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From:   Song Liu <songliubraving@fb.com>
To:     Arnaldo Carvalho de Melo <acme@redhat.com>
CC:     Jiri Olsa <jolsa@redhat.com>, Song Liu <song@kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        "Arnaldo Carvalho de Melo" <acme@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        "jolsa@kernel.org" <jolsa@kernel.org>
Subject: Re: [PATCH v2 3/3] perf-stat: introduce config
 stat.bpf-counter-events
Thread-Topic: [PATCH v2 3/3] perf-stat: introduce config
 stat.bpf-counter-events
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Date:   Thu, 8 Apr 2021 19:41:51 +0000
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> On Apr 8, 2021, at 11:50 AM, Arnaldo Carvalho de Melo <acme@redhat.com> w=
rote:
>=20
> Em Thu, Apr 08, 2021 at 08:24:47PM +0200, Jiri Olsa escreveu:
>> On Thu, Apr 08, 2021 at 06:08:20PM +0000, Song Liu wrote:
>>>=20
>>>=20
>>>> On Apr 8, 2021, at 10:45 AM, Jiri Olsa <jolsa@redhat.com> wrote:
>>>>=20
>>>> On Thu, Apr 08, 2021 at 05:28:10PM +0000, Song Liu wrote:
>>>>>=20
>>>>>=20
>>>>>> On Apr 8, 2021, at 10:20 AM, Jiri Olsa <jolsa@redhat.com> wrote:
>>>>>>=20
>>>>>> On Thu, Apr 08, 2021 at 04:39:33PM +0000, Song Liu wrote:
>>>>>>>=20
>>>>>>>=20
>>>>>>>> On Apr 8, 2021, at 4:47 AM, Jiri Olsa <jolsa@redhat.com> wrote:
>>>>>>>>=20
>>>>>>>> On Tue, Apr 06, 2021 at 05:36:01PM -0700, Song Liu wrote:
>>>>>>>>> Currently, to use BPF to aggregate perf event counters, the user =
uses
>>>>>>>>> --bpf-counters option. Enable "use bpf by default" events with a =
config
>>>>>>>>> option, stat.bpf-counter-events. This is limited to hardware even=
ts in
>>>>>>>>> evsel__hw_names.
>>>>>>>>>=20
>>>>>>>>> This also enables mixed BPF event and regular event in the same s=
esssion.
>>>>>>>>> For example:
>>>>>>>>>=20
>>>>>>>>> perf config stat.bpf-counter-events=3Dinstructions
>>>>>>>>> perf stat -e instructions,cs
>>>>>>>>>=20
>>>>>>>>=20
>>>>>>>> so if we are mixing events now, how about uing modifier for bpf co=
unters,
>>>>>>>> instead of configuring .perfconfig list we could use:
>>>>>>>>=20
>>>>>>>> perf stat -e instructions:b,cs
>>>>>>>>=20
>>>>>>>> thoughts?
>>>>>>>>=20
>>>>>>>> the change below adds 'b' modifier and sets 'evsel::bpf_counter',
>>>>>>>> feel free to use it
>>>>>>>=20
>>>>>>> I think we will need both 'b' modifier and .perfconfig configuratio=
n.=20
>>>>>>> For systems with BPF-managed perf events running in the background,=
=20
>>>>>>=20
>>>>>> hum, I'm not sure I understand what that means.. you mean there
>>>>>> are tools that run perf stat so you don't want to change them?
>>>>>=20
>>>>> We have tools that do perf_event_open(). I will change them to use=20
>>>>> BPF managed perf events for "cycles" and "instructions". Since these=
=20
>>>>> tools are running 24/7, perf-stat on the system should use BPF manage=
d
>>>>> "cycles" and "instructions" by default.=20
>>>>=20
>>>> well if you are already changing the tools why not change them to add
>>>> modifier.. but I don't mind adding that .perfconfig stuff if you need
>>>> that
>>>=20
>>> The tools I mentioned here don't use perf-stat, they just use=20
>>> perf_event_open() and read the perf events fds. We want a config to mak=
e
>>=20
>> just curious, how those tools use perf_event_open?
>=20
> I.e. do they use tools/lib/perf/? :-)

Not right now. I do hope we can eventually let them use libperf. But I=20
haven't figured out the best path forward.=20

>=20
> I guess they will use it now for getting that "struct perf_event_attr_map=
_entry" and
> the map name define.
>=20
>>> "cycles" to use BPF by default, so that when the user (not these tools)
>>> runs perf-stat, it will share PMCs with those events by default.=20
>=20
>> I'm sorry but I still don't see the usecase.. if you need to change both=
 tools,
>> you can change them to use bpf-managed event, why bother with the list?
>=20
> He wants users not to bother if they are using bpf based counters, this w=
ill happen
> automagically after they set their ~/.perfconfig with some command line S=
ong provides.
>=20
> Then they will be using bpf counters that won't get exclusive access to t=
hose
> scarce counters, the tooling they are using will use bpf-counters and all=
 will
> be well.
>=20
> Right Song?

Yes, exactly. The config automatically switches ad-hoc perf-stat runs (for =
debug,=20
performance tuning, etc.) to bpf managed counters.=20

Thanks,
Song