[PATCH] rhashtable: specialize default comparison for constant parameters

[Usama Arif <usama.arif@linux.dev>]

The inline lookup and insert helpers receive the rhashtable parameters by
value. With a static const parameter block, key_offset and key_len are
compile-time constants at the call site.

The default comparison throws that information away by reading both fields
back from ht->p. Fixed-size keys consequently load the parameters and call
bcmp() for every object in the bucket, even when the compiler could use
scalar comparisons instead.

The commit "sched_ext: Specialize the DSQ hashtable compare" [1] resulted
in a 2.9x faster DSQ lookup after replacing this path for its u64 key
with a scalar comparison. Doing that through obj_cmpfn requires every
fixed-key user to provide its own callback.

Pass the call-site parameters to rhashtable_compare(), as
rht_key_get_hash() already does. Use them when key_len is a compile-time
constant. Retain the ht->p path for dynamic parameter blocks. A constant
zero key_len continues to take the runtime length from ht->p.key_len.

This specializes the default comparison for all fixed-size users. With
Clang 22 on x86-64, bcmp() disappears from sched_ext, mac80211, NFSd,
TIPC, NFQUEUE, VFS superblock, pidfs, SysV IPC and hardware-breakpoint
table paths. The affected build_policy.o, sta_info.o and nfsd filecache.o
text shrinks by 512, 400 and 352 bytes respectively. Four- and eight-byte
keys become direct scalar comparisons; six-byte MAC addresses become a
four-byte and a two-byte comparison.

Measure the VFS case with ustat() on a mounted ramfs. This exercises
user_get_super() and its super_dev lookup. Across ten interleaved baseline
and patched VM boot pairs, with five 3 million call samples per boot, the
average per-boot median latency drops from 477.2 to 462.6 ns per call, or
3.0%. All ten pairs improved.

No functional change intended.

[1] https://lore.kernel.org/all/20260921171928.1639407-1-usama.arif@linux.dev/

Suggested-by: Mykola Lysenko <nickolay.lysenko@gmail.com>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 include/linux/rhashtable.h | 20 +++++++++++++++-----
 lib/rhashtable.c           |  5 +++--
 2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index 57a2a29bef0e8..c9872965b753c 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -598,13 +598,23 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
 	for (pos = list; pos && rht_entry(tpos, pos, member);		\
 	     pos = rcu_dereference_all(pos->next))
 
-static inline int rhashtable_compare(struct rhashtable_compare_arg *arg,
-				     const void *obj)
+/*
+ * Use constant params from inlined callers to specialize memcmp(). If params
+ * isn't constant, it must be equal to ht->p.
+ */
+static __always_inline int rhashtable_compare(struct rhashtable_compare_arg *arg,
+					      const void *obj,
+					      const struct rhashtable_params params)
 {
 	struct rhashtable *ht = arg->ht;
 	const char *ptr = obj;
 
-	return memcmp(ptr + ht->p.key_offset, arg->key, ht->p.key_len);
+	if (!__builtin_constant_p(params.key_len))
+		return memcmp(ptr + ht->p.key_offset, arg->key,
+			      ht->p.key_len);
+
+	return memcmp(ptr + params.key_offset, arg->key,
+		      params.key_len ? : ht->p.key_len);
 }
 
 /* Internal function, do not use. */
@@ -632,7 +642,7 @@ static __always_inline struct rhash_head *__rhashtable_lookup(
 		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)))
+			    rhashtable_compare(&arg, rht_obj(ht, he), params))
 				continue;
 			return he;
 		}
@@ -797,7 +807,7 @@ static __always_inline void *__rhashtable_insert_fast(
 		if (!key ||
 		    (params.obj_cmpfn ?
 		     params.obj_cmpfn(&arg, rht_obj(ht, head)) :
-		     rhashtable_compare(&arg, rht_obj(ht, head)))) {
+		     rhashtable_compare(&arg, rht_obj(ht, head), params))) {
 			pprev = &head->next;
 			continue;
 		}
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 6362896e4f099..9d9f03b59c823 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -548,7 +548,7 @@ static void *rhashtable_lookup_one(struct rhashtable *ht,
 		if (!key ||
 		    (ht->p.obj_cmpfn ?
 		     ht->p.obj_cmpfn(&arg, rht_obj(ht, head)) :
-		     rhashtable_compare(&arg, rht_obj(ht, head)))) {
+		     rhashtable_compare(&arg, rht_obj(ht, head), ht->p))) {
 			pprev = &head->next;
 			continue;
 		}
@@ -710,7 +710,8 @@ static struct rhash_head *__rhashtable_next_in_table(
 		rht_for_each_rcu(he, tbl, b) {
 			bool match = params.obj_cmpfn
 				     ? !params.obj_cmpfn(&arg, rht_obj(ht, he))
-				     : !rhashtable_compare(&arg, rht_obj(ht, he));
+				     : !rhashtable_compare(&arg, rht_obj(ht, he),
+						   params);
 			if (found) {
 				if (match)
 					continue;
-- 
2.53.0-Meta