linux-kernel - Re: [PATCH] perf-stat: introduce bperf, share hardware PMCs with BPF

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Date:   Mon, 15 Mar 2021 18:24:10 +0000
From:   Song Liu <songliubraving@...com>
To:     Jiri Olsa <jolsa@...hat.com>
CC:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        "acme@...hat.com" <acme@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "jolsa@...nel.org" <jolsa@...nel.org>
Subject: Re: [PATCH] perf-stat: introduce bperf, share hardware PMCs with BPF



> On Mar 15, 2021, at 7:09 AM, Jiri Olsa <jolsa@...hat.com> wrote:
> 
> On Mon, Mar 15, 2021 at 07:51:11AM +0000, Song Liu wrote:
> 
> SNIP

[...]

>> 
>> It is mostly to cover corner cases, like something else used the same 
>> name. 
> 
> about that.. we just take the object fd assuming it's map,
> should we test it somehow?
> 
>  map_fd = bpf_obj_get(path);
> 
> if it's not the map we expect, I think we should generate
> another name without forcing user to run again with --attr-map
> 
> but still warn, so other perf session can use the new name

The auto failover is an interesting idea. But I guess we still need 
--attr-map. Another use case is when the user mounted bpffs to a 
different path. Alternatively, maybe we can teach perf to search all 
bpffs mount points for perf_attr_map? 

> 
> SNIP
> 
>>>> +static int bperf_sync_counters(struct evsel *evsel)
>>>> +{
>>>> +	struct perf_cpu_map *all_cpus = perf_cpu_map__new(NULL);
>>>> +	int num_cpu, i, cpu;
>>>> +
>>>> +	if (!all_cpus)
>>>> +		return -1;
>>>> +
>>>> +	num_cpu = all_cpus->nr;
>>>> +	for (i = 0; i < num_cpu; i++) {
>>>> +		cpu = all_cpus->map[i];
>>>> +		bperf_trigger_reading(evsel->bperf_leader_prog_fd, cpu);
>>>> +	}
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static int bperf__enable(struct evsel *evsel)
>>>> +{
>>>> +	struct bperf_follower_bpf *skel = evsel->follower_skel;
>>>> +	__u32 num_cpu_bpf = libbpf_num_possible_cpus();
>>> 
>>> we have cpu__max_cpu for that
>> 
>> libbpf calls for percpu array use libbpf_num_possible_cpus. So I guess it 
>> is better to use the same here. The two are identical at the moment though.
> 
> then in the bperf__read you take that array and update
> perf_counts, which is based on perf's cpus, so they mix
> anyway
> 
> I'd like to keep perf code using perf's cpus api.. could
> we just check at the begining that libbpf_num_possible_cpus
> returns same number as cpu__max_cpu (if not, we have a
> problem anyway) and use perf's cpu api

Let me try cpu__max_cpu. 

Thanks,
Song