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Subject[PATCH] futex: eliminate cache miss from futex_hash()
Date
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.

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

: 31 d1 xor %edx,%ecx
: c1 ca 12 ror $0x12,%edx
: 29 d1 sub %edx,%ecx
-: 48 8b 15 25 c8 e5 00 mov 0xe5c825(%rip),%rdx # ffffffff81f149c8 <futex_hashsize>
+: 48 8b 15 35 c8 e5 00 mov 0xe5c835(%rip),%rdx # ffffffff81f149d8 <__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 e4 5e 02 01 add 0x1025ee4(%rip),%rax # ffffffff820de0a0 <futex_queues>
+: 48 03 05 14 c8 e5 00 add 0xe5c814(%rip),%rax # ffffffff81f149d0 <__futex_data>
: c3 retq
: 0f 1f 00 nopl (%rax)

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
Resending since this was never picked up - and I assume it's actually
ok. Also, this time the alignment is spelled 2*sizeof(long) to avoid
wasting 8 bytes on 32bit.

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

diff --git a/kernel/futex.c b/kernel/futex.c
index 6e443efc65f4..dfc86e93c31d 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.
--
2.1.3



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