On Fri, Feb 10, 2017 at 12:58:30PM -0600, Tom Zanussi wrote: > Hi Masami, > > On Fri, 2017-02-10 at 18:34 +0900, Masami Hiramatsu wrote: > > On Fri, 10 Feb 2017 13:16:17 +0900 > > Namhyung Kim <namhyung@xxxxxxxxxx> wrote: > > > > > > ==== > > > > Example - wakeup latency > > > > ==== > > > > > > > > This basically implements the -RT latency_hist 'wakeup_latency' > > > > histogram using the synthetic events, variables, and actions > > > > described. The output below is from a run of cyclictest using the > > > > following command: > > > > > > > > # rt-tests/cyclictest -p 80 -n -s -t 2 > > > > > > > > What we're measuring the latency of is the time between when a > > > > thread (of cyclictest) is awakened and when it's scheduled in. To > > > > do that we add triggers to sched_wakeup and sched_switch with the > > > > appropriate variables, and on a matching sched_switch event, > > > > generate a synthetic 'wakeup_latency' event. Since it's just > > > > another trace event like any other, we can also define a histogram > > > > on that event, the output of which is what we see displayed when > > > > reading the wakeup_latency 'hist' file. > > > > > > > > First, we create a synthetic event called wakeup_latency, that > > > > references 3 variables from other events: > > > > > > > > # echo 'wakeup_latency lat=sched_switch:wakeup_lat \ > > > > pid=sched_switch:woken_pid \ > > > > prio=sched_switch:woken_prio' >> \ > > > > /sys/kernel/debug/tracing/synthetic_events > > > > > > > > Next we add a trigger to sched_wakeup, which saves the value of the > > > > 'common_timestamp' when that event is hit in a variable, ts0. Note > > > > that this happens only when 'comm==cyclictest'. > > > > > > > > Also, 'common_timestamp' is a new field defined on every event (if > > > > needed - if there are no users of timestamps in a trace, timestamps > > > > won't be saved and there's no additional overhead from that). > > > > > > > > # echo 'hist:keys=pid:ts0=common_timestamp.usecs if \ > > > > comm=="cyclictest"' >> \ > > > > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger > > > > > > > > Next, we add a trigger to sched_switch. When the pid being switched > > > > to matches the pid woken up by a previous sched_wakeup event, this > > > > event grabs the ts0 saved on that event, takes the difference > > > > between it and the current sched_switch's common_timestamp, and > > > > assigns it to a new 'wakeup_lat' variable. It also saves a couple > > > > other variables and then invokes the onmatch().trace() action which > > > > generates a new wakeup_latency event using those variables. > > > > > > > > # echo 'hist:keys=woken_pid=next_pid:woken_prio=next_prio:\ > > > > wakeup_lat=common_timestamp.usecs-ts0:onmatch().trace(wakeup_latency) \ > > > > if next_comm=="cyclictest"' >> \ > > > > /sys/kernel/debug/tracing/events/sched/sched_switch/trigger > > > > > > As Masami said, I think the syntax is a bit hard to understand. Also > > > it'd be nice to access an event field directly (i.e. not by adding a > > > field in a hist). Maybe we can use a prefix like '$' to identify hist > > > fields.. > > > > Ah that's a nice idea! > > > > > > > > How about below? > > > > > > # echo 'wakeup_latency \ > > > lat=sched_switch.$wakeup_lat \ > > > pid=sched_switch.next_pid \ > > > prio=sched_switch.next_prio' >> \ > > > /sys/kernel/debug/tracing/synthetic_events > > > > Should we define these parameter assignment at this.point? > > > > I think this syntax binds wakeup_latency event to sched_switch too tight. I > > mean, if someone kicks this event from some other event, it may easily lose > > values. > > So, at this point, we will define event name and what parameters it has, > > until binding this event to onmatch(). > > > > Right, I agree this binding doesn't need to be done here, good idea to > defer it as below... > > > > # echo 'hist: \ > > > keys=pid: \ > > > ts0=common_timestamp.usec \ > > > if comm=="cyclictest"' >> \ > > > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger > > > > > > # echo 'hist: \ > > > keys=next_pid: \ > > > wakeup_lat=common_timestamp.usec-$ts0: \ > > > onmatch(sched_wakeup).trace(wakeup_latency) \ > > > > This one seems much better for me, but I would like to ask you call event > > directly from onmatch, like as > > > > "onmatch(sched_wakeup).wakeup_latency(wakeup_lat,next_pid,next_prio)" > > > > At this point, kernel will finalize the wakeup_latency event with wakeup_lat, > > next_pid and next_prio. > > > > Yes, I like this much better - things are no longer so implicit and > therefore subject to confusion, and the syntax itself makes more sense, > even if it is a bit more verbose on the trigger, which is fine. I thought about it too, but it needs to add some kind of type checking then. What if another hist generates the event with totally different info? Thanks, Namhyung -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
