Re: [RFC PATCH 0/3] tracing: Support poll on event hist file

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

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=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@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>
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>