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Usama Arif
8c515ff875
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> Acked-by: Tejun Heo <tj@kernel.org> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>