Re: [RFC] NUMA futex hashing

Eric Dumazet
2006-08-08T09:14:53Z

On Tuesday 08 August 2006 09:07, Ravikiran G Thirumalai wrote:
> Current futex hash scheme is not the best for NUMA. The futex hash table
> is an array of struct futex_hash_bucket, which is just a spinlock and a
> list_head -- this means multiple spinlocks on the same cacheline and on
> NUMA machines, on the same internode cacheline. If futexes of two
> unrelated threads running on two different nodes happen to hash onto
> adjacent hash buckets, or buckets on the same internode cacheline, then we
> have the internode cacheline bouncing between nodes.
>
> Here is a simple scheme which maintains per-node hash tables for futexes.
>
> In this scheme, a private futex is assigned to the node id of the futex's
> KVA. The reasoning is, the futex KVA is allocated from the node as
> indicated by memory policy set by the process, and that should be a good
> 'home node' for that futex. Of course this helps workloads where all the
> threads of a process are bound to the same node, but it seems reasonable to
> run all threads of a process on the same node.
>
> A shared futex is assigned a home node based on jhash2 itself. Since inode
> and offset are used as the key, the same inode offset is used to arrive at
> the home node of a shared futex. This distributes private futexes across
> all nodes.
>
> Comments? Suggestions? Particularly regarding shared futexes. Any policy
> suggestions?
>

Your patch seems fine, but I have one comment.

For non NUMA machine, we would have one useless indirection to get the
futex_queues pointer.

static struct futex_hash_bucket *futex_queues[1];

I think it is worth to redesign your patch so that this extra-indirection is
needed only for NUMA machines.

#if defined(CONFIG_NUMA)
static struct futex_hash_bucket *futex_queues[MAX_NUMNODES];
#define FUTEX_QUEUES(nodeid, hash) \
&futex_queues[nodeid][hash & ((1 << FUTEX_HASHBITS)-1)];
#else
static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];
# define FUTEX_QUEUES(nodeid, hash) \
&futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)];
#endif

Thank you

> Thanks,
> Kiran
>
> Note: This patch needs to have kvaddr_to_nid() reintroduced. This was
> taken out in git commit 9f3fd602aef96c2a490e3bfd669d06475aeba8d8
>
> Index: linux-2.6.18-rc3/kernel/futex.c
> ===================================================================
> --- linux-2.6.18-rc3.orig/kernel/futex.c 2006-08-02 12:11:34.000000000
> -0700 +++ linux-2.6.18-rc3/kernel/futex.c 2006-08-02 16:48:47.000000000
> -0700 @@ -137,20 +137,35 @@ struct futex_hash_bucket {
> struct list_head chain;
> };
>
> -static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];
> +static struct futex_hash_bucket *futex_queues[MAX_NUMNODES] __read_mostly;
>
> /* Futex-fs vfsmount entry: */
> static struct vfsmount *futex_mnt;
>
> /*
> * We hash on the keys returned from get_futex_key (see below).
> + * With NUMA aware futex hashing, we have per-node hash tables.
> + * We determine the home node of a futex based on the KVA -- if the futex
> + * is a private futex. For shared futexes, we use jhash2 itself on the
> + * futex_key to arrive at a home node.
> */
> static struct futex_hash_bucket *hash_futex(union futex_key *key)
> {
> + int nodeid;
> 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)];
> + if (key->both.offset & 0x1) {
> + /*
> + * Shared futex: Use any of the 'possible' nodes as home node.
> + */
> + nodeid = hash & (MAX_NUMNODES -1);
> + BUG_ON(!node_possible(nodeid));
> + } else
> + /* Private futex */
> + nodeid = kvaddr_to_nid(key->both.ptr);
> +
> + return &futex_queues[nodeid][hash & ((1 << FUTEX_HASHBITS)-1)];
> }
>
> /*
> @@ -1909,13 +1924,25 @@ static int __init init(void)
> {
> unsigned int i;
>
> + int nid;
> +
> + for_each_node(nid)
> + {
> + futex_queues[nid] = kmalloc_node(
> + (sizeof(struct futex_hash_bucket) *
> + (1 << FUTEX_HASHBITS)),
> + GFP_KERNEL, nid);
> + if (!futex_queues[nid])
> + panic("futex_init: Allocation of multi-node futex_queues failed");
> + for (i = 0; i < (1 << FUTEX_HASHBITS); i++) {
> + INIT_LIST_HEAD(&futex_queues[nid][i].chain);
> + spin_lock_init(&futex_queues[nid][i].lock);
> + }
> + }
> +
> register_filesystem(&futex_fs_type);
> futex_mnt = kern_mount(&futex_fs_type);
>
> - for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
> - INIT_LIST_HEAD(&futex_queues[i].chain);
> - spin_lock_init(&futex_queues[i].lock);
> - }
> return 0;
> }
> __initcall(init);
>
> -
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