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

    >
    >> + u32 cpu = bpf_get_smp_processor_id();
    >> + u64 start_time = bpf_ktime_get_ns();
    >> +
    >> + bpf_loop(nr_loops, loop_update_callback, &key, 0);
    >> + percpu_time[cpu & 255] = bpf_ktime_get_ns() - start_time;
    >> + return 0;
    >> +}
    >> --
    >> 2.20.1
    >>

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