Re: [PATCH 3/3] perf stat: Enable BPF counter with --for-each-cgroup

Song Liu
2021-06-24T21:41:23Z

> On Jun 24, 2021, at 2:01 PM, Namhyung Kim <namhyung@kernel.org> wrote:
>
> On Thu, Jun 24, 2021 at 9:20 AM Song Liu <songliubraving@fb.com> wrote:
>>>>> +
>>>>> +// single set of global perf events to measure
>>>>> +struct {
>>>>> + __uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
>>>>> + __uint(key_size, sizeof(__u32));
>>>>> + __uint(value_size, sizeof(int));
>>>>> + __uint(max_entries, 1);
>>>>> +} events SEC(".maps");
>>>>> +
>>>>> +// from logical cpu number to event index
>>>>> +// useful when user wants to count subset of cpus
>>>>> +struct {
>>>>> + __uint(type, BPF_MAP_TYPE_HASH);
>>>>> + __uint(key_size, sizeof(__u32));
>>>>> + __uint(value_size, sizeof(__u32));
>>>>> + __uint(max_entries, 1);
>>>>> +} cpu_idx SEC(".maps");
>>>>
>>>> How about we make cpu_idx a percpu array and use 0,1 for
>>>> disable/enable profiling on this cpu?
>>>
>>> No, it's to calculate an index to the cgrp_readings map which
>>> has the event x cpu x cgroup number of elements.
>>>
>>> It controls enabling events with a global (bss) variable.
>>
>> If we make cgrp_idx a per cpu array, we probably don't need the
>> cpu_idx map?
>
> Right.
>
>>
>>>
>>>>
>>>>> +
>>>>> +// from cgroup id to event index
>>>>> +struct {
>>>>> + __uint(type, BPF_MAP_TYPE_HASH);
>>>>> + __uint(key_size, sizeof(__u64));
>>>>> + __uint(value_size, sizeof(__u32));
>>>>> + __uint(max_entries, 1);
>>>>> +} cgrp_idx SEC(".maps");
>>>>> +
>>>>> +// per-cpu event snapshots to calculate delta
>>>>> +struct {
>>>>> + __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
>>>>> + __uint(key_size, sizeof(__u32));
>>>>> + __uint(value_size, sizeof(struct bpf_perf_event_value));
>>>>> +} prev_readings SEC(".maps");
>>>>> +
>>>>> +// aggregated event values for each cgroup
>>>>> +// will be read from the user-space
>>>>> +struct {
>>>>> + __uint(type, BPF_MAP_TYPE_ARRAY);
>>>>> + __uint(key_size, sizeof(__u32));
>>>>> + __uint(value_size, sizeof(struct bpf_perf_event_value));
>>>>> +} cgrp_readings SEC(".maps");
>>>>
>>>> Maybe also make this a percpu array? This should make the BPF program
>>>> faster.
>>>
>>> Maybe. But I don't know how to access the elements
>>> in a per-cpu map from userspace.
>>
>> Please refer to bperf__read() reading accum_readings. Basically, we read
>> one index of all CPUs with one bpf_map_lookup_elem().
>
> Thanks! So when I use a per-cpu array with 3 elements, I can access
> to cpu/elem entries in a row like below, right?
>
> 0/0, 0/1, 0/2, 1/0, 1/1, 1/2, 2/0, 2/1, 2/2, 3/0, ...

I am not sure I am following here.

Say the system have 10 cpus, and the array has 3 elements. We can do:

__u32 values[10]; /* assuming both key and value are __u32 */
__u32 elem;
int cpu;

for (elem = 0; elem < 3; elem++) {
bpf_map_lookup_elem(map_fd, &elem, values);
for (cpu = 0; cpu < 10; cpu++)
values[cpu] /* this is the value for cpu/elem */
}

Thanks,
Song