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Message-Id: <20240703080814.c8bbe5b27ad70a5520262a6d@kernel.org>
Date: Wed, 3 Jul 2024 08:08:14 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Tom Zanussi <zanussi@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Shuah Khan <shuah@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] tracing: Support poll on event hist file
On Sun, 30 Jun 2024 16:07:43 -0500
Tom Zanussi <zanussi@...nel.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@...nel.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@...nel.org>
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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