linux-kernel - [PATCH v2 2/4] sbitmap: remove swap

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Date:   Sun, 22 Nov 2020 15:35:46 +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 v2 2/4] sbitmap: remove swap_lock

map->swap_lock protects map->cleared from concurrent modification,
however sbitmap_deferred_clear() is already atomically drains it, so
it's guaranteed to not loose bits on concurrent
sbitmap_deferred_clear().

A one threaded tag heavy test on top of nullbk showed ~1.5% t-put
increase, and 3% -> 1% cycle reduction of sbitmap_get() according to perf.

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 include/linux/sbitmap.h |  5 -----
 lib/sbitmap.c           | 14 +++-----------
 2 files changed, 3 insertions(+), 16 deletions(-)

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index e40d019c3d9d..74cc6384715e 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -32,11 +32,6 @@ struct sbitmap_word {
 	 * @cleared: word holding cleared bits
 	 */
 	unsigned long cleared ____cacheline_aligned_in_smp;
-
-	/**
-	 * @swap_lock: Held while swapping word <-> cleared
-	 */
-	spinlock_t swap_lock;
 } ____cacheline_aligned_in_smp;
 
 /**
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index c1c8a4e69325..4fd877048ba8 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -15,13 +15,9 @@
 static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
 {
 	unsigned long mask, val;
-	bool ret = false;
-	unsigned long flags;
 
-	spin_lock_irqsave(&map->swap_lock, flags);
-
-	if (!map->cleared)
-		goto out_unlock;
+	if (!READ_ONCE(map->cleared))
+		return false;
 
 	/*
 	 * First get a stable cleared mask, setting the old mask to 0.
@@ -35,10 +31,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
 		val = map->word;
 	} while (cmpxchg(&map->word, val, val & ~mask) != val);
 
-	ret = true;
-out_unlock:
-	spin_unlock_irqrestore(&map->swap_lock, flags);
-	return ret;
+	return true;
 }
 
 int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
@@ -80,7 +73,6 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
 	for (i = 0; i < sb->map_nr; i++) {
 		sb->map[i].depth = min(depth, bits_per_word);
 		depth -= sb->map[i].depth;
-		spin_lock_init(&sb->map[i].swap_lock);
 	}
 	return 0;
 }
-- 
2.24.0