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Tue, 22 Sep 2026 00:21:04 -0700 (PDT) Received: from mail.gmail.com ([2a04:ee41:4:b2de:1ac0:4dff:fe0f:3782]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49fdad53b47sm27942955e9.13.2026.09.22.00.21.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Sep 2026 00:21:03 -0700 (PDT) Date: Tue, 22 Sep 2026 07:31:33 +0000 From: Anton Protopopov To: Mykyta Yatsenko Cc: bpf@vger.kernel.org, ast@kernel.org, andrii@kernel.org, daniel@iogearbox.net, kernel-team@meta.com, eddyz87@gmail.com, memxor@gmail.com, Mykyta Yatsenko Subject: Re: [PATCH bpf-next v2] bpf: Speed up htab lookups for u32/u64 keys Message-ID: References: <20260921-hashtab_fast_hashfn-v2-1-79aa962ec56b@meta.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260921-hashtab_fast_hashfn-v2-1-79aa962ec56b@meta.com> On 26/09/21 02:28PM, Mykyta Yatsenko wrote: > From: Mykyta Yatsenko > > Four- and eight-byte keys are common enough to warrant specialized htab > lookups. Fold JHASH_INITVAL and the key length into hashrnd once when > allocating maps with these key sizes. > > For JITed BPF_MAP_TYPE_HASH lookups, htab_map_gen_lookup() also knows the > map's fixed key size. Select u32- and u64-specific lookup entry points so > the compiler can specialize both hash setup and key comparison. Other key > sizes continue to use the generic lookup entry point. > > This preserves the jhash2() result and lookup semantics while avoiding > generic length handling and repeated state setup. > > Measure this with six runs of: > > ./bench -w3 -d10 -a bpf-hashmap-lookup \ > --key_size KEY_SIZE --max_entries MAX_ENTRIES \ > --nr_entries NR_ENTRIES --nr_loops NR_LOOPS \ > --map_flags 0x40 > > inside a vng guest with two vCPUs and 4 GiB of RAM. Run the baseline > first and the optimized kernel second. Use these workloads: > > max_entries nr_entries nr_loops > small 512 256 8,388,608 > medium 10,000 5,000 8,000,000 > large 100,000 50,000 8,000,000 > > Mean throughput in million lookups per second is: > > Baseline: > key size (bytes) > 1 4 8 10 > small 89.45 91.21 103.40 91.74 > medium 125.97 80.25 91.94 78.52 > large 140.67 53.56 58.68 47.31 > > Optimized: > key size (bytes) > 1 4 8 10 > small 86.95 233.63 168.65 90.88 > medium 124.97 150.20 137.76 77.51 > large 131.06 80.51 76.04 47.25 > > Change: > key size (bytes) > 1 4 8 10 > small -2.79% +156.15% +63.11% -0.94% > medium -0.79% +87.17% +49.83% -1.28% > large -6.84% +50.32% +29.58% -0.12% > > The u32 specialization improves lookup throughput by 50-156%, and the > u64 specialization improves it by 30-63%. Ten-byte controls remain > within a 1.3% slowdown. One-byte controls range from -6.8% to -0.8%. > > Signed-off-by: Mykyta Yatsenko > Acked-by: Anton Protopopov > --- > Changes in v2: > - Added variants of __htab_map_lookup_elem() functions with inlined > u32/u64 key sizes, use them in htab_map_gen_lookup(); > improved lookup perf significantly (Alexei) > - Link to v1: https://patch.msgid.link/20260921-hashtab_fast_hashfn-v1-1-f92ae72cefcd@meta.com > --- > kernel/bpf/hashtab.c | 49 ++++++++++++++++++++++++++++++++++++++++++------- > 1 file changed, 42 insertions(+), 7 deletions(-) > > diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c > index 6f331c80130d..77c96105eda8 100644 > --- a/kernel/bpf/hashtab.c > +++ b/kernel/bpf/hashtab.c > @@ -612,6 +612,13 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr) > htab->hashrnd = 0; > else > htab->hashrnd = get_random_u32(); > + /* > + * Fold the jhash constant and key length into hashrnd once for the > + * fixed-size fast paths instead of doing it on every lookup. > + */ > + if (htab->map.key_size == sizeof(u32) || > + htab->map.key_size == sizeof(u64)) > + htab->hashrnd += JHASH_INITVAL + htab->map.key_size; > > htab_init_buckets(htab); > > @@ -679,9 +686,19 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr) > > static inline u32 htab_map_hash(const void *key, u32 key_len, u32 hashrnd) > { > - if (likely(key_len % 4 == 0)) > + const u32 *k = key; > + u32 b; > + > + if (key_len == sizeof(u32)) > + b = 0; > + else if (key_len == sizeof(u64)) > + b = k[1]; > + else if (likely(key_len % 4 == 0)) > return jhash2(key, key_len / 4, hashrnd); > - return jhash(key, key_len, hashrnd); > + else > + return jhash(key, key_len, hashrnd); > + > + return __jhash_nwords(k[0], b, 0, hashrnd); > > static inline struct bucket *__select_bucket(struct bpf_htab *htab, u32 hash) > @@ -735,17 +752,15 @@ static struct htab_elem *lookup_nulls_elem_raw(struct hlist_nulls_head *head, > * The return value is adjusted by BPF instructions > * in htab_map_gen_lookup(). > */ > -static void *__htab_map_lookup_elem(struct bpf_map *map, void *key) > +static __always_inline void *__htab_lookup(struct bpf_map *map, void *key, u32 key_size) > { > struct bpf_htab *htab = container_of(map, struct bpf_htab, map); > struct hlist_nulls_head *head; > struct htab_elem *l; > - u32 hash, key_size; > + u32 hash; > > WARN_ON_ONCE(!bpf_rcu_lock_held()); > > - key_size = map->key_size; > - > hash = htab_map_hash(key, key_size, htab->hashrnd); > > head = select_bucket(htab, hash); > @@ -755,6 +770,21 @@ static void *__htab_map_lookup_elem(struct bpf_map *map, void *key) > return l; > } > > +static void *__htab_map_lookup_elem(struct bpf_map *map, void *key) > +{ > + return __htab_lookup(map, key, map->key_size); > +} > + > +static void *__htab_map_lookup_elem_u32(struct bpf_map *map, void *key) > +{ > + return __htab_lookup(map, key, sizeof(u32)); > +} > + > +static void *__htab_map_lookup_elem_u64(struct bpf_map *map, void *key) > +{ > + return __htab_lookup(map, key, sizeof(u64)); > +} > + > static void *htab_map_lookup_elem(struct bpf_map *map, void *key) > { > struct htab_elem *l = __htab_map_lookup_elem(map, key); > @@ -783,7 +813,12 @@ static int htab_map_gen_lookup(struct bpf_map *map, struct bpf_insn *insn_buf) > > BUILD_BUG_ON(!__same_type(&__htab_map_lookup_elem, > (void *(*)(struct bpf_map *map, void *key))NULL)); > - *insn++ = BPF_EMIT_CALL(__htab_map_lookup_elem); > + if (map->key_size == sizeof(u32)) > + *insn++ = BPF_EMIT_CALL(__htab_map_lookup_elem_u32); > + else if (map->key_size == sizeof(u64)) > + *insn++ = BPF_EMIT_CALL(__htab_map_lookup_elem_u64); > + else > + *insn++ = BPF_EMIT_CALL(__htab_map_lookup_elem); I am trying to remember why I didn't do this when I was touching this code, I was definitely thinking about it... Given your changes, it is worth to approach xxh3 again, it works way better for bigger key sizes, see [1]. [1] https://archive.fosdem.org/2023/schedule/event/bpf_hashing/ > *insn++ = BPF_JMP_IMM(BPF_JEQ, ret, 0, 1); > *insn++ = BPF_ALU64_IMM(BPF_ADD, ret, > offsetof(struct htab_elem, key) + > > --- > base-commit: 6e1ac8758fcd07bee12ccaff0087cc66930a4187 > change-id: 20260916-hashtab_fast_hashfn-9e52b3c2276e > > Best regards, > -- > Mykyta Yatsenko >