Messages in this thread | | | From | Song Liu <> | Subject | Re: [PATCH 3/3] perf stat: Enable BPF counter with --for-each-cgroup | Date | Thu, 24 Jun 2021 22:15:52 +0000 |
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> 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? [...]
>>>>> 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, Song
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