Re: [PATCH v7 24/37] tracing: Add support for 'synthetic' events

[Tom Zanussi <tom.zanussi@linux.intel.com>]

Hi  Namhyung,

On Tue, 2017-12-12 at 23:42 +0900, Namhyung Kim wrote:
> On Wed, Dec 06, 2017 at 04:38:05PM -0600, Tom Zanussi wrote:
> > Synthetic events are user-defined events generated from hist trigger
> > variables saved from one or more other events.
> > 
> > To define a synthetic event, the user writes a simple specification
> > consisting of the name of the new event along with one or more
> > variables and their type(s), to the tracing/synthetic_events file.
> > 
> > For instance, the following creates a new event named 'wakeup_latency'
> > with 3 fields: lat, pid, and prio:
> > 
> >     # echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
> >       /sys/kernel/debug/tracing/synthetic_events
> > 
> > Reading the tracing/synthetic_events file lists all the
> > currently-defined synthetic events, in this case the event we defined
> > above:
> > 
> >     # cat /sys/kernel/debug/tracing/synthetic_events
> >     wakeup_latency u64 lat; pid_t pid; int prio
> > 
> > At this point, the synthetic event is ready to use, and a histogram
> > can be defined using it:
> > 
> >     # echo 'hist:keys=pid,prio,lat.log2:sort=pid,lat' >> \
> >     /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
> > 
> > The new event is created under the tracing/events/synthetic/ directory
> > and looks and behaves just like any other event:
> > 
> >     # ls /sys/kernel/debug/tracing/events/synthetic/wakeup_latency
> >       enable  filter  format  hist  id  trigger
> > 
> > Although a histogram can be defined for it, nothing will happen until
> > an action tracing that event via the trace_synth() function occurs.
> > The trace_synth() function is very similar to all the other trace_*
> > invocations spread throughout the kernel, except in this case the
> > trace_ function and its corresponding tracepoint isn't statically
> > generated but defined by the user at run-time.
> > 
> > How this can be automatically hooked up via a hist trigger 'action' is
> > discussed in a subsequent patch.
> > 
> > Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
> > [fix noderef.cocci warnings, sizeof pointer for kcalloc of event->fields]
> > Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
> > ---
> 
> [SNIP]
> > +static enum print_line_t print_synth_event(struct trace_iterator *iter,
> > +					   int flags,
> > +					   struct trace_event *event)
> > +{
> > +	struct trace_array *tr = iter->tr;
> > +	struct trace_seq *s = &iter->seq;
> > +	struct synth_trace_event *entry;
> > +	struct synth_event *se;
> > +	unsigned int i, n_u64;
> > +	char print_fmt[32];
> > +	const char *fmt;
> > +
> > +	entry = (struct synth_trace_event *)iter->ent;
> > +	se = container_of(event, struct synth_event, call.event);
> > +
> > +	trace_seq_printf(s, "%s: ", se->name);
> > +
> > +	for (i = 0, n_u64 = 0; i < se->n_fields; i++) {
> > +		if (trace_seq_has_overflowed(s))
> > +			goto end;
> > +
> > +		fmt = synth_field_fmt(se->fields[i]->type);
> > +
> > +		/* parameter types */
> > +		if (tr->trace_flags & TRACE_ITER_VERBOSE)
> > +			trace_seq_printf(s, "%s ", fmt);
> > +
> > +		snprintf(print_fmt, sizeof(print_fmt), "%%s=%s%%s", fmt);
> > +
> > +		/* parameter values */
> > +		if (se->fields[i]->is_string) {
> > +			trace_seq_printf(s, print_fmt, se->fields[i]->name,
> > +					 (char *)(long)entry->fields[n_u64],
> 
> Hmm.. shouldn't it be (char *)&entry->fields[n_u64] ?
> 

Yes, thanks for pointing this out - looks like I need to add something
that enables string-containing synthetic events and looks at the trace
file display to my testing..

Tom

> Thanks,
> Namhyung
> 
> 
> > +					 i == se->n_fields - 1 ? "" : " ");
> > +			n_u64 += STR_VAR_LEN_MAX / sizeof(u64);
> > +		} else {
> > +			trace_seq_printf(s, print_fmt, se->fields[i]->name,
> > +					 entry->fields[n_u64],
> > +					 i == se->n_fields - 1 ? "" : " ");
> > +			n_u64++;
> > +		}
> > +	}
> > +end:
> > +	trace_seq_putc(s, '\n');
> > +
> > +	return trace_handle_return(s);
> > +}