linux-kernel - [RFC PATCH v3 4/9] futex: Hash only the address for private futexes.

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Message-ID: <20241115172035.795842-5-bigeasy@linutronix.de>
Date: Fri, 15 Nov 2024 17:58:45 +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: [RFC PATCH v3 4/9] futex: Hash only the address for private futexes.

futex_hash() passes the whole futex_key to jhash2. The first two member
are passed as the first argument and the offset as the "initial value".

For private futexes, the mm-part is always the same and it is used only
within the process. By excluding the mm part from the hash, we reduce
the length passed to jhash2 from 4 (16 / 4) to 2 (8 / 2). This avoids
the __jhash_mix() part of jhash.

The resulting code is smaller and based on testing this variant performs
as good as the original or slightly better.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/futex/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 2f5087fde57ef..5b66b6e52aeb5 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -134,8 +134,8 @@ struct futex_hash_bucket *futex_hash(union futex_key *key)
 	if (fhb && futex_key_is_private(key)) {
 		u32 hash_mask = current->mm->futex_hash_mask;
 
-		hash = jhash2((u32 *)key,
-			      offsetof(typeof(*key), both.offset) / 4,
+		hash = jhash2((void *)&key->private.address,
+			      sizeof(key->private.address) / 4,
 			      key->both.offset);
 		return &fhb[hash & hash_mask];
 	}
-- 
2.45.2