Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751273AbaALXcU (ORCPT ); Sun, 12 Jan 2014 18:32:20 -0500 Received: from g4t0016.houston.hp.com ([15.201.24.19]:12400 "EHLO g4t0016.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751207AbaALXbp (ORCPT ); Sun, 12 Jan 2014 18:31:45 -0500 From: Davidlohr Bueso To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, dvhart@linux.intel.com, peterz@infradead.org, tglx@linutronix.de, paulmck@linux.vnet.ibm.com, efault@gmx.de, jeffm@suse.com, torvalds@linux-foundation.org, jason.low2@hp.com, Waiman.Long@hp.com, tom.vaden@hp.com, scott.norton@hp.com, aswin@hp.com, davidlohr@hp.com Subject: [PATCH v6 2/5] futex: Larger hash table Date: Sun, 12 Jan 2014 15:31:23 -0800 Message-Id: <1389569486-25487-3-git-send-email-davidlohr@hp.com> X-Mailer: git-send-email 1.8.1.4 In-Reply-To: <1389569486-25487-1-git-send-email-davidlohr@hp.com> References: <1389569486-25487-1-git-send-email-davidlohr@hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Davidlohr Bueso Currently, the futex global hash table suffers from it's fixed, smallish (for today's standards) size of 256 entries, as well as its lack of NUMA awareness. Large systems, using many futexes, can be prone to high amounts of collisions; where these futexes hash to the same bucket and lead to extra contention on the same hb->lock. Furthermore, cacheline bouncing is a reality when we have multiple hb->locks residing on the same cacheline and different futexes hash to adjacent buckets. This patch keeps the current static size of 16 entries for small systems, or otherwise, 256 * ncpus (or larger as we need to round the number to a power of 2). Note that this number of CPUs accounts for all CPUs that can ever be available in the system, taking into consideration things like hotpluging. While we do impose extra overhead at bootup by making the hash table larger, this is a one time thing, and does not shadow the benefits of this patch. Furthermore, as suggested by tglx, by cache aligning the hash buckets we can avoid access across cacheline boundaries and also avoid massive cache line bouncing if multiple cpus are hammering away at different hash buckets which happen to reside in the same cache line. Also, similar to other core kernel components (pid, dcache, tcp), by using alloc_large_system_hash() we benefit from its NUMA awareness and thus the table is distributed among the nodes instead of in a single one. For a custom microbenchmark that pounds on the uaddr hashing -- making the wait path fail at futex_wait_setup() returning -EWOULDBLOCK for large amounts of futexes, we can see the following benefits on a 80-core, 8-socket 1Tb server: +---------+--------------------+------------------------+-----------------------+-------------------------------+ | threads | baseline (ops/sec) | aligned-only (ops/sec) | large table (ops/sec) | large table+aligned (ops/sec) | +---------+--------------------+------------------------+-----------------------+-------------------------------+ |     512 |              32426 | 50531  (+55.8%)        | 255274  (+687.2%)     | 292553  (+802.2%)             | |     256 |              65360 | 99588  (+52.3%)        | 443563  (+578.6%)     | 508088  (+677.3%)             | |     128 |             125635 | 200075 (+59.2%)        | 742613  (+491.1%)     | 835452  (+564.9%)             | |      80 |             193559 | 323425 (+67.1%)        | 1028147 (+431.1%)     | 1130304 (+483.9%)             | |      64 |             247667 | 443740 (+79.1%)        | 997300  (+302.6%)     | 1145494 (+362.5%)             | |      32 |             628412 | 721401 (+14.7%)        | 965996  (+53.7%)      | 1122115 (+78.5%)              | +---------+--------------------+------------------------+-----------------------+-------------------------------+ Cc: Ingo Molnar Reviewed-by: Darren Hart Acked-by: Peter Zijlstra Cc: Thomas Gleixner Reviewed-by: Paul E. McKenney Cc: Mike Galbraith Cc: Jeff Mahoney Cc: Linus Torvalds Cc: Scott Norton Cc: Tom Vaden Cc: Aswin Chandramouleeswaran Reviewed-by: Waiman Long Reviewed-and-tested-by: Jason Low Signed-off-by: Davidlohr Bueso --- kernel/futex.c | 26 +++++++++++++++++++------- 1 file changed, 19 insertions(+), 7 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index 085f5fa..577481d 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -63,6 +63,7 @@ #include #include #include +#include #include @@ -70,8 +71,6 @@ 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 +148,11 @@ 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<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)]; } /* @@ -2719,7 +2720,18 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val, static int __init futex_init(void) { u32 curval; - int i; + unsigned long i; + +#if 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 @@ -2734,7 +2746,7 @@ static int __init futex_init(void) if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT) futex_cmpxchg_enabled = 1; - for (i = 0; i < ARRAY_SIZE(futex_queues); i++) { + for (i = 0; i < futex_hashsize; i++) { plist_head_init(&futex_queues[i].chain); spin_lock_init(&futex_queues[i].lock); } -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/