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

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Message-Id: <20210724193145.c63b44aa843e05ed9c0b4fdc@kernel.org>
Date:   Sat, 24 Jul 2021 19:31:45 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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 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,

---
 kernel/trace/trace_events_hist.c | 69 ++++++++++++++------------------
 1 file changed, 31 insertions(+), 38 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 14b840de1326..2fab91a22628 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -492,6 +492,27 @@ struct snapshot_context {
 	void			*key;
 };
 
+static const char *no_comm = "(no comm)";
+
+static u64 hist_field_execname(struct hist_field *hist_field,
+			       struct tracing_map_elt *elt,
+			       struct trace_buffer *buffer,
+			       struct ring_buffer_event *rbe,
+			       void *event)
+{
+	struct hist_elt_data *elt_data;
+
+	if (WARN_ON_ONCE(!elt))
+		return (u64)(unsigned long)no_comm;
+
+	elt_data = elt->private_data;
+
+	if (WARN_ON_ONCE(!elt_data->comm))
+		return (u64)(unsigned long)no_comm;
+
+	return (u64)(unsigned long)(elt_data->comm);
+}
+
 static void track_data_free(struct track_data *track_data)
 {
 	struct hist_elt_data *elt_data;
@@ -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;
+		hist_field->size = MAX_FILTER_STR_VAL;
+		hist_field->is_signed = 0;
+
+		hist_field->type = "char[]";
+		hist_field->fn = hist_field_execname;
+		goto out;
+	}
+
 	if (WARN_ON_ONCE(!field))
 		goto out;
 
@@ -3703,41 +3734,6 @@ static int create_val_field(struct hist_trigger_data *hist_data,
 	return __create_val_field(hist_data, val_idx, file, NULL, field_str, 0);
 }
 
-static const char *no_comm = "(no comm)";
-
-static u64 hist_field_execname(struct hist_field *hist_field,
-			       struct tracing_map_elt *elt,
-			       struct trace_buffer *buffer,
-			       struct ring_buffer_event *rbe,
-			       void *event)
-{
-	struct hist_elt_data *elt_data;
-
-	if (WARN_ON_ONCE(!elt))
-		return (u64)(unsigned long)no_comm;
-
-	elt_data = elt->private_data;
-
-	if (WARN_ON_ONCE(!elt_data->comm))
-		return (u64)(unsigned long)no_comm;
-
-	return (u64)(unsigned long)(elt_data->comm);
-}
-
-/* Convert a var that points to common_pid.execname to a string */
-static void update_var_execname(struct hist_field *hist_field)
-{
-	hist_field->flags = HIST_FIELD_FL_STRING | HIST_FIELD_FL_VAR |
-		HIST_FIELD_FL_EXECNAME;
-	hist_field->size = MAX_FILTER_STR_VAL;
-	hist_field->is_signed = 0;
-
-	kfree_const(hist_field->type);
-	hist_field->type = "char[]";
-
-	hist_field->fn = hist_field_execname;
-}
-
 static int create_var_field(struct hist_trigger_data *hist_data,
 			    unsigned int val_idx,
 			    struct trace_event_file *file,
@@ -3762,9 +3758,6 @@ static int create_var_field(struct hist_trigger_data *hist_data,
 
 	ret = __create_val_field(hist_data, val_idx, file, var_name, expr_str, flags);
 
-	if (!ret && hist_data->fields[val_idx]->flags & HIST_FIELD_FL_EXECNAME)
-		update_var_execname(hist_data->fields[val_idx]);
-
 	if (!ret && hist_data->fields[val_idx]->flags & HIST_FIELD_FL_STRING)
 		hist_data->fields[val_idx]->var_str_idx = hist_data->n_var_str++;
 
-- 
2.25.1



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