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

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Date:   Tue, 1 Aug 2023 10:33:40 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ze Gao <zegao2021@...il.com>,
        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

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