linux-kernel - Re: [RFC PATCH 1/3] sched, tracing: report task state in symbolic chars instead

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Date:   Tue, 25 Jul 2023 22:16:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ze Gao <zegao2021@...il.com>, Ingo Molnar <mingo@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, Ze Gao <zegao@...cent.com>
Subject: Re: [RFC PATCH 1/3] sched, tracing: report task state in symbolic
 chars instead

On Tue, Jul 25, 2023 at 01:59:38PM -0400, Steven Rostedt wrote:
> > @@ -232,7 +235,7 @@ TRACE_EVENT(sched_switch,
> >  		__array(	char,	prev_comm,	TASK_COMM_LEN	)
> >  		__field(	pid_t,	prev_pid			)
> >  		__field(	int,	prev_prio			)
> > -		__field(	long,	prev_state			)
> > +		__field(	char,	prev_state			)
> >  		__array(	char,	next_comm,	TASK_COMM_LEN	)
> >  		__field(	pid_t,	next_pid			)
> >  		__field(	int,	next_prio			)
> 
> This will break userspace. Just because you update libtraceevent
> doesn't mean that it will get to all the users of it. There's still
> tools that have the old method hard coded and doesn't use the library.
> 
> Now, because the old tools still do the parsing of this format, we can
> add a new field called prev_state_char that will give you this. Now to
> save space, we should change prev_state to int (can't make it short as
> there's that test for "+" which does sometimes happen). I believe we
> can make prev_prio and next prio into shorts (and possibly chars!).

Or just leave the thing alone?