linux-kernel - Re: [PATCH v2 3/3] tracing: Add support for dynamic strings to synthetic events

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Message-Id: <20201002161703.3961a96e84314fe31157f308@kernel.org>
Date:   Fri, 2 Oct 2020 16:17:03 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Tom Zanussi <zanussi@...nel.org>
Cc:     rostedt@...dmis.org, axelrasmussen@...gle.com, mhiramat@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] tracing: Add support for dynamic strings to
 synthetic events

Hi Tom,

On Wed, 30 Sep 2020 13:40:52 -0500
Tom Zanussi <zanussi@...nel.org> wrote:

> Currently, sythetic events only support static string fields such as:
> 
>   # echo 'test_latency u64 lat; char somename[32]' > /sys/kernel/debug/tracing/synthetic_events
> 
> Which is fine, but wastes a lot of space in the event.
> 
> It also prevents the most commonly-defined strings in the existing
> trace events e.g. those defined using __string(), from being passed to
> synthetic events via the trace() action.
> 
> With this change, synthetic events with dynamic fields can be defined:
> 
>   # echo 'test_latency u64 lat; char somename[]' > /sys/kernel/debug/tracing/synthetic_events

Could you add a testcase (and update existing one) of ftracetest
for this new feature too?

> 
> And the trace() action can be used to generate events using either
> dynamic or static strings:
> 
>   # echo 'hist:keys=name:lat=common_timestamp.usecs-$ts0:onmatch(sys.event).test_latency($lat,name)' > /sys/kernel/debug/tracing/events
> 
> The synthetic event dynamic strings are implemented in the same way as
> the existing __data_loc strings and appear as such in the format file.
> 
> Signed-off-by: Tom Zanussi <zanussi@...nel.org>
> ---
>  Documentation/trace/events.rst      |  15 +-
>  Documentation/trace/histogram.rst   |  18 +++
>  kernel/trace/synth_event_gen_test.c |  18 ++-
>  kernel/trace/trace_events_hist.c    |   9 ++
>  kernel/trace/trace_events_synth.c   | 239 ++++++++++++++++++++++++----
>  kernel/trace/trace_synth.h          |   4 +

And you might also need to update tracefs/README so that user
can check whether the kernel supports dynamic string or not.


Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>