linux-kernel - Re: [RFC PATCH v4 4/7] sched, tracing: reorganize fields of switch event struct

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Date:   Wed, 2 Aug 2023 11:05:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Ze Gao <zegao2021@...il.com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.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 v4 4/7] sched, tracing: reorganize fields of switch
 event struct

On Wed,  2 Aug 2023 08:09:59 -0400
Ze Gao <zegao2021@...il.com> wrote:

> From: Ze Gao <zegao2021@...il.com>
> 
> 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>
> Signed-off-by: Ze Gao <zegao@...cent.com>
> ---
>  include/trace/events/sched.h | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> index fbb99a61f714..7d34db20b2c6 100644
> --- a/include/trace/events/sched.h
> +++ b/include/trace/events/sched.h
> @@ -187,7 +187,7 @@ DEFINE_EVENT(sched_wakeup_template, sched_wakeup_new,
>  	     TP_ARGS(p));
>  
>  #ifdef CREATE_TRACE_POINTS
> -static inline long __trace_sched_switch_state(bool preempt,
> +static inline int __trace_sched_switch_state(bool preempt,
>  					      unsigned int prev_state,
>  					      struct task_struct *p)
>  {
> @@ -229,23 +229,23 @@ TRACE_EVENT(sched_switch,
>  	TP_ARGS(preempt, prev, next, prev_state),
>  
>  	TP_STRUCT__entry(
> -		__array(	char,	prev_comm,	TASK_COMM_LEN	)
>  		__field(	pid_t,	prev_pid			)
> -		__field(	int,	prev_prio			)
> -		__field(	long,	prev_state			)
> -		__array(	char,	next_comm,	TASK_COMM_LEN	)
>  		__field(	pid_t,	next_pid			)
> -		__field(	int,	next_prio			)
> +		__field(	short,	prev_prio			)
> +		__field(	short,	next_prio			)
> +		__field(	int,	prev_state			)

I was talking with Peter on IRC and since the biggest number that
prev_state can be is TASK_REPORT_MASK which is 0x100, I think we can make
prev_state into a short as well.

-- Steve


> +		__array(	char,	prev_comm,	TASK_COMM_LEN	)
> +		__array(	char,	next_comm,	TASK_COMM_LEN	)
>  	),
>  
>  	TP_fast_assign(
> -		memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
> -		__entry->prev_pid	= prev->pid;
> -		__entry->prev_prio	= prev->prio;
> -		__entry->prev_state	= __trace_sched_switch_state(preempt, prev_state, prev);
> +		__entry->prev_pid		= prev->pid;
> +		__entry->next_pid		= next->pid;
> +		__entry->prev_prio		= (short) prev->prio;
> +		__entry->next_prio		= (short) next->prio;
> +		__entry->prev_state		= __trace_sched_switch_state(preempt, prev_state, prev);
>  		memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
> -		__entry->next_pid	= next->pid;
> -		__entry->next_prio	= next->prio;
> +		memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
>  		/* XXX SCHED_DEADLINE */
>  	),
>