsigned long mask, val;
 	bool ret = false;
 	unsigned long flags;
 
-	spin_lock_irqsave(&sb->map[index].swap_lock, flags);
+	spin_lock_irqsave(&map->swap_lock, flags);
 
-	if (!sb->map[index].cleared)
+	if (!map->cleared)
 		goto out_unlock;
 
 	/*
 	 * First get a stable cleared mask, setting the old mask to 0.
 	 */
-	mask = xchg(&sb->map[index].cleared, 0);
+	mask = xchg(&map->cleared, 0);
 
 	/*
 	 * Now clear the masked bits in our free word
 	 */
 	do {
-		val = sb->map[index].word;
-	} while (cmpxchg(&sb->map[index].word, val, val & ~mask) != val);
+		val = map->word;
+	} while (cmpxchg(&map->word, val, val & ~mask) != val);
 
 	ret = true;
 out_unlock:
-	spin_unlock_irqrestore(&sb->map[index].swap_lock, flags);
+	spin_unlock_irqrestore(&map->swap_lock, flags);
 	return ret;
 }
 
-- 
2.24.0