Messages in this thread | | | From | Namhyung Kim <> | Date | Mon, 21 Jun 2021 18:43:29 -0700 | Subject | Re: [PATCHSET v2 0/3] perf stat: Enable BPF counters with --for-each-cgroup |
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Hi Song,
On Wed, Jun 16, 2021 at 11:33 PM Song Liu <songliubraving@fb.com> wrote: > > > > > On Jun 16, 2021, at 3:32 PM, Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Wed, Jun 16, 2021 at 09:33:42AM -0700, Namhyung Kim wrote: > > > >>> That seems unfortunate; there's no bpf helper to iterate cgroup > >>> hierarchy? > >> > >> I couldn't find one.. > > > > Song, is that something that would make sense to have? > > I think we can solve this with bpf_get_current_ancestor_cgroup_id and > a bounded loop. Like: > > /* get diff_reading, which is reading - prev_reading */ > > for (i = 0; i < 10 /* at most 10 levels */; i++) { > __u64 cgroup_id = bpf_get_current_ancestor_cgroup_id(i); > if (!cgroup_id) > break; > /* add diff_reading to cgroup_id */ > }
OK, but I'm not sure 0 id is guaranteed.
> > > > >>>> * there's no reliable way to trigger running the BPF program > >>> > >>> You can't attach to the PERF_COUNT_SW_CGROUP_SWITCHES event? > >> > >> I did it. But the BPF test run seems not to work with perf_event. > >> So it needs to trigger a cgroup switch manually.. > > > > AFAICT it should be possible to set a bpf prog on a software event. > > perf_event_set_bpf_prog() will take the first branch > > (!perf_event_is_tracing()) and call perf_event_set_bpf_handler(). > > > > That should then result in running the bpf program every time the event > > would generate a sample. > > > > So if you configure the event to sample on every single event, it should > > then run your program every time. > > > > This is all from looking at the code, because I really can't operate any > > of that for real. I suspect Song can help out. > > > > The alternative is to attach a BPF program to the sched_switch > > tracepoint and do the cgroup filter in BPF. > > We can create a raw_tp BPF program just for BPF_PROG_TEST_RUN (now also called > BPF_PROG_RUN). The program should be the same as current on_switch program. > We don't have to attach the program, just use BPF_PROG_RUN to trigger it. > > Would something like this work?
Oh, I think it'd work. Thanks for the suggestion!
Thanks, Namhyung
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