Re: [PATCH v7 22/37] tracing: Add variable reference handling to hist triggers

[Namhyung Kim]

Hi Tom,

On Wed, Dec 06, 2017 at 04:38:03PM -0600, Tom Zanussi wrote:
> Add the necessary infrastructure to allow the variables defined on one
> event to be referenced in another.  This allows variables set by a
> previous event to be referenced and used in expressions combining the
> variable values saved by that previous event and the event fields of
> the current event.  For example, here's how a latency can be
> calculated and saved into yet another variable named 'wakeup_lat':
> 
>     # echo 'hist:keys=pid,prio:ts0=common_timestamp ...
>     # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp-$ts0 ...
> 
> In the first event, the event's timetamp is saved into the variable
> ts0.  In the next line, ts0 is subtracted from the second event's
> timestamp to produce the latency.
> 
> Further users of variable references will be described in subsequent
> patches, such as for instance how the 'wakeup_lat' variable above can
> be displayed in a latency histogram.
> 
> Signed-off-by: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>
> ---

[SNIP]
> @@ -313,10 +529,150 @@ static struct hist_field *find_var(struct hist_trigger_data *hist_data,
>  	return NULL;
>  }
>  
> +static struct trace_event_file *find_var_file(struct trace_array *tr,
> +					      char *system,
> +					      char *event_name,
> +					      char *var_name)
> +{
> +	struct hist_trigger_data *var_hist_data;
> +	struct hist_var_data *var_data;
> +	struct trace_event_call *call;
> +	struct trace_event_file *file, *found = NULL;
> +	const char *name;
> +
> +	list_for_each_entry(var_data, &tr->hist_vars, list) {
> +		var_hist_data = var_data->hist_data;
> +		file = var_hist_data->event_file;
> +		if (file == found)
> +			continue;
> +		call = file->event_call;
> +		name = trace_event_name(call);
> +
> +		if (!system || !event_name) {
> +			if (find_var(var_hist_data, file, var_name)) {

Is find_var() really needed?  I guess find_var_field() could do the
job with lower overhead..

> +				if (found) {
> +					return NULL;
> +				}
> +
> +				found = file;
> +			}
> +			continue;
> +		}
> +
> +		if (strcmp(event_name, name) != 0)
> +			continue;
> +		if (strcmp(system, call->class->system) != 0)
> +			continue;

Also it doesn't need to iterate the loop when system and event name is
given.  Please see below


> +
> +		found = file;
> +		break;
> +	}
> +
> +	return found;
> +}


How about this?

find_var_file()
{
	if (system)
		return find_event_file(tr, system, event);

	list_for_each_entry(var_data, &tr->hist_vars, list) {
		var_hist_data = var_data->hist_data;
		file = var_hist_data->event_file;
		if (file == found)
			continue;

		if (find_var_field(var_hist_data, var_name)) {
			if (found)
				return NULL;
			
			found = file;
		}
	}
}


Thanks,
Namhyung