From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5E2153B47D5; Sat, 12 Sep 2026 09:35:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789205743; cv=none; b=VqCK8vxPI89y7KcFwZKITbgktUn1sGDeeU3QVgiDvBLryBPalDIE7ssx2ajymr0+nAEZQM+Vu0+7ZNt5clxmXjDhwis5zvpPQs46O9yK9490oIeEurrJUibIQAsqN63Sms5q2L9PaWO0YsqmZysbkROkxkcaCsNe6rzNziZk698= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789205743; c=relaxed/simple; bh=F7FEYw4CTD3tnW1swQtYRK1JqFgLA97pwVq21NuWkD8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=FHayJgt8luDy+Nrd/KmuJ48cCCWTtChnfza/UQIw1dHq8qtlCmVhQCZFQWsNU5Nv8pztTcev9kaJw3c99QdHhB51LDnppXkjOHPA5RozWVAqzPrtF7OeV1UG7/NhBzhqQ6VTBq+OqWYieIFd6Kb/v1C+2y7c43RbUGPCtfxaNaE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=Pt+flJ1n; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="Pt+flJ1n" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CEE4D1F000FF; Sat, 12 Sep 2026 09:35:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1789205741; bh=4Nfo/nfcEurMmL1jOgnQqueqcJUjS5RrCNWq45CMnew=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Pt+flJ1nx5q42MIo//nUK04Cfxokzgxumfqn6mTmUtCn/fD162rrmVR8qX2qrrT65 /CwxCjWirQbFoWwd7V1xRBs+P7rq3LFvRKeQnqpEX8k7omSSE8x2QojwEzQsROoNMM jFrCpfB9cdqqdGqlfy2mTJNiQPKkTrUFJcp37B3U= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Breno Leitao , "Peter Zijlstra (Intel)" , Thomas Gleixner , Sasha Levin Subject: [PATCH 6.18 0046/1518] futex: Optimize futex hash bucket access patterns Date: Sat, 12 Sep 2026 08:36:55 +0200 Message-ID: <20260912065624.478844865@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260912065623.398859879@linuxfoundation.org> References: <20260912065623.398859879@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Peter Zijlstra [ Upstream commit a734d9fca84e1d4fa0cb442ef5f84c88f8212d32 ] Breno reported significant c2c HITM in a futex hash heavy workload. It turns out that the hash bucket to private hash table reverse pointer (futex_hash_bucket::priv) was to blame. Notably when the hash buckets are heavily contended, the: 'fph = bh->priv;' load in futex_hash() will typically miss and consequently become quite expensive. Since this load in particular is quite superfluous, removing it is fairly straight forward. However, removing it does not in fact achieve anything much. The pain moves to the next user, notably: futex_hash_put(). Therefore rework the whole private hash refcounting to avoid needing this back pointer (and removing it). Instead of passing around 'struct futex_hash_bucket *hb', pass around a new structure that contains it and the related 'struct futex_private_hash *fph' pointer in tandem. Funnily this turns out to remove more code than it adds and significantly improves futex hash performance (as measured by 'perf bench futex hash'): SKL dual socket 112 threads: Baseline Patched shared (16k) 1571857 1641435 + 4.4% autosize (512) 646390 903371 +39.7% -b 256 464395 587014 +26.4% -b 512 715687 995943 +39.2% -b 1024 995085 1396328 +40.3% -b 2048 1293114 1668395 +29.0% -b 4096 2124438 2240228 + 5.5% Zen3 dual socket 256 threads: Baseline Patched shared (16k) 1275840 1381279 + 8.2% autosize (512) 1252745 1482179 +18.3% -b 256 856274 955455 +11.5% -b 512 1267490 1544010 +21.8% -b 1024 1424013 1625424 +14.1% -b 2048 1505181 1669342 +10.9% -b 4096 1465993 1688932 +15.2% AMD EPYC 9D64 (Zen4, single socket) 176 threads: Baseline Patched Delta shared (16k) 1,230,599 1,368,655 +11.2% autosize (1024) 1,285,440 1,556,946 +21.1% -b 256 1,341,471 1,520,303 +13.3% -b 512 1,438,330 1,599,319 +11.2% -b 1024 1,443,772 1,622,493 +12.4% -b 2048 1,472,108 1,643,975 +11.7% -b 4096 1,333,098 1,570,897 +17.8% Reported-by: Breno Leitao Signed-off-by: Peter Zijlstra (Intel) Tested-by: Breno Leitao Tested-by: Thomas Gleixner Link: https://patch.msgid.link/20260610135510.GB1430057@noisy.programming.kicks-ass.net [ Stable adaptation: use mm->futex_phash in __futex_hash(), since this branch does not contain the mm_struct futex field consolidation. Preserve the final-put use-after-free fix by saving fph->mm before futex_ref_put(), and restore the NULL guard needed by the direct private-hash put callers. futex_key_is_private() is already in futex.h; no new functions are introduced. Keep the bucket-reference conversion so 912edebe8501 applies without changes. ] Stable-dep-of: 912edebe8501 ("futex: Provide rt_mutex_.*_schedule() equivalents for futex scheduling") Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman --- kernel/futex/core.c | 103 +++++++++++++++--------------------------------- kernel/futex/futex.h | 28 ++++++------- kernel/futex/pi.c | 21 +++++---- kernel/futex/requeue.c | 20 ++++----- kernel/futex/waitwake.c | 17 +++++-- 5 files changed, 80 insertions(+), 109 deletions(-) --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -125,7 +125,7 @@ late_initcall(fail_futex_debugfs); #endif /* CONFIG_FAIL_FUTEX */ static struct futex_hash_bucket * -__futex_hash(union futex_key *key, struct futex_private_hash *fph); +__futex_hash(union futex_key *key, struct futex_private_hash *fph, struct futex_private_hash **fph_p); #ifdef CONFIG_FUTEX_PRIVATE_HASH static bool futex_ref_get(struct futex_private_hash *fph); @@ -141,35 +141,14 @@ static bool futex_private_hash_get(struc void futex_private_hash_put(struct futex_private_hash *fph) { - struct mm_struct *mm = fph->mm; - - if (futex_ref_put(fph)) - wake_up_var(mm); -} - -/** - * futex_hash_get - Get an additional reference for the local hash. - * @hb: ptr to the private local hash. - * - * Obtain an additional reference for the already obtained hash bucket. The - * caller must already own an reference. - */ -void futex_hash_get(struct futex_hash_bucket *hb) -{ - struct futex_private_hash *fph = hb->priv; + struct mm_struct *mm; if (!fph) return; - WARN_ON_ONCE(!futex_private_hash_get(fph)); -} -void futex_hash_put(struct futex_hash_bucket *hb) -{ - struct futex_private_hash *fph = hb->priv; - - if (!fph) - return; - futex_private_hash_put(fph); + mm = fph->mm; + if (futex_ref_put(fph)) + wake_up_var(mm); } static struct futex_hash_bucket * @@ -177,14 +156,6 @@ __futex_hash_private(union futex_key *ke { u32 hash; - if (!futex_key_is_private(key)) - return NULL; - - if (!fph) - fph = rcu_dereference(key->private.mm->futex_phash); - if (!fph || !fph->hash_mask) - return NULL; - hash = jhash2((void *)&key->private.address, sizeof(key->private.address) / 4, key->both.offset); @@ -205,13 +176,12 @@ static void futex_rehash_private(struct spin_lock(&hb_old->lock); plist_for_each_entry_safe(this, tmp, &hb_old->chain, list) { - plist_del(&this->list, &hb_old->chain); futex_hb_waiters_dec(hb_old); WARN_ON_ONCE(this->lock_ptr != &hb_old->lock); - hb_new = __futex_hash(&this->key, new); + hb_new = __futex_hash(&this->key, new, NULL); futex_hb_waiters_inc(hb_new); /* * The new pointer isn't published yet but an already @@ -265,9 +235,8 @@ static void futex_pivot_hash(struct mm_s } } -struct futex_private_hash *futex_private_hash(void) +struct futex_private_hash *futex_private_hash(struct mm_struct *mm) { - struct mm_struct *mm = current->mm; /* * Ideally we don't loop. If there is a replacement in progress * then a new private hash is already prepared and a reference can't be @@ -293,18 +262,17 @@ again: goto again; } -struct futex_hash_bucket *futex_hash(union futex_key *key) +struct futex_bucket_ref futex_hash(union futex_key *key) { - struct futex_private_hash *fph; - struct futex_hash_bucket *hb; - again: scoped_guard(rcu) { - hb = __futex_hash(key, NULL); - fph = hb->priv; + struct futex_private_hash *fph = NULL; + struct futex_hash_bucket *hb; + + hb = __futex_hash(key, NULL, &fph); if (!fph || futex_private_hash_get(fph)) - return hb; + return (struct futex_bucket_ref){ .hb = hb, .fph = fph }; } futex_pivot_hash(key->private.mm); goto again; @@ -312,15 +280,9 @@ again: #else /* !CONFIG_FUTEX_PRIVATE_HASH */ -static struct futex_hash_bucket * -__futex_hash_private(union futex_key *key, struct futex_private_hash *fph) -{ - return NULL; -} - -struct futex_hash_bucket *futex_hash(union futex_key *key) +struct futex_bucket_ref futex_hash(union futex_key *key) { - return __futex_hash(key, NULL); + return (struct futex_bucket_ref){ .hb = __futex_hash(key, NULL, NULL), .fph = NULL }; } #endif /* CONFIG_FUTEX_PRIVATE_HASH */ @@ -398,6 +360,8 @@ static int futex_mpol(struct mm_struct * * __futex_hash - Return the hash bucket * @key: Pointer to the futex key for which the hash is calculated * @fph: Pointer to private hash if known + * @fph_p: Pointer to a private hash pointer; output for the private hash + * used when set. * * We hash on the keys returned from get_futex_key (see below) and return the * corresponding hash bucket. @@ -406,18 +370,22 @@ static int futex_mpol(struct mm_struct * * global hash is returned. */ static struct futex_hash_bucket * -__futex_hash(union futex_key *key, struct futex_private_hash *fph) +__futex_hash(union futex_key *key, struct futex_private_hash *fph, struct futex_private_hash **fph_p) { int node = key->both.node; u32 hash; - if (node == FUTEX_NO_NODE) { - struct futex_hash_bucket *hb; - - hb = __futex_hash_private(key, fph); - if (hb) - return hb; +#ifdef CONFIG_FUTEX_PRIVATE_HASH + if (node == FUTEX_NO_NODE && futex_key_is_private(key)) { + if (!fph) + fph = rcu_dereference(key->private.mm->futex_phash); + if (fph && fph->hash_mask) { + if (fph_p) + *fph_p = fph; + return __futex_hash_private(key, fph); + } } +#endif hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / sizeof(u32), @@ -1362,7 +1330,7 @@ static void exit_pi_state_list(struct ta * on the mutex. */ WARN_ON(curr != current); - guard(private_hash)(); + guard(private_hash)(current->mm); /* * We are a ZOMBIE and nobody can enqueue itself on * pi_state_list anymore, but we have to be careful @@ -1374,7 +1342,8 @@ static void exit_pi_state_list(struct ta pi_state = list_entry(next, struct futex_pi_state, list); key = pi_state->key; if (1) { - CLASS(hb, hb)(&key); + CLASS(hbr, hbr)(&key); + auto hb = hbr.hb; /* * We can race against put_pi_state() removing itself from the @@ -1556,12 +1525,8 @@ void futex_exec_done(struct task_struct tsk->futex_state = FUTEX_STATE_OK; } -static void futex_hash_bucket_init(struct futex_hash_bucket *fhb, - struct futex_private_hash *fph) +static void futex_hash_bucket_init(struct futex_hash_bucket *fhb) { -#ifdef CONFIG_FUTEX_PRIVATE_HASH - fhb->priv = fph; -#endif atomic_set(&fhb->waiters, 0); plist_head_init(&fhb->chain); spin_lock_init(&fhb->lock); @@ -1866,7 +1831,7 @@ static int futex_hash_allocate(unsigned fph->mm = mm; for (i = 0; i < hash_slots; i++) - futex_hash_bucket_init(&fph->queues[i], fph); + futex_hash_bucket_init(&fph->queues[i]); if (custom) { struct wait_bit_queue_entry __wbq_entry; @@ -2069,7 +2034,7 @@ static int __init futex_init(void) BUG_ON(!table); for (i = 0; i < hashsize; i++) - futex_hash_bucket_init(&table[i], NULL); + futex_hash_bucket_init(&table[i]); futex_queues[n] = table; } --- a/kernel/futex/futex.h +++ b/kernel/futex/futex.h @@ -144,7 +144,6 @@ struct futex_hash_bucket { atomic_t waiters; spinlock_t lock; struct plist_head chain; - struct futex_private_hash *priv; } ____cacheline_aligned_in_smp; /* @@ -184,7 +183,7 @@ typedef void (futex_wake_fn)(struct wake * @requeue_pi_key: the requeue_pi target futex key * @bitset: bitset for the optional bitmasked wakeup * @requeue_state: State field for futex_requeue_pi() - * @drop_hb_ref: Waiter should drop the extra hash bucket reference if true + * @drop_fph: Waiter should drop the extra private hash reference when set * @requeue_wait: RCU wait for futex_requeue_pi() (RT only) * * We use this hashed waitqueue, instead of a normal wait_queue_entry_t, so @@ -211,7 +210,7 @@ struct futex_q { union futex_key *requeue_pi_key; u32 bitset; atomic_t requeue_state; - bool drop_hb_ref; + struct futex_private_hash *drop_fph; #ifdef CONFIG_PREEMPT_RT struct rcuwait requeue_wait; #endif @@ -231,28 +230,29 @@ extern struct hrtimer_sleeper * futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout, int flags, u64 range_ns); -extern struct futex_hash_bucket *futex_hash(union futex_key *key); -#ifdef CONFIG_FUTEX_PRIVATE_HASH -extern void futex_hash_get(struct futex_hash_bucket *hb); -extern void futex_hash_put(struct futex_hash_bucket *hb); +struct futex_bucket_ref { + struct futex_hash_bucket *hb; + struct futex_private_hash *fph; +}; -extern struct futex_private_hash *futex_private_hash(void); +#ifdef CONFIG_FUTEX_PRIVATE_HASH +extern struct futex_private_hash *futex_private_hash(struct mm_struct *mm); extern void futex_private_hash_put(struct futex_private_hash *fph); #else /* !CONFIG_FUTEX_PRIVATE_HASH */ -static inline void futex_hash_get(struct futex_hash_bucket *hb) { } -static inline void futex_hash_put(struct futex_hash_bucket *hb) { } -static inline struct futex_private_hash *futex_private_hash(void) { return NULL; } +static inline struct futex_private_hash *futex_private_hash(struct mm_struct *mm) { return NULL; } static inline void futex_private_hash_put(struct futex_private_hash *fph) { } #endif -DEFINE_CLASS(hb, struct futex_hash_bucket *, - if (_T) futex_hash_put(_T), +extern struct futex_bucket_ref futex_hash(union futex_key *key); + +DEFINE_CLASS(hbr, struct futex_bucket_ref, + if (_T.fph) futex_private_hash_put(_T.fph), futex_hash(key), union futex_key *key); DEFINE_CLASS(private_hash, struct futex_private_hash *, if (_T) futex_private_hash_put(_T), - futex_private_hash(), void); + futex_private_hash(mm), struct mm_struct *mm); /** * futex_match - Check whether two futex keys are equal --- a/kernel/futex/pi.c +++ b/kernel/futex/pi.c @@ -1005,7 +1005,8 @@ retry: retry_private: if (1) { - CLASS(hb, hb)(&q.key); + CLASS(hbr, hbr)(&q.key); + auto hb = hbr.hb; futex_q_lock(&q, hb); @@ -1068,7 +1069,7 @@ retry_private: * the thread, performing resize, will block on hb->lock during * the requeue. */ - futex_hash_put(no_free_ptr(hb)); + futex_private_hash_put(no_free_ptr(hbr.fph)); /* * Must be done before