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

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Date:   Thu, 8 Apr 2021 19:45:22 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Song Liu <songliubraving@...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 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

> 
> > 
> >> .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/

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

jirka