On Sun, 30 Jun 2024 16:07:43 -0500 Tom Zanussi zanussi@kernel.org 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=to...
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@kernel.org
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@kernel.org