linux-kernel - Re: [PATCH 1/5] tracing: use union to simplify the trace_event

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Date:   Thu, 9 Jul 2020 17:57:39 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] tracing: use union to simplify the
 trace_event_functions initialization

On Fri,  3 Jul 2020 10:06:08 +0800
Wei Yang <richard.weiyang@...ux.alibaba.com> wrote:

> There are for 4 fields in trace_event_functions with the same type of
> trace_print_func. Initialize them in register_trace_event() one by one
> looks redundant.

I have mixed emotions about this patch. Yeah, it consolidates it a bit,
but it also makes it less easy to know what it is doing.

All this patch is doing is optimizing the initialization path, which is
done once when an event is registered. It's error prone, as you would
need to make sure to map the array with the functions. Something like
this is only reasonable if it is used more often, which here it's a
single spot.

So no, I can't take this patch.

-- Steve



> 
> Let's take advantage of union to simplify the procedure.
> 
> Signed-off-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
> ---
>  include/linux/trace_events.h | 13 +++++++++----
>  kernel/trace/trace_output.c  | 14 +++++---------
>  2 files changed, 14 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> index 5c6943354049..1a421246f4a2 100644
> --- a/include/linux/trace_events.h
> +++ b/include/linux/trace_events.h
> @@ -122,10 +122,15 @@ typedef enum print_line_t (*trace_print_func)(struct trace_iterator *iter,
>  				      int flags, struct trace_event *event);
>  
>  struct trace_event_functions {
> -	trace_print_func	trace;
> -	trace_print_func	raw;
> -	trace_print_func	hex;
> -	trace_print_func	binary;
> +	union {
> +		struct {
> +			trace_print_func	trace;
> +			trace_print_func	raw;
> +			trace_print_func	hex;
> +			trace_print_func	binary;
> +		};
> +		trace_print_func print_funcs[4];
> +	};
>  };
>  
>  struct trace_event {
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index 73976de7f8cc..47bf9f042b97 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -728,7 +728,7 @@ void trace_event_read_unlock(void)
>  int register_trace_event(struct trace_event *event)
>  {
>  	unsigned key;
> -	int ret = 0;
> +	int i, ret = 0;
>  
>  	down_write(&trace_event_sem);
>  
> @@ -770,14 +770,10 @@ int register_trace_event(struct trace_event *event)
>  			goto out;
>  	}
>  
> -	if (event->funcs->trace == NULL)
> -		event->funcs->trace = trace_nop_print;
> -	if (event->funcs->raw == NULL)
> -		event->funcs->raw = trace_nop_print;
> -	if (event->funcs->hex == NULL)
> -		event->funcs->hex = trace_nop_print;
> -	if (event->funcs->binary == NULL)
> -		event->funcs->binary = trace_nop_print;
> +	for (i = 0; i < ARRAY_SIZE(event->funcs->print_funcs); i++) {
> +		if (!event->funcs->print_funcs[i])
> +			event->funcs->print_funcs[i] = trace_nop_print;
> +	}
>  
>  	key = event->type & (EVENT_HASHSIZE - 1);
>