linux-kernel - Re: [PATCH v2 2/2] tracing: Allow execnames to be passed as args for synthetic events

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Date:   Sun, 25 Jul 2021 11:18:30 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tom Zanussi <zanussi@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH v2 2/2] tracing: Allow execnames to be passed as args
 for synthetic events

Hi,

On Sat, 24 Jul 2021 19:31:45 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Hi Steve,
> 
> On Thu, 22 Jul 2021 21:24:38 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Fri, 23 Jul 2021 10:11:33 +0900
> > Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > 
> > > I understand. As far as I can see the code, it looks a bit complicated.
> > > To simplify it, I need to understand the spec for "hist_field"
> > > for keys and for vars. And maybe need to split both case.
> > 
> > I'll give you a hint that took me a bit to figure out.
> > 
> > 1) The execname is saved at the start of the histogram and not by one
> > of the ->fn() functions.
> > 
> > It's saved by hist_trigger_elt_data_init() if the elt_data->comm is
> > allocated. That function is part of the "tracing_map_ops" which gets
> > assigned by tracing_map_create() (in tracing_map.c) as the "elt_init"
> > function, which is called when getting a new elt element by
> > get_free_elt().
> > 
> > 2) That elt_data->comm is only allocated if it finds a "hist_field"
> > that has HIST_FIELD_FL_EXECNAME flag set. It currently only looks for
> > that flag in the "keys" fields, which means that .execname is useless
> > for everything else. This patch changed it to search all hist_fields so
> > that it can find that flag if a variable has it set (which I added).
> 
> Thanks for the hints, but actually, that part looks good to me.
> 
> So, what I pointed was the part of update_var_execname(). Below diff
> is what I intended.
> This moves HIST_FIELD_FL_EXECNAME setup in the create_hist_field()
> as same as other flags, and removed the add-hoc update_var_execname()
> fixup function.
> 
> I confirmed it passed the ftracetest trigger testcases and your
> example code.
> 
> Thank you,
> 

I found a bug in this change.

[..]
> @@ -1682,6 +1703,16 @@ static struct hist_field *create_hist_field(struct hist_trigger_data *hist_data,
>  		goto out;
>  	}
>  
> +	if ((flags & HIST_FIELD_FL_EXECNAME) && var_name) {
> +		flags |= HIST_FIELD_FL_STRING | HIST_FIELD_FL_VAR;

Here, we don't need to check 'var_name' and remove HIST_FIELD_FL_VAR, since it must be set in the flag.

	if (flags & HIST_FIELD_FL_EXECNAME) {
		flags |= HIST_FIELD_FL_STRING;


> +		hist_field->size = MAX_FILTER_STR_VAL;
> +		hist_field->is_signed = 0;
> +
> +		hist_field->type = "char[]";
> +		hist_field->fn = hist_field_execname;
> +		goto out;
> +	}
> +



Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>