linux-kernel - [tip:locking/core] futex: Force hot variables into a single cache line

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Date:	Tue, 22 Sep 2015 07:27:33 -0700
From:	tip-bot for Rasmus Villemoes <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	mingo@...nel.org, tglx@...utronix.de, fengguang.wu@...el.com,
	dave@...olabs.net, linux-kernel@...r.kernel.org,
	bigeasy@...utronix.de, peterz@...radead.org,
	linux@...musvillemoes.dk, hpa@...or.com
Subject: [tip:locking/core] futex:
  Force hot variables into a single cache line

Commit-ID:  ac742d37180bee83bc433be087b66a17af2883b9
Gitweb:     http://git.kernel.org/tip/ac742d37180bee83bc433be087b66a17af2883b9
Author:     Rasmus Villemoes <linux@...musvillemoes.dk>
AuthorDate: Wed, 9 Sep 2015 23:36:40 +0200
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Tue, 22 Sep 2015 16:23:15 +0200

futex: Force hot variables into a single cache line

futex_hash() references two global variables: the base pointer
futex_queues and the size of the array futex_hashsize. The latter is
marked __read_mostly, while the former is not, so they are likely to
end up very far from each other. This means that futex_hash() is
likely to encounter two cache misses.

We could mark futex_queues as __read_mostly as well, but that doesn't
guarantee they'll end up next to each other (and even if they do, they
may still end up in different cache lines). So put the two variables
in a small singleton struct with sufficient alignment and mark that as
__read_mostly.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: kbuild test robot <fengguang.wu@...el.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Link: http://lkml.kernel.org/r/1441834601-13633-1-git-send-email-linux@rasmusvillemoes.dk
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/futex.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 6e443ef..dfc86e9 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -255,9 +255,18 @@ struct futex_hash_bucket {
 	struct plist_head chain;
 } ____cacheline_aligned_in_smp;
 
-static unsigned long __read_mostly futex_hashsize;
+/*
+ * The base of the bucket array and its size are always used together
+ * (after initialization only in hash_futex()), so ensure that they
+ * reside in the same cacheline.
+ */
+static struct {
+	struct futex_hash_bucket *queues;
+	unsigned long            hashsize;
+} __futex_data __read_mostly __aligned(2*sizeof(long));
+#define futex_queues   (__futex_data.queues)
+#define futex_hashsize (__futex_data.hashsize)
 
-static struct futex_hash_bucket *futex_queues;
 
 /*
  * Fault injections for futexes.
--
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