Hi Masami, On Wed, 2024-06-26 at 00:16 +0900, Masami Hiramatsu (Google) wrote: > Hi, > > Here is an RFC patch to support polling on event 'hist' file. > > There has been interest in allowing user programs to monitor kernel > events in real time. Ftrace provides `trace_pipe` interface to wait > on events in the ring buffer, but it is needed to wait until filling > up a page with events in the ring buffer. We can also peek the > `trace` file periodically, but that is inefficient way to monitor > a randomely happening event. > > This patch set allows user to `poll`(or `select`, `epoll`) on event > histogram interface. As you know each event has its own `hist` file > which shows histograms generated by trigger action. So user can set > a new hist trigger on any event you want to monitor, and poll on the > `hist` file until it is updated. > > There are 2 poll events are supported, POLLIN and POLLPRI. POLLIN > means that there are any readable update on `hist` file and this > event will be flashed only when you call read(). So, this is > useful if you want to read the histogram periodically. > The other POLLPRI event is for monitoring trace event. Like the > POLLIN, this will be returned when the histogram is updated, but > you don't need to read() the file and use poll() again. > > Note that this waits for histogram update (not event arrival), thus > you must set a histogram on the event at first. > > Here is an example usage: > > ---- > TRACEFS=/sys/kernel/tracing > EVENT=$TRACEFS/events/sched/sched_process_free > > # setup histogram trigger and enable event > echo "hist:key=comm" >> $EVENT/trigger > echo 1 > $EVENT/enable > > # Wait for update > poll $EVENT/hist > > # Event arrived. > echo "process free event is comming" > tail $TRACEFS/trace > ---- > > The 'poll' command is in the selftest patch. > > You can take this series also from here; > > https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/log/?h=topic/event-hist-poll > > Thank you, I think this is a clever use of the histogram files, and will be very useful for real-time monitoring apps. I'm looking forward to using it myself - thanks for doing this. For the whole series, Reviewed-by: Tom Zanussi <zanussi@xxxxxxxxxx> > > --- > > Masami Hiramatsu (Google) (3): > tracing/hist: Add poll(POLLIN) support on hist file > tracing/hist: Support POLLPRI event for poll on histogram > selftests/tracing: Add hist poll() support test > > > include/linux/trace_events.h | 5 + > kernel/trace/trace_events.c | 18 ++++ > kernel/trace/trace_events_hist.c | 101 > +++++++++++++++++++- > tools/testing/selftests/ftrace/Makefile | 3 + > tools/testing/selftests/ftrace/poll.c | 34 +++++++ > .../ftrace/test.d/trigger/trigger-hist-poll.tc | 46 +++++++++ > 6 files changed, 204 insertions(+), 3 deletions(-) > create mode 100644 tools/testing/selftests/ftrace/poll.c > create mode 100644 > tools/testing/selftests/ftrace/test.d/trigger/trigger-hist-poll.tc > > -- > Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
