linux-kernel - Re: [RFC PATCH v3 4/6] sched, tracing: reorganize fields of switch event struct

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Message-ID: <CAD8CoPAPsCyg=P=pDmzjM3qZt5qcTp7Q=AL2w3_ZwGCnNpgu2g@mail.gmail.com>
Date:   Wed, 2 Aug 2023 11:06:05 +0800
From:   Ze Gao <zegao2021@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org,
        linux-trace-devel@...r.kernel.org, Ze Gao <zegao@...cent.com>
Subject: Re: [RFC PATCH v3 4/6] sched, tracing: reorganize fields of switch
 event struct

Thanks for clarifying this ! Steven. This is really helpful.

Regards,
Ze

On Tue, Aug 1, 2023 at 10:33 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Tue, 1 Aug 2023 13:46:50 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
>
> > On Tue, Aug 01, 2023 at 05:01:22PM +0800, Ze Gao wrote:
> > > Report priorities in 'short' and prev_state in 'int' to save
> > > some buffer space. And also reorder the fields so that we take
> > > struct alignment into consideration to make the record compact.
> > >
> > > Suggested-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> >
> > I don't see a single line describing the effort you've done to audit
> > consumers of this tracepoint.
> >
> > *IF* you're wanting to break this tracepoint ABI, because seriously
> > that's what it is, then you get to invest the time and effort to audit
> > the users.
>
> The known major users that I am aware of is raesdaemon,
> powertop/latencytop, perf, trace-cmd and some bpf tools. The bpf tooling is
> known to update per kernel. The others all use libtraceevent that can
> handle this change.
>
> What other tools are there? There's Perfetto, but it also looks at tracefs
> to examine where the values are. There's LTTng, but I believe it uses the
> raw tracepoint directly and doesn't look at the layout of the ftrace/perf
> buffers.
>
> All other tooling I am slightly aware of uses libtracefs and libtraceveent,
> as I've been giving many talks on how to use those libraries.
>
> -- Steve