Messages in this thread | | | From | Namhyung Kim <> | Date | Thu, 24 Jun 2021 15:21:16 -0700 | Subject | Re: [PATCH 3/3] perf stat: Enable BPF counter with --for-each-cgroup |
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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
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