[PATCH 6.18.y 2/3] futex: Optimize futex hash bucket access patterns

[Sasha Levin <sashal@kernel.org>]

From: Peter Zijlstra <peterz@infradead.org>

[ 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 <leitao@debian.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Breno Leitao <leitao@debian.org>
Tested-by: Thomas Gleixner <tglx@kernel.org>
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 <sashal@kernel.org>
---
 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(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 0ecbd1ad5fcd3..69f340cddfd1f 100644
--- 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(struct futex_private_hash *fph)
 
 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 *key, struct futex_private_hash *fph)
 {
 	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 futex_private_hash *old,
 
 		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_struct *mm)
 	}
 }
 
-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 @@ struct futex_private_hash *futex_private_hash(void)
 	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 @@ struct futex_hash_bucket *futex_hash(union futex_key *key)
 
 #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 *mm, unsigned long addr)
  * __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 *mm, unsigned long addr)
  * 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 task_struct *curr)
 	 * 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 task_struct *curr)
 		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)
 	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 int hash_slots, unsigned int flags)
 	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;
 	}
diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index d126365a36ca6..17893fe1ce400 100644
--- 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_q_head *wake_q, struct futex_q *q);
  * @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
diff --git a/kernel/futex/pi.c b/kernel/futex/pi.c
index 7110453848be1..f1b4f3e3e805e 100644
--- a/kernel/futex/pi.c
+++ b/kernel/futex/pi.c
@@ -1005,7 +1005,8 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
 
 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 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
 		 * 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 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
 
 		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 @@ int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
 	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 @@ int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
 			 * 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;
diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c
index b597cb3d17fc1..79823ad136830 100644
--- a/kernel/futex/requeue.c
+++ b/kernel/futex/requeue.c
@@ -241,8 +241,8 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
 	 * 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);
 
@@ -459,8 +459,10 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 
 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);
@@ -832,7 +834,8 @@ int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
 	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);
@@ -902,11 +905,8 @@ int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
 	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) {
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index e2bbe5509ec27..67eeaabbeb3d9 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -169,7 +169,8 @@ int futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
 	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 @@ int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
 
 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 futex_vector *vs, int count, int *woken)
 	 * 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 @@ int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *woken)
 		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 @@ int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
 
 retry_private:
 	if (1) {
-		CLASS(hb, hb)(&q->key);
+		CLASS(hbr, hbr)(&q->key);
+		auto hb = hbr.hb;
 
 		futex_q_lock(q, hb);
 
-- 
2.53.0