Sometimes, the result of the rhashtable_lookup() is expected to be found.
Therefore, we can use likely() for such cases.

Following new functions are introduced, which will use likely or unlikely
during the lookup:

 rhashtable_lookup_likely
 rhltable_lookup_likely

A micro-benchmark is made for these new functions: lookup a existed entry
repeatedly for 100000000 times, and rhashtable_lookup_likely() gets ~30%
speedup.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
---
v2:
- remove the unlikely, as it's not suitable

This patch base on the patch that I commit before:
  rhashtable: use __always_inline for rhashtable

The new functions that we introduced can be used by other modules, and I'm
not sure if it is a good idea to do it in this series, as they belong to
different tree. So I decide to do it in the target tree after this patch
merged.
---
 include/linux/rhashtable.h | 70 +++++++++++++++++++++++++++++++-------
 1 file changed, 58 insertions(+), 12 deletions(-)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index e740157f3cd7..5b42dcdef23f 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -355,12 +355,25 @@ static inline void rht_unlock(struct bucket_table *tbl,
 	local_irq_restore(flags);
 }
 
-static inline struct rhash_head *__rht_ptr(
-	struct rhash_lock_head *p, struct rhash_lock_head __rcu *const *bkt)
+enum rht_lookup_freq {
+	RHT_LOOKUP_NORMAL,
+	RHT_LOOKUP_LIKELY,
+};
+
+static __always_inline struct rhash_head *__rht_ptr(
+	struct rhash_lock_head *p, struct rhash_lock_head __rcu *const *bkt,
+	const enum rht_lookup_freq freq)
 {
-	return (struct rhash_head *)
-		((unsigned long)p & ~BIT(0) ?:
-		 (unsigned long)RHT_NULLS_MARKER(bkt));
+	unsigned long p_val = (unsigned long)p & ~BIT(0);
+
+	BUILD_BUG_ON(!__builtin_constant_p(freq));
+
+	if (freq == RHT_LOOKUP_LIKELY)
+		return (struct rhash_head *)
+			(likely(p_val) ? p_val : (unsigned long)RHT_NULLS_MARKER(bkt));
+	else
+		return (struct rhash_head *)
+			(p_val ?: (unsigned long)RHT_NULLS_MARKER(bkt));
 }
 
 /*
@@ -370,10 +383,17 @@ static inline struct rhash_head *__rht_ptr(
  *   rht_ptr_exclusive() dereferences in a context where exclusive
  *            access is guaranteed, such as when destroying the table.
  */
+static __always_inline struct rhash_head *__rht_ptr_rcu(
+	struct rhash_lock_head __rcu *const *bkt,
+	const enum rht_lookup_freq freq)
+{
+	return __rht_ptr(rcu_dereference(*bkt), bkt, freq);
+}
+
 static inline struct rhash_head *rht_ptr_rcu(
 	struct rhash_lock_head __rcu *const *bkt)
 {
-	return __rht_ptr(rcu_dereference(*bkt), bkt);
+	return __rht_ptr_rcu(bkt, RHT_LOOKUP_NORMAL);
 }
 
 static inline struct rhash_head *rht_ptr(
@@ -381,13 +401,15 @@ static inline struct rhash_head *rht_ptr(
 	struct bucket_table *tbl,
 	unsigned int hash)
 {
-	return __rht_ptr(rht_dereference_bucket(*bkt, tbl, hash), bkt);
+	return __rht_ptr(rht_dereference_bucket(*bkt, tbl, hash), bkt,
+			 RHT_LOOKUP_NORMAL);
 }
 
 static inline struct rhash_head *rht_ptr_exclusive(
 	struct rhash_lock_head __rcu *const *bkt)
 {
-	return __rht_ptr(rcu_dereference_protected(*bkt, 1), bkt);
+	return __rht_ptr(rcu_dereference_protected(*bkt, 1), bkt,
+			 RHT_LOOKUP_NORMAL);
 }
 
 static inline void rht_assign_locked(struct rhash_lock_head __rcu **bkt,
@@ -588,7 +610,8 @@ static inline int rhashtable_compare(struct rhashtable_compare_arg *arg,
 /* Internal function, do not use. */
 static __always_inline struct rhash_head *__rhashtable_lookup(
 	struct rhashtable *ht, const void *key,
-	const struct rhashtable_params params)
+	const struct rhashtable_params params,
+	const enum rht_lookup_freq freq)
 {
 	struct rhashtable_compare_arg arg = {
 		.ht = ht,
@@ -599,12 +622,13 @@ static __always_inline struct rhash_head *__rhashtable_lookup(
 	struct rhash_head *he;
 	unsigned int hash;
 
