linux-kernel - Re: [PATCH v2 3/3] perf-stat: introduce config stat.bpf-counter-events

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Message-ID: <FDBEC7CA-7F74-4494-93B3-7E5758E5DED9@fb.com>
Date:   Thu, 8 Apr 2021 18:08:20 +0000
From:   Song Liu <songliubraving@...com>
To:     Jiri Olsa <jolsa@...hat.com>
CC:     Song Liu <song@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        "jolsa@...nel.org" <jolsa@...nel.org>
Subject: Re: [PATCH v2 3/3] perf-stat: introduce config
 stat.bpf-counter-events



> On Apr 8, 2021, at 10:45 AM, Jiri Olsa <jolsa@...hat.com> wrote:
> 
> On Thu, Apr 08, 2021 at 05:28:10PM +0000, Song Liu wrote:
>> 
>> 
>>> On Apr 8, 2021, at 10:20 AM, Jiri Olsa <jolsa@...hat.com> wrote:
>>> 
>>> On Thu, Apr 08, 2021 at 04:39:33PM +0000, Song Liu wrote:
>>>> 
>>>> 
>>>>> On Apr 8, 2021, at 4:47 AM, Jiri Olsa <jolsa@...hat.com> wrote:
>>>>> 
>>>>> 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 events in
>>>>>> evsel__hw_names.
>>>>>> 
>>>>>> This also enables mixed BPF event and regular event in the same sesssion.
>>>>>> For example:
>>>>>> 
>>>>>> perf config stat.bpf-counter-events=instructions
>>>>>> perf stat -e instructions,cs
>>>>>> 
>>>>> 
>>>>> so if we are mixing events now, how about uing modifier for bpf counters,
>>>>> instead of configuring .perfconfig list we could use:
>>>>> 
>>>>> perf stat -e instructions:b,cs
>>>>> 
>>>>> thoughts?
>>>>> 
>>>>> the change below adds 'b' modifier and sets 'evsel::bpf_counter',
>>>>> feel free to use it
>>>> 
>>>> I think we will need both 'b' modifier and .perfconfig configuration. 
>>>> For systems with BPF-managed perf events running in the background, 
>>> 
>>> 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?
>> 
>> We have tools that do perf_event_open(). I will change them to use 
>> BPF managed perf events for "cycles" and "instructions". Since these 
>> tools are running 24/7, perf-stat on the system should use BPF managed
>> "cycles" and "instructions" by default. 
> 
> 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

The tools I mentioned here don't use perf-stat, they just use 
perf_event_open() and read the perf events fds. We want a config to make
"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. 

> 
>> 
>>> 
>>>> .perfconfig makes sure perf-stat sessions will share PMCs with these 
>>>> background monitoring tools. 'b' modifier, on the other hand, is useful
>>>> when the user knows there is opportunity to share the PMCs. 
>>>> 
>>>> Does this make sense? 
>>> 
>>> if there's reason for that then sure.. but let's not limit that just
>>> on HARDWARE events only.. there are RAW events with the same demand
>>> for this feature.. why don't we let user define any event for this?
>> 
>> I haven't found a good way to config RAW events. I guess RAW events 
>> could use 'b' modifier? 
> any event uing the pmu notation like cpu/instructions/

Can we do something like "perf config stat.bpf-counter-events=cpu/*" means 
all "cpu/xx" events use BPF by default?

Thanks,
Song

> 
> we can allow any event to be BPF-managed, right? IIUC we don't care,
> the code will work with any event

Yes, the code works with any event.