Re: [PATCH 23/32] tracing: Add 'onmatch' hist trigger action support

[Tom Zanussi]

Hi Namhyung,

On Wed, 2017-07-26 at 11:40 +0900, Namhyung Kim wrote:
> Hi Tom,
> 
> On Mon, Jun 26, 2017 at 05:49:24PM -0500, Tom Zanussi wrote:
> > Add an 'onmatch(matching.event).<synthetic_event_name>(param list)'
> > hist trigger action which is invoked with the set of variables or
> > event fields named in the 'param list'.  The result is the generation
> > of a synthetic event that consists of the values contained in those
> > variables and/or fields at the time the invoking event was hit.
> > 
> > As an example the below defines a simple synthetic event using a
> > variable defined on the sched_wakeup_new event, and shows the event
> > definition with unresolved fields, since the sched_wakeup_new event
> > with the testpid variable hasn't been defined yet:
> > 
> >     # echo 'wakeup_new_test pid_t pid; int prio' >> \
> >       /sys/kernel/debug/tracing/synthetic_events
> > 
> >     # cat /sys/kernel/debug/tracing/synthetic_events
> >       wakeup_new_test pid_t pid; int prio
> > 
> > The following hist trigger both defines a testpid variable and
> > specifies an onmatch() trace action that uses that variable along with
> > a non-variable field to generate a wakeup_new_test synthetic event
> > whenever a sched_wakeup_new event occurs, which because of the 'if
> > comm == "cyclictest"' filter only happens when the executable is
> > cyclictest:
> > 
> >     # echo 'hist:keys=testpid=pid:\
> >       onmatch(sched.sched_wakeup_new).wakeup_new_test($testpid, prio) \
> >         if comm=="cyclictest"' >> \
> >       /sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger
> > 
> > Creating and displaying a histogram based on those events is now just
> > a matter of using the fields and new synthetic event in the
> > tracing/events/synthetic directory, as usual:
> > 
> >     # echo 'hist:keys=pid,prio:sort=pid,prio' >> \
> >       /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/trigger
> > 
> > Signed-off-by: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>
> > ---
> 
> [SNIP]
> >  static void hist_trigger_elt_data_free(struct tracing_map_elt *elt)
> >  {
> > +	struct hist_trigger_data *hist_data = elt->map->private_data;
> >  	struct hist_elt_data *private_data = elt->private_data;
> > +	unsigned int i, n_str;
> > +
> > +	n_str = hist_data->n_field_var_str;
> > +
> > +	for (i = 0; i < n_str; i++)
> > +		kfree(private_data->field_var_str[i]);
> >  
> >  	kfree(private_data->comm);
> >  	kfree(private_data);
> > @@ -1537,7 +1627,7 @@ static int hist_trigger_elt_data_alloc(struct tracing_map_elt *elt)
> >  	unsigned int size = TASK_COMM_LEN + 1;
> >  	struct hist_elt_data *elt_data;
> >  	struct hist_field *key_field;
> > -	unsigned int i;
> > +	unsigned int i, n_str;
> >  
> >  	elt->private_data = elt_data = kzalloc(sizeof(*elt_data), GFP_KERNEL);
> >  	if (!elt_data)
> > @@ -1557,6 +1647,16 @@ static int hist_trigger_elt_data_alloc(struct tracing_map_elt *elt)
> >  		}
> >  	}
> >  
> > +	n_str = hist_data->n_field_var_str;
> > +
> > +	for (i = 0; i < n_str; i++) {
> > +		elt_data->field_var_str[i] = kzalloc(size, GFP_KERNEL);
> 
> So the max length of a string variable is TASK_COMM_LEN, right?
> 

Yes, but I think it should be a bit larger - looking at existing events,
I also see 32 as a common length, so it would probably be good to up it
to that.  Unless you have a better idea...

> In addition, isn't it necessary for hist_trigger_elt_data_copy() to
> copy the field_var_str array?
> 

Yep, missed that, will add.

> 
> > +		if (!elt_data->field_var_str[i]) {
> > +			hist_trigger_elt_data_free(elt);
> > +			return -ENOMEM;
> > +		}
> > +	}
> > +
> >  	return 0;
> >  }
> 
> [SNIP]
> > +static bool compatible_keys(struct hist_trigger_data *target_hist_data,
> > +			    struct hist_trigger_data *hist_data,
> > +			    unsigned int n_keys)
> > +{
> > +	struct hist_field *target_hist_field, *hist_field;
> > +	unsigned int n, i, j;
> > +
> > +	if (hist_data->n_fields - hist_data->n_vals != n_keys)
> > +		return false;
> > +
> > +	i = hist_data->n_vals;
> > +	j = target_hist_data->n_vals;
> > +
> > +	for (n = 0; n < n_keys; n++) {
> > +		hist_field = hist_data->fields[i + n];
> > +		target_hist_field = hist_data->fields[j + n];
> 
> Shouldn't it be 'target_hist_field = target_hist_data->fields[j + n]'?
>                                      ^^^^^^^^^^^^^^^^
> 

Yeah, thanks for point out the typo.

> > +
> > +		if (strcmp(hist_field->type, target_hist_field->type) != 0)
> > +			return false;
> > +		if (hist_field->size != target_hist_field->size)
> > +			return false;
> > +		if (hist_field->is_signed != target_hist_field->is_signed)
> > +			return false;
> > +	}
> > +
> > +	return true;
> > +}
> > +
> > +static struct hist_trigger_data *
> > +find_compatible_hist(struct hist_trigger_data *target_hist_data,
> > +		     struct trace_event_file *file)
> > +{
> > +	struct hist_trigger_data *hist_data;
> > +	struct event_trigger_data *test;
> > +	unsigned int n_keys;
> > +
> > +	n_keys = target_hist_data->n_fields - target_hist_data->n_vals;
> > +
> > +	list_for_each_entry_rcu(test, &file->triggers, list) {
> > +		if (test->cmd_ops->trigger_type == ETT_EVENT_HIST) {
> > +			hist_data = test->private_data;
> > +
> > +			if (compatible_keys(target_hist_data, hist_data, n_keys))
> > +				return hist_data;
> > +		}
> > +	}
> > +
> > +	return NULL;
> > +}
> > +
> > +static struct trace_event_file *event_file(char *system, char *event_name)
> > +{
> > +	struct trace_event_file *file;
> > +	struct trace_array *tr;
> > +
> > +	tr = top_trace_array();
> > +	if (!tr)
> > +		return ERR_PTR(-ENODEV);
> > +
> > +	file = find_event_file(tr, system, event_name);
> > +	if (!file)
> > +		return ERR_PTR(-EINVAL);
> > +
> > +	return file;
> > +}
> > +
> > +static struct hist_field *
> > +create_field_var_hist(struct hist_trigger_data *target_hist_data,
> > +		      char *system, char *event_name, char *field_name)
> 
> IIUC this is needed to create a new hist on a match_event only to
> provide a variable for a field, right?  I guess it's needed because
> adding a new variable is dangerous/unsafe for a running hist.
> 

Exactly.  I'll add a comment to make that clear.

Thanks,

Tom