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SubjectRe: [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@goodmis.org> wrote:

> On Fri, 23 Jul 2021 10:11:33 +0900
> Masami Hiramatsu <mhiramat@kernel.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@kernel.org>

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