Messages in this thread | | | From | Namhyung Kim <> | Date | Thu, 24 Jun 2021 15:06:46 -0700 | Subject | Re: [PATCH 3/3] perf stat: Enable BPF counter with --for-each-cgroup |
| |
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.
> > > >> > >>> > >>>> > >>>>> + > >>>>> +// 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 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
Thanks, Namhyung
| |