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
>>