linux-kernel - [PATCH v6 12/15] futex: Use a hashmask instead of hashsize.

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Message-ID: <20241218111618.268028-13-bigeasy@linutronix.de>
Date: Wed, 18 Dec 2024 12:09:50 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH v6 12/15] futex: Use a hashmask instead of hashsize.

The global hash uses futex_hashsize to save the amount of the hash
buckets that have been allocated during system boot. On each
futex_hash() invocation this number is substracted by one to get the
mask. This can be optimized by saving directly the mask avoiding the
substraction on each futex_hash() invocation.

Rename futex_hashsize to futex_hashmask and save the mask of the
allocated hash map.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/futex/core.c | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 95a177f0c5d68..0fdbf691ec95b 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -52,10 +52,10 @@
  */
 static struct {
 	struct futex_hash_bucket *queues;
-	unsigned long            hashsize;
+	unsigned long            hashmask;
 } __futex_data __read_mostly __aligned(2*sizeof(long));
 #define futex_queues   (__futex_data.queues)
-#define futex_hashsize (__futex_data.hashsize)
+#define futex_hashmask (__futex_data.hashmask)
 
 struct futex_hash_bucket_private {
 	rcuref_t	users;
@@ -273,7 +273,7 @@ struct futex_hash_bucket *futex_hash(union futex_key *key)
 	hash = jhash2((u32 *)key,
 		      offsetof(typeof(*key), both.offset) / 4,
 		      key->both.offset);
-	return &futex_queues[hash & (futex_hashsize - 1)];
+	return &futex_queues[hash & futex_hashmask];
 }
 
 void futex_hash_put(struct futex_hash_bucket *hb)
@@ -1370,8 +1370,8 @@ static int futex_hash_allocate(unsigned int hash_slots)
 		hash_slots = 16;
 	if (hash_slots < 2)
 		hash_slots = 2;
-	if (hash_slots > futex_hashsize)
-		hash_slots = futex_hashsize;
+	if (hash_slots > futex_hashmask + 1)
+		hash_slots = futex_hashmask + 1;
 	if (!is_power_of_2(hash_slots))
 		hash_slots = rounddown_pow_of_two(hash_slots);
 
@@ -1427,7 +1427,7 @@ int futex_hash_allocate_default(void)
 
 	buckets = roundup_pow_of_two(4 * threads);
 	buckets = max(buckets, 16);
-	buckets = min(buckets, futex_hashsize);
+	buckets = min(buckets, futex_hashmask + 1);
 	if (current_buckets >= buckets)
 		return 0;
 
@@ -1467,24 +1467,25 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3)
 
 static int __init futex_init(void)
 {
+	unsigned long i, hashsize;
 	unsigned int futex_shift;
-	unsigned long i;
 
 #ifdef CONFIG_BASE_SMALL
-	futex_hashsize = 16;
+	hashsize = 16;
 #else
-	futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
+	hashsize = roundup_pow_of_two(256 * num_possible_cpus());
 #endif
 
 	futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
-					       futex_hashsize, 0, 0,
+					       hashsize, 0, 0,
 					       &futex_shift, NULL,
-					       futex_hashsize, futex_hashsize);
-	futex_hashsize = 1UL << futex_shift;
+					       hashsize, hashsize);
+	hashsize = 1UL << futex_shift;
 
-	for (i = 0; i < futex_hashsize; i++)
+	for (i = 0; i < hashsize; i++)
 		futex_hash_bucket_init(&futex_queues[i], 0);
 
+	futex_hashmask = hashsize - 1;
 	return 0;
 }
 core_initcall(futex_init);
-- 
2.45.2