linux-kernel - Re: [PATCH v7 05/16] tracing: Generalize hist trigger onmax and save action

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Message-ID: <1543875722.2018.22.camel@kernel.org>
Date:   Mon, 03 Dec 2018 16:22:02 -0600
From:   Tom Zanussi <zanussi@...nel.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     rostedt@...dmis.org, tglx@...utronix.de, mhiramat@...nel.org,
        vedang.patel@...el.com, bigeasy@...utronix.de,
        joel@...lfernandes.org, mathieu.desnoyers@...icios.com,
        julia@...com, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH v7 05/16] tracing: Generalize hist trigger onmax and
 save action

Hi Namhyung,

On Fri, 2018-11-23 at 11:50 +0900, Namhyung Kim wrote:
> Hi Tom,
> 
> On Wed, Nov 14, 2018 at 02:18:02PM -0600, Tom Zanussi wrote:
> > From: Tom Zanussi <tom.zanussi@...ux.intel.com>
> > 

[snip]

> > 
 enum handler_id {
> >  	HANDLER_ONMATCH = 1,
> >  	HANDLER_ONMAX,
> > @@ -349,14 +358,18 @@ struct action_data {
> >  
> >  		struct {
> >  			char			*var_str;
> > -			unsigned int		max_var_ref_id
> > x;
> > -			struct hist_field	*max_var;
> > -			struct hist_field	*var;
> > -		} onmax;
> > +			struct hist_field	*var_ref;
> > +			unsigned int		var_ref_idx;
> 
> I have a question.  It's confusing for me there are many indexes for
> a
> variable (ref).  The hist_field already has var.idx, var_idx and
> var_ref_idx in it.  But you also added an external var_ref_idx along
> with the var_ref.  Also I see another var_ref_idx in the action data.
> Is all that really needed?  Could you please add some comment then?
> 

Below is a patch with some comments I'll merge into the next version
that I hope will help make things more clear.  Basically, the
hist_field.var_idx isn't used so I've removed it and therefore that
source of confusion, while var.idx is the variable's unique 'handle' in
the tracing_map, used when getting and setting the variable.  And then
there are the several versions of var_ref_idx used for different
purposes depending on the context, but all of them are indices into the
array of variable values collected when a trigger is hit.  For example,
the var_ref_idx defined inside track_data is the index that points to
the tracked var value, which the action can use directly, and the
var_ref_idx alongside the synth fields in action_data is the index of
the first param used when generating a synthetic event, and so on.

Tom 

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 818944391d97..5310ef73f023 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -39,6 +39,16 @@ enum field_op_id {
 	FIELD_OP_UNARY_MINUS,
 };
 
+/*
+ * A hist_var (histogram variable) contains variable information for
+ * hist_fields having the HIST_FIELD_FL_VAR or HIST_FIELD_FL_VAR_REF
+ * flag set.  A hist_var has a variable name e.g. ts0, and is
+ * associated with a given histogram trigger, as specified by
+ * hist_data.  The hist_var idx is the unique index assigned to the
+ * variable by the hist trigger's tracing_map.  The idx is what is
+ * used to set a variable's value and, by a variable reference, to
+ * retrieve it.
+ */
 struct hist_var {
 	char				*name;
 	struct hist_trigger_data	*hist_data;
@@ -60,7 +70,15 @@ struct hist_field {
 	char				*system;
 	char				*event_name;
 	char				*name;
-	unsigned int			var_idx;
+
+	/*
+	 * When a histogram trigger is hit, if it has any references
+	 * to variables, the values of those variables are collected
+	 * into a var_ref_vals array by resolve_var_refs().  The
+	 * current value of each variable is read from the tracing_map
+	 * using the hist field's hist_var.idx and entered into the
+	 * var_ref_idx entry i.e. var_ref_vals[var_ref_idx].
+	 */
 	unsigned int			var_ref_idx;
 	bool                            read_once;
 };
@@ -350,6 +368,14 @@ struct action_data {
 	unsigned int		n_params;
 	char			*params[SYNTH_FIELDS_MAX];
 
+	/*
+	 * When a histogram trigger is hit, the values of any
+	 * references to variables, including variables being passed
+	 * as parameters to synthetic events, are collected into a
+	 * var_ref_vals array.  This var_ref_idx is the index of the
+	 * first param in the array to be passed to the synthetic
+	 * event invocation.
+	 */
 	unsigned int		var_ref_idx;
 	struct synth_event	*synth_event;
 	bool			use_trace_keyword;
@@ -362,10 +388,29 @@ struct action_data {
 		} match_data;
 
 		struct {
+			/*
+			 * var_str and var_ref refer to the variable
+			 * being tracked e.g onmax($var).
+			 */
 			char			*var_str;
 			struct hist_field	*var_ref;
+
+			/*
+			 * When a histogram trigger is hit, the values
+			 * of any references to variables, including
+			 * variables being tracked e.g. onmax($var),
+			 * are collected into a var_ref_vals array.
+			 * This var_ref_idx is the index of the
+			 * tracked var value in var_ref_vals, which
+			 * the action can make use of directly.
+			 */
 			unsigned int		var_ref_idx;
 
+			/*
+			 * This track_var contains the 'invisible'
+			 * tracking variable created to keep the
+			 * current e.g. max value.
+			 */
 			struct hist_field	*track_var;
 
 			check_track_val_fn_t	check_val;