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

[Namhyung Kim <namhyung@kernel.org>]

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, ...

Thanks,
Namhyung