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SubjectRe: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket plist in futex_wake()
On Wed, Nov 27, 2013 at 11:44:38PM -0800, Davidlohr Bueso wrote:
> How about both enlarging the table _and_ aligning the buckets? As you
> know, increasing the size of the table also benefits (particularly in
> larger systems) in having more spinlocks. So we reduce the amount of
> collisions and alleviate contention on the hb->lock. Btw, do you have
> any particular concerns about the larger hash table patch?

My only concern was the amount of #ifdef.

Wouldn't something like the below also work?


---
kernel/futex.c | 20 +++++++++++++++-----
1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 80ba086f021d..2e1a9294f751 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -70,8 +70,6 @@

int __read_mostly futex_cmpxchg_enabled;

-#define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)
-
/*
* Futex flags used to encode options to functions and preserve them across
* restarts.
@@ -149,9 +147,11 @@ static const struct futex_q futex_q_init = {
struct futex_hash_bucket {
spinlock_t lock;
struct plist_head chain;
-};
+} ____cacheline_aligned_in_smp;

-static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];
+static unsigned long __read_mostly futex_hashsize;
+
+static struct futex_hash_bucket *futex_queues;

/*
* We hash on the keys returned from get_futex_key (see below).
@@ -161,7 +161,7 @@ static struct futex_hash_bucket *hash_futex(union futex_key *key)
u32 hash = jhash2((u32*)&key->both.word,
(sizeof(key->both.word)+sizeof(key->both.ptr))/4,
key->both.offset);
- return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)];
+ return &futex_queues[hash & (futex_hashsize - 1)];
}

/*
@@ -2735,6 +2735,16 @@ static int __init futex_init(void)
u32 curval;
int i;

+#ifdef CONFIG_BASE_SMALL
+ futex_hashsize = 16;
+#else
+ futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
+#endif
+
+ futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
+ futex_hashsize, 0, futex_hashsize < 256 ? HASH_SMALL : 0,
+ NULL, NULL, futex_hashsize, futex_hashsize);
+
/*
* This will fail and we want it. Some arch implementations do
* runtime detection of the futex_atomic_cmpxchg_inatomic()

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