+	BUILD_BUG_ON(!__builtin_constant_p(freq));
 	tbl = rht_dereference_rcu(ht->tbl, ht);
 restart:
 	hash = rht_key_hashfn(ht, tbl, key, params);
 	bkt = rht_bucket(tbl, hash);
 	do {
-		rht_for_each_rcu_from(he, rht_ptr_rcu(bkt), tbl, hash) {
+		rht_for_each_rcu_from(he, __rht_ptr_rcu(bkt, freq), tbl, hash) {
 			if (params.obj_cmpfn ?
 			    params.obj_cmpfn(&arg, rht_obj(ht, he)) :
 			    rhashtable_compare(&arg, rht_obj(ht, he)))
@@ -643,11 +667,22 @@ static __always_inline void *rhashtable_lookup(
 	struct rhashtable *ht, const void *key,
 	const struct rhashtable_params params)
 {
-	struct rhash_head *he = __rhashtable_lookup(ht, key, params);
+	struct rhash_head *he = __rhashtable_lookup(ht, key, params,
+						    RHT_LOOKUP_NORMAL);
 
 	return he ? rht_obj(ht, he) : NULL;
 }
 
+static __always_inline void *rhashtable_lookup_likely(
+	struct rhashtable *ht, const void *key,
+	const struct rhashtable_params params)
+{
+	struct rhash_head *he = __rhashtable_lookup(ht, key, params,
+						    RHT_LOOKUP_LIKELY);
+
+	return likely(he) ? rht_obj(ht, he) : NULL;
+}
+
 /**
  * rhashtable_lookup_fast - search hash table, without RCU read lock
  * @ht:		hash table
@@ -693,11 +728,22 @@ static __always_inline struct rhlist_head *rhltable_lookup(
 	struct rhltable *hlt, const void *key,
 	const struct rhashtable_params params)
 {
-	struct rhash_head *he = __rhashtable_lookup(&hlt->ht, key, params);
+	struct rhash_head *he = __rhashtable_lookup(&hlt->ht, key, params,
+						    RHT_LOOKUP_NORMAL);
 
 	return he ? container_of(he, struct rhlist_head, rhead) : NULL;
 }
 
+static __always_inline struct rhlist_head *rhltable_lookup_likely(
+	struct rhltable *hlt, const void *key,
+	const struct rhashtable_params params)
+{
+	struct rhash_head *he = __rhashtable_lookup(&hlt->ht, key, params,
+						    RHT_LOOKUP_LIKELY);
+
+	return likely(he) ? container_of(he, struct rhlist_head, rhead) : NULL;
+}
+
 /* Internal function, please use rhashtable_insert_fast() instead. This
  * function returns the existing element already in hashes if there is a clash,
  * otherwise it returns an error via ERR_PTR().
-- 
2.51.0

On Sat, Oct 11, 2025 at 09:48:55AM +0800, Menglong Dong wrote:
> Sometimes, the result of the rhashtable_lookup() is expected to be found.
> Therefore, we can use likely() for such cases.
> 
> Following new functions are introduced, which will use likely or unlikely
> during the lookup:
> 
>  rhashtable_lookup_likely
>  rhltable_lookup_likely
> 
> A micro-benchmark is made for these new functions: lookup a existed entry
> repeatedly for 100000000 times, and rhashtable_lookup_likely() gets ~30%
> speedup.
> 
> Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
> ---
> v2:
> - remove the unlikely, as it's not suitable
> 
> This patch base on the patch that I commit before:
>   rhashtable: use __always_inline for rhashtable
> 
> The new functions that we introduced can be used by other modules, and I'm
> not sure if it is a good idea to do it in this series, as they belong to
> different tree. So I decide to do it in the target tree after this patch
> merged.
> ---
>  include/linux/rhashtable.h | 70 +++++++++++++++++++++++++++++++-------
>  1 file changed, 58 insertions(+), 12 deletions(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt