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

[Alexei Starovoitov]

On Wed, Jun 1, 2022 at 1:17 PM Feng Zhou <zhoufeng.zf@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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?