Re: Re: Re: [PATCH v4 2/2] selftest/bpf/benchs: Add bpf_map benchmark

[Feng Zhou <zhoufeng.zf@bytedance.com>]

在 2022/6/1 下午7:37, Alexei Starovoitov 写道:
> On Wed, Jun 1, 2022 at 1:17 PM Feng Zhou <zhoufeng.zf@bytedance.com> wrote:
>> 在 2022/6/1 下午5:53, Alexei Starovoitov 写道:
>>> On Wed, Jun 1, 2022 at 10:42 AM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>>>> +struct {
>>>> +       __uint(type, BPF_MAP_TYPE_HASH);
>>>> +       __type(key, u32);
>>>> +       __type(value, u64);
>>>> +       __uint(max_entries, MAX_ENTRIES);
>>>> +} hash_map_bench SEC(".maps");
>>>> +
>>>> +u64 __attribute__((__aligned__(256))) percpu_time[256];
>>> aligned 256 ?
>>> What is the point?
>> I didn't think too much about it here, just referenced it from
>> tools/testing/selftests/bpf/progs/bloom_filter_bench.c
>>
>>>> +u64 nr_loops;
>>>> +
>>>> +static int loop_update_callback(__u32 index, u32 *key)
>>>> +{
>>>> +       u64 init_val = 1;
>>>> +
>>>> +       bpf_map_update_elem(&hash_map_bench, key, &init_val, BPF_ANY);
>>>> +       return 0;
>>>> +}
>>>> +
>>>> +SEC("fentry/" SYS_PREFIX "sys_getpgid")
>>>> +int benchmark(void *ctx)
>>>> +{
>>>> +       u32 key = bpf_get_prandom_u32() % MAX_ENTRIES + MAX_ENTRIES;
>>> What is the point of random ?
>>> just key = MAX_ENTRIES would be the same, no?
>>> or key = -1 ?
>> If all threads on different cpu trigger sys_getpgid and lookup the same
>> key, it will cause
>> "ret = htab_lock_bucket(htab, b, hash, &flags);"
>> the lock competition here is fierce, and unnecessary overhead is
>> introduced,
>> and I don't want it to interfere with the test.
> I see.
> but using random leaves it to chance.
> Use cpu+max_entries then?

Ok, will do. Thanks.