linux-kernel - Re: [PATCH 3/3] perf stat: Enable BPF counter with --for-each-cgroup

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Date:   Thu, 24 Jun 2021 21:40:39 +0000
From:   Song Liu <songliubraving@...com>
To:     Namhyung Kim <namhyung@...nel.org>
CC:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.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: [PATCH 3/3] perf stat: Enable BPF counter with --for-each-cgroup



> On Jun 24, 2021, at 2:01 PM, Namhyung Kim <namhyung@...nel.org> wrote:
> 
> On Thu, Jun 24, 2021 at 9:20 AM Song Liu <songliubraving@...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.
> 
>> 
>>> 
>>>> 
>>>>> +
>>>>> +// 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,
Song