linux-kernel - Re: [PATCHSET 0/4] perf stat: Add --multiply-cgroup option

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Date:   Fri, 11 Sep 2020 11:35:30 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ian Rogers <irogers@...gle.com>
Subject: Re: [PATCHSET 0/4] perf stat: Add --multiply-cgroup option

Hi Andi,

On Fri, Sep 11, 2020 at 1:05 AM Andi Kleen <andi@...stfloor.org> wrote:
>
> On Tue, Sep 08, 2020 at 01:42:24PM +0900, Namhyung Kim wrote:
> > When we profile cgroup events with perf stat, it's very annoying to
> > specify events and cgroups on the command line as it requires the
> > mapping between events and cgroups.  (Note that perf record can use
> > cgroup sampling but it's not usable for perf stat).
>
> The problem is real, but I don't really like your solution.
> The option is ugly. Should rather be solved with some suitable
> syntax in the expression parser to express: apply to all,
> instead of adding adhoc options like this.

Yeah, I'd admit that I'm terrible at naming. :)
I'm open to any suggestions as I'm also bad at making new syntax..

But as Ian said, my approach takes care of the common case
without dealing with controversial syntax changes.

>
> There are some additional problems that really need to be eventually
> solved too:
>
> - If you use the old syntax and some cgroups are not covered you don't
> get any warning. At least that should be fixed too.

I don't follow.  Do you mean like when -G option and --multiply-cgroup option
is used at the same time?  Or when -G option doesn't match to -e option?

>
> - And of course if everything works it is still very slow for the kernel
> because there are so many perf events to handle. Long term we probably
> need some more flexible way to just specify for given perf events which set of
> cgroups they should apply, so that sharing and low overhead monitoring
> of many cgroups is possible I hate to say it, but maybe some eBPF filter
> is the solution here.

Yes, I'm also interested in improving that too.

Thanks
Namhyung