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

[Tom Zanussi]

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>