[PATCH v2 bpf-next] bpf: optimize hashmap lookups when key_size is divisible by 4

[Anton Protopopov <aspsk@isovalent.com>]

The BPF hashmap uses the jhash() hash function. There is an optimized version
of this hash function which may be used if hash size is a multiple of 4. Apply
this optimization to the hashmap in a similar way as it is done in the bloom
filter map.

On practice the optimization is only noticeable for smaller key sizes, which,
however, is sufficient for many applications. An example is listed in the
following table of measurements (a hashmap of 65536 elements was used):

    --------------------------------------------------------------------
    | key_size | fullness | lookups /sec | lookups (opt) /sec |   gain |
    --------------------------------------------------------------------
    |        4 |      25% |      42.990M |            46.000M |   7.0% |
    |        4 |      50% |      37.910M |            39.094M |   3.1% |
    |        4 |      75% |      34.486M |            36.124M |   4.7% |
    |        4 |     100% |      31.760M |            32.719M |   3.0% |
    --------------------------------------------------------------------
    |        8 |      25% |      43.855M |            49.626M |  13.2% |
    |        8 |      50% |      38.328M |            42.152M |  10.0% |
    |        8 |      75% |      34.483M |            38.088M |  10.5% |
    |        8 |     100% |      31.306M |            34.686M |  10.8% |
    --------------------------------------------------------------------
    |       12 |      25% |      38.398M |            43.770M |  14.0% |
    |       12 |      50% |      33.336M |            37.712M |  13.1% |
    |       12 |      75% |      29.917M |            34.440M |  15.1% |
    |       12 |     100% |      27.322M |            30.480M |  11.6% |
    --------------------------------------------------------------------
    |       16 |      25% |      41.491M |            41.921M |   1.0% |
    |       16 |      50% |      36.206M |            36.474M |   0.7% |
    |       16 |      75% |      32.529M |            33.027M |   1.5% |
    |       16 |     100% |      29.581M |            30.325M |   2.5% |
    --------------------------------------------------------------------
    |       20 |      25% |      34.240M |            36.787M |   7.4% |
    |       20 |      50% |      30.328M |            32.663M |   7.7% |
    |       20 |      75% |      27.536M |            29.354M |   6.6% |
    |       20 |     100% |      24.847M |            26.505M |   6.7% |
    --------------------------------------------------------------------
    |       24 |      25% |      36.329M |            40.608M |  11.8% |
    |       24 |      50% |      31.444M |            35.059M |  11.5% |
    |       24 |      75% |      28.426M |            31.452M |  10.6% |
    |       24 |     100% |      26.278M |            28.741M |   9.4% |
    --------------------------------------------------------------------
    |       28 |      25% |      31.540M |            31.944M |   1.3% |
    |       28 |      50% |      27.739M |            28.063M |   1.2% |
    |       28 |      75% |      24.993M |            25.814M |   3.3% |
    |       28 |     100% |      23.513M |            23.500M |  -0.1% |
    --------------------------------------------------------------------
    |       32 |      25% |      32.116M |            33.953M |   5.7% |
    |       32 |      50% |      28.879M |            29.859M |   3.4% |
    |       32 |      75% |      26.227M |            26.948M |   2.7% |
    |       32 |     100% |      23.829M |            24.613M |   3.3% |
    --------------------------------------------------------------------
    |       64 |      25% |      22.535M |            22.554M |   0.1% |
    |       64 |      50% |      20.471M |            20.675M |   1.0% |
    |       64 |      75% |      19.077M |            19.146M |   0.4% |
    |       64 |     100% |      17.710M |            18.131M |   2.4% |
    --------------------------------------------------------------------

The following script was used to gather the results (SMT & frequency off):

    cd tools/testing/selftests/bpf
    for key_size in 4 8 12 16 20 24 28 32 64; do
            for nr_entries in `seq 16384 16384 65536`; do
                    fullness=$(printf '%3s' $((nr_entries*100/65536)))
                    echo -n "key_size=$key_size: $fullness% full: "
                    sudo ./bench -d2 -a bpf-hashmap-lookup --key_size=$key_size --nr_entries=$nr_entries --max_entries=65536 --nr_loops=2000000 --map_flags=0x40 | grep cpu
            done
            echo
    done

Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
---

v1->v2:
  - simplify/optimize code by just testing the (key_len%4 == 0) in hot path (Alexei)

 kernel/bpf/hashtab.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 96b645bba3a4..00c253b84bf5 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -607,6 +607,8 @@ 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))
+		return jhash2(key, key_len / 4, hashrnd);
 	return jhash(key, key_len, hashrnd);
 }
 
-- 
2.34.1