linux-kernel - Re: [PATCH v6 0/3] tracing: Support poll on event hist file

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Message-Id: <20241029100356.099a6e639846aae4d0a6083f@kernel.org>
Date: Tue, 29 Oct 2024 10:03:56 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Shuah Khan <shuah@...nel.org>, Tom
 Zanussi <zanussi@...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: [PATCH v6 0/3] tracing: Support poll on event hist file

On Wed, 16 Oct 2024 19:49:15 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:

> Overview
> --------
> 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.

Note: This `hist` is not disabled by tracing_on interface, because
that interface only disables `recording`.
Thus to monitor events via this interface, user must ensure the
tracing_on is enabled, also, set the same "filter" to the hist
action.

Thank you,

> 
> 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.
> 
> Usage
> -----
> 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 pri $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,
> ---
> 
> 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                       |   14 +++
>  kernel/trace/trace_events.c                        |   14 +++
>  kernel/trace/trace_events_hist.c                   |  100 +++++++++++++++++++-
>  tools/testing/selftests/ftrace/Makefile            |    2 
>  tools/testing/selftests/ftrace/poll.c              |   74 +++++++++++++++
>  .../ftrace/test.d/trigger/trigger-hist-poll.tc     |   74 +++++++++++++++
>  6 files changed, 275 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>