linux-kernel - Re: [PATCHSET v2 0/3] perf stat: Enable BPF counters with --for-each-cgroup

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Date:   Thu, 17 Jun 2021 06:33:28 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Namhyung Kim <namhyung@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCHSET v2 0/3] perf stat: Enable BPF counters with
 --for-each-cgroup



> On Jun 16, 2021, at 3:32 PM, Peter Zijlstra <peterz@...radead.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 */
	}

> 
>>>> * 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?

Thanks,
Song