On Sun, 30 Jun 2024 16:07:43 -0500 Tom Zanussi <zanussi@xxxxxxxxxx> wrote: > 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> Thanks Tom! I found an issue in the selftests (can not support old stable kernel) so let me update it. Thank you, > > > > > --- > > > > 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> > -- Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
