Re: [PATCH RFC] futex: avoid false sharing between hb->chain and the bucket lock

[Thomas Gleixner]

Breno!

On Tue, Jun 09 2026 at 08:28, Breno Leitao wrote:
> On Tue, Jun 09, 2026 at 12:46:03PM +0200, Peter Zijlstra wrote:
>> On Fri, Jun 05, 2026 at 09:53:12AM -0700, Breno Leitao wrote:
>>   perf bench futex hash			 192479  195523  +1.5%
>>   perf bench futex hash -b 256		3453734 3987880 +15.5%
>> 
>> And then I do see the improvement from your patch, but I really cannot
>> make sense of your reasoning for it.
>
> So, let me rephrase it. The bucket cacheline takes hits from four access
> patterns - the three I listed (waiters_pending readers, lock spinners,
> lock-holder chain writes) plus the lockless `fph = hb->priv` load on the
> futex_hash() fast path, which is what c2c surfaced. That priv load is the
> dominant HITM source on baseline, not the chain writes I emphasized. 

Ok. That makes a lot more sense now.

>> > Cost: one extra cacheline (56 B padding) per bucket. Would it be
>> > acceptable?
>> 
>> I'm really not sure, it *doubles* the futex memory cost.
>
> I think it's worth the trade. The global hash scales linearly with
> num_possible_cpus(), so the extra bytes track the same curve as the machines
> that actually need the fix
>
> in simpler words, a box big enough to feel this contention has plenty of RAM
> headroom to absorb it.

Well, it's not only about the global hash. The per process private hash
is affected too.

Can you try the completely untested below?

Thanks,

        tglx
---
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -124,7 +124,7 @@ 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);
 
 #ifdef CONFIG_FUTEX_PRIVATE_HASH
 static bool futex_ref_get(struct futex_private_hash *fph);
@@ -179,22 +179,25 @@ void futex_hash_put(struct futex_hash_bu
 }
 
 static struct futex_hash_bucket *
-__futex_hash_private(union futex_key *key, struct futex_private_hash *fph)
+__futex_hash_private(union futex_key *key, struct futex_private_hash **fph)
 {
+	struct futex_private_hash *lfph = *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)
+	if (!lfph)
+		lfph = rcu_dereference(key->private.mm->futex_phash);
+	if (!lfph || !lfph->hash_mask)
 		return NULL;
 
+	*fph = lfph;
+
 	hash = jhash2((void *)&key->private.address,
 		      sizeof(key->private.address) / 4,
 		      key->both.offset);
-	return &fph->queues[hash & fph->hash_mask];
+	return &lfph->queues[hash & lfph->hash_mask];
 }
 
 static void futex_rehash_private(struct futex_private_hash *old,
@@ -217,7 +220,7 @@ static void futex_rehash_private(struct
 
 			WARN_ON_ONCE(this->lock_ptr != &hb_old->lock);
 
-			hb_new = __futex_hash(&this->key, new);
+			hb_new = __futex_hash(&this->key, &new);
 			futex_hb_waiters_inc(hb_new);
 			/*
 			 * The new pointer isn't published yet but an already
@@ -301,13 +304,12 @@ struct futex_private_hash *futex_private
 
 struct futex_hash_bucket *futex_hash(union futex_key *key)
 {
-	struct futex_private_hash *fph;
+	struct futex_private_hash *fph = NULL;
 	struct futex_hash_bucket *hb;
 
 again:
 	scoped_guard(rcu) {
-		hb = __futex_hash(key, NULL);
-		fph = hb->priv;
+		hb = __futex_hash(key, &fph);
 
 		if (!fph || futex_private_hash_get(fph))
 			return hb;
@@ -319,7 +321,7 @@ struct futex_hash_bucket *futex_hash(uni
 #else /* !CONFIG_FUTEX_PRIVATE_HASH */
 
 static struct futex_hash_bucket *
-__futex_hash_private(union futex_key *key, struct futex_private_hash *fph)
+__futex_hash_private(union futex_key *key, struct futex_private_hash **fph)
 {
 	return NULL;
 }
@@ -412,7 +414,7 @@ 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)
 {
 	int node = key->both.node;
 	u32 hash;