we enqueue the waiter, here is unfortunately * under the hb lock, but that *should* work because it does nothing. @@ -1158,11 +1159,9 @@ no_block: futex_unqueue_pi(&q); spin_unlock(q.lock_ptr); - if (q.drop_hb_ref) { - CLASS(hb, hb)(&q.key); - /* Additional reference from futex_unlock_pi() */ - futex_hash_put(hb); - } + + /* Additional reference from futex_unlock_pi() */ + futex_private_hash_put(q.drop_fph); goto out; out_unlock_put_key: @@ -1218,7 +1217,8 @@ retry: if (ret) return ret; - CLASS(hb, hb)(&key); + CLASS(hbr, hbr)(&key); + auto hb = hbr.hb; spin_lock(&hb->lock); retry_hb: @@ -1275,8 +1275,9 @@ retry_hb: * Acquire a reference for the leaving waiter to ensure * valid futex_q::lock_ptr. */ - futex_hash_get(hb); - top_waiter->drop_hb_ref = true; + if (futex_key_is_private(&key)) + top_waiter->drop_fph = futex_private_hash(key.private.mm); + __futex_unqueue(top_waiter); raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); goto retry_hb; --- a/kernel/futex/requeue.c +++ b/kernel/futex/requeue.c @@ -249,8 +249,8 @@ void requeue_pi_wake_futex(struct futex_ * Acquire a reference for the waiter to ensure valid * futex_q::lock_ptr. */ - futex_hash_get(hb); - q->drop_hb_ref = true; + if (futex_key_is_private(key)) + q->drop_fph = futex_private_hash(key->private.mm); q->lock_ptr = &hb->lock; task = READ_ONCE(q->task); @@ -467,8 +467,10 @@ retry: retry_private: if (1) { - CLASS(hb, hb1)(&key1); - CLASS(hb, hb2)(&key2); + CLASS(hbr, hbr1)(&key1); + CLASS(hbr, hbr2)(&key2); + auto hb1 = hbr1.hb; + auto hb2 = hbr2.hb; futex_hb_waiters_inc(hb2); double_lock_hb(hb1, hb2); @@ -840,7 +842,8 @@ int futex_wait_requeue_pi(u32 __user *ua switch (futex_requeue_pi_wakeup_sync(&q)) { case Q_REQUEUE_PI_IGNORE: { - CLASS(hb, hb)(&q.key); + CLASS(hbr, hbr)(&q.key); + auto hb = hbr.hb; /* The waiter is still on uaddr1 */ spin_lock(&hb->lock); ret = handle_early_requeue_pi_wakeup(hb, &q, to); @@ -910,11 +913,8 @@ int futex_wait_requeue_pi(u32 __user *ua default: BUG(); } - if (q.drop_hb_ref) { - CLASS(hb, hb)(&q.key); - /* Additional reference from requeue_pi_wake_futex() */ - futex_hash_put(hb); - } + /* Additional reference from requeue_pi_wake_futex() */ + futex_private_hash_put(q.drop_fph); out: if (to) { --- a/kernel/futex/waitwake.c +++ b/kernel/futex/waitwake.c @@ -169,7 +169,8 @@ int futex_wake(u32 __user *uaddr, unsign if ((flags & FLAGS_STRICT) && !nr_wake) return 0; - CLASS(hb, hb)(&key); + CLASS(hbr, hbr)(&key); + auto hb = hbr.hb; /* Make sure we really have tasks to wakeup */ if (!futex_hb_waiters_pending(hb)) @@ -266,8 +267,10 @@ retry: retry_private: if (1) { - CLASS(hb, hb1)(&key1); - CLASS(hb, hb2)(&key2); + CLASS(hbr, hbr1)(&key1); + CLASS(hbr, hbr2)(&key2); + auto hb1 = hbr1.hb; + auto hb2 = hbr2.hb; double_lock_hb(hb1, hb2); op_ret = futex_atomic_op_inuser(op, uaddr2); @@ -409,7 +412,7 @@ int futex_wait_multiple_setup(struct fut * Make sure to have a reference on the private_hash such that we * don't block on rehash after changing the task state below. */ - guard(private_hash)(); + guard(private_hash)(current->mm); /* * Enqueuing multiple futexes is tricky, because we need to enqueue @@ -446,7 +449,8 @@ retry: u32 val = vs[i].w.val; if (1) { - CLASS(hb, hb)(&q->key); + CLASS(hbr, hbr)(&q->key); + auto hb = hbr.hb; futex_q_lock(q, hb); ret = futex_get_value_locked(&uval, uaddr); @@ -620,7 +624,8 @@ retry: retry_private: if (1) { - CLASS(hb, hb)(&q->key); + CLASS(hbr, hbr)(&q->key); + auto hb = hbr.hb; futex_q_lock(q, hb);