[PATCH 1/2] futex: eliminate cache miss from futex_hash()

[Rasmus Villemoes]

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. So put the two variables
in a small singleton struct and mark that as __read_mostly.

A diff of the disassembly shows what I'd expect:

@@ -213289,14 +213289,14 @@
 :      31 d1                   xor    %edx,%ecx
 :      c1 ca 12                ror    $0x12,%edx
 :      29 d1                   sub    %edx,%ecx
-:      48 8b 15 95 61 e4 00    mov    0xe46195(%rip),%rdx        # ffffffff81eff9e8 <futex_hashsize>
+:      48 8b 15 a5 61 e4 00    mov    0xe461a5(%rip),%rdx        # ffffffff81eff9f8 <__futex_data+0x8>
 :      31 c8                   xor    %ecx,%eax
 :      c1 c9 08                ror    $0x8,%ecx
 :      29 c8                   sub    %ecx,%eax
 :      48 83 ea 01             sub    $0x1,%rdx
 :      48 21 d0                and    %rdx,%rax
 :      48 c1 e0 06             shl    $0x6,%rax
-:      48 03 05 74 dc 00 01    add    0x100dc74(%rip),%rax        # ffffffff820c74e0 <futex_queues>
+:      48 03 05 84 61 e4 00    add    0xe46184(%rip),%rax        # ffffffff81eff9f0 <__futex_data>
 :      c3                      retq
 :      0f 1f 00                nopl   (%rax)

Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---

If this is worth applying, one could consider giving dentry_hashtable,
inode_hashtable and their friends the same treatment. The variables
are all __read_mostly, but may still end up in separate cache lines
(even if the linker places them next to each other).

Even better would be to have some alternatives-like mechanism for
replacing the code with instructions using immediates. But that's far
more complicated...

 kernel/futex.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 2579e407ff67..c5f33bf78293 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -254,9 +254,18 @@ 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(16);
+#define futex_queues   (__futex_data.queues)
+#define futex_hashsize (__futex_data.hashsize)
 
-static struct futex_hash_bucket *futex_queues;
 
 static inline void futex_get_mm(union futex_key *key)
 {
-- 
2.1.3

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