Re: [PATCH v8 24/37] tracing: Add support for 'synthetic' events

Namhyung Kim
2018-01-10T05:44:24Z

Hi Tom,

On Thu, Dec 21, 2017 at 10:02:46AM -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 int release_all_synth_events(void)
> +{
> + struct list_head release_events;
> + struct synth_event *event, *e;
> + int ret = 0, err = 0;
> +
> + INIT_LIST_HEAD(&release_events);
> +
> + mutex_lock(&synth_event_mutex);
> +
> + list_for_each_entry(event, &synth_event_list, list) {
> + if (event->ref) {
> + mutex_unlock(&synth_event_mutex);
> + return -EBUSY;
> + }
> + }
> +
> + list_for_each_entry_safe(event, e, &synth_event_list, list)
> + list_move(&event->list, &release_events);

list_splice_init() ?

> +
> + mutex_unlock(&synth_event_mutex);
> +
> + list_for_each_entry_safe(event, e, &release_events, list) {
> + list_del(&event->list);
> +
> + ret = unregister_synth_event(event);
> + add_or_delete_synth_event(event, !ret);
> + }
> +
> + if (err)
> + ret = err;

The err was never used.

> +
> + return ret;
> +}