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Date:   Tue, 25 Jul 2023 22:16:03 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Ze Gao <zegao2021@gmail.com>, Ingo Molnar <mingo@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        linux-trace-kernel@vger.kernel.org, Ze Gao <zegao@tencent.com>
Subject: Re: [RFC PATCH 1/3] sched, tracing: report task state in symbolic
 chars instead
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References: <20230725072254.32045-1-zegao@tencent.com>
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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?