linux-kernel - Re: [PATCH 1/2] tracing/hist: simplify contains

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Date:   Wed, 22 Mar 2023 13:13:51 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        mhiramat@...nel.org, zanussi@...nel.org
Subject: Re: [PATCH 1/2] tracing/hist: simplify contains_operator()

On Sat, Mar 18, 2023 at 03:12:08PM -0400, Steven Rostedt wrote:
> On Thu,  2 Mar 2023 17:17:54 +0000
> Mark Rutland <mark.rutland@....com> wrote:
> 
> FYI, we follow Linus's preference that subjects start with a capital
> letter. Unless of course you are a socialist and dislike capitalism?
> 
>   tracing/hist: Simplify contains_operator()
> 

Sorry; I always get this wrong since many other trees do everything lower case
(or support total commit message anarchy). I'll go fix that up.

> 
> > In a subsequent patch we'll add additional operators for histogram
> > expressions.
> 
> Refrain from using "subsequent patch", instead say:
> 
>  Simplify contains_operator() in order to support additional operators
>  for histogram expressions.

Sure.

> 
> > 
> > In preparation for adding additional operators, this patch refactors
> > contains_operator() to consider each operator within a precedence group
> > independently by using the 'sep' pointer as the current rightmost
> > operator, and removing the separate op pointers.
> > 
> > Within each precedence group, this allows operators to be checked
> > independently with a consistent pattern:
> > 
> > 	op = strrchr(str, $OP_CHAR);
> > 	if (op > *sep) {
> > 		*sep = op;
> > 		field_op = $FIELD_OP_TYPE;
> > 	}
> > 
> > This makes it easy to add new operators of the same precedence without
> > needing to check multiple pointers, and without needing a final switch
> > statement to recover the relevant pointer.
> > 
> > There should be no functional change as a result of this patch.
> > 
> > Signed-off-by: Mark Rutland <mark.rutland@....com>
> > Cc: Masami Hiramatsu <mhiramat@...nel.org>
> > Cc: Steven Rostedt (Google) <rostedt@...dmis.org>
> > Cc: Tom Zanussi <zanussi@...nel.org>
> > Cc: linux-trace-kernel@...r.kernel.org
> > ---
> >  kernel/trace/trace_events_hist.c | 80 ++++++++++++--------------------
> >  1 file changed, 30 insertions(+), 50 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> > index 10d36f751fcd..a308da2cde2f 100644
> > --- a/kernel/trace/trace_events_hist.c
> > +++ b/kernel/trace/trace_events_hist.c
> > @@ -1813,13 +1813,15 @@ static char *expr_str(struct hist_field *field, unsigned int level)
> >  static int contains_operator(char *str, char **sep)
> >  {
> >  	enum field_op_id field_op = FIELD_OP_NONE;
> > -	char *minus_op, *plus_op, *div_op, *mult_op;
> > +	char *op;
> >  
> > +	*sep = NULL;
> 
> Hmm!

Ugh, sorry, I had completely glossed over the:

	if (sep) {
		...
		// assignments to *sep here
		...
	}

... in the existing code.

I'll go rework that...

> 
> >  
> >  	/*
> > -	 * Report the last occurrence of the operators first, so that the
> > -	 * expression is evaluated left to right. This is important since
> > -	 * subtraction and division are not associative.
> > +	 * For operators of the same precedence report the last occurrence of
> > +	 * the operators first, so that the expression is evaluated left to
> > +	 * right. This is important since subtraction and division are not
> > +	 * associative.
> >  	 *
> >  	 *	e.g
> >  	 *		64/8/4/2 is 1, i.e 64/8/4/2 = ((64/8)/4)/2
> > @@ -1830,68 +1832,46 @@ static int contains_operator(char *str, char **sep)
> >  	 * First, find lower precedence addition and subtraction
> >  	 * since the expression will be evaluated recursively.
> >  	 */
> > -	minus_op = strrchr(str, '-');
> > -	if (minus_op) {
> > +	op = strrchr(str, '-');
> > +	if (op > *sep) {
> 
> Why compare to *sep if it is always NULL?

As in the commit message, that was just so that every check for an operator had
the same shape. I can certainly drop this for the first check and just have:

	op = strrchr(str, '-');
	if (op) {
		...
	}

> 
> Oh! But later in the code we have:
> 
> 	if (contains_operator(field, NULL) || is_var_ref(field))
> 
> I wonder how *sep = NULL will handle that?

Yep, I got this wrong. I'll go rejig that.

Thanks,
Mark.