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

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Date:   Thu, 8 Apr 2021 15:50:12 -0300
From:   Arnaldo Carvalho de Melo <acme@...hat.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Song Liu <songliubraving@...com>, Song Liu <song@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        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

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:
> > 
> > 
> > > 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
> 
> just curious, how those tools use perf_event_open?

I.e. do they use tools/lib/perf/? :-)

I guess they will use it now for getting that "struct perf_event_attr_map_entry" and
the map name define.
 
> > "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. 
 
> 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?

He wants users not to bother if they are using bpf based counters, this will happen
automagically after they set their ~/.perfconfig with some command line Song provides.

Then they will be using bpf counters that won't get exclusive access to those
scarce counters, the tooling they are using will use bpf-counters and all will
be well.

Right Song?

- Arnaldo