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 10:33:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ze Gao <zegao2021@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        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 03:22:52PM +0800, Ze Gao wrote:

> @@ -197,13 +198,6 @@ static inline long __trace_sched_switch_state(bool preempt,
>  	BUG_ON(p != current);
>  #endif /* CONFIG_SCHED_DEBUG */
>  
> -	/*
> -	 * Preemption ignores task state, therefore preempted tasks are always
> -	 * RUNNING (we will not have dequeued if state != RUNNING).
> -	 */
> -	if (preempt)
> -		return TASK_REPORT_MAX;
> -
>  	/*
>  	 * task_state_index() uses fls() and returns a value from 0-8 range.
>  	 * Decrement it by 1 (except TASK_RUNNING state i.e 0) before using
> @@ -212,7 +206,16 @@ static inline long __trace_sched_switch_state(bool preempt,
>  	 */
>  	state = __task_state_index(prev_state, p->exit_state);
>  
> -	return state ? (1 << (state - 1)) : state;
> +	/*
> +	 * Preemption ignores task state, therefore preempted tasks are always
> +	 * RUNNING (we will not have dequeued if state != RUNNING).
> +	 * Here, we use 'p' to denote this case and only for this case.
> +	 */
> +	if (preempt)
> +		return 'p';
> +

I don't get this move, why compute state before this return?

> +
> +	return task_index_to_char(state);
>  }
>  #endif /* CREATE_TRACE_POINTS */
>  
> @@ -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 is a format change and will likely break a ton of programs :/