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

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Jun 24, 2021 at 3:16 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Jun 24, 2021, at 3:06 PM, Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Thu, Jun 24, 2021 at 2:41 PM Song Liu <songliubraving@fb.com> wrote:
> >>
> >>
> >>
> >>> 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.
> >
> > Maybe not.  Sometimes we want to profile a subset of cpus only.
> > In that case, cpu != idx then I think we still need this.
>
> We can only attach the bpf program on selected CPUs. Say, we want
> CPUs 1, 3, 5. We just do
>
>         for (i in [1, 3, 5]) {
>                 link = bpf_program__attach_perf_event(skel->progs.on_switch,
>                                                       FD(cgrp_switch, i));
>                 /* */
>         }
>
> The value arrays are still for all cpu, but they will just report zero
> for CPU 0, 2, 4, ....
>
> Would this work?

Yeah, that's exactly what I do, and I'd like to have a compact map
eliminating the unused entries (cpus).  But now I think that I can
keep it with a full cpus and just don't use them.


>
> >>>>> 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 for the explanation, I didn't think that way.
> > I thought it like below:
> >
> >    __u32 elem, value;
> >
> >    for (elem = 0; elem < 3 * 10; elem++) {
> >        bpf_map_lookup_elem(map_fd, &elem, &value);
> >    }
> >
> > So in this case, the actual value size is like below, right?
> >
> >  value-size = map-value-size * number-of-cpu
>
> This is right (for user space).

Thanks for your clarification!

Namhyung