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Subject: Re: [PATCH v2 3/3] tracing: Add support for dynamic strings to
 synthetic events
From:   Tom Zanussi <zanussi@kernel.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     rostedt@goodmis.org, axelrasmussen@google.com,
        linux-kernel@vger.kernel.org
Date:   Fri, 02 Oct 2020 17:06:17 -0500
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References: <cover.1601490263.git.zanussi@kernel.org>
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         <20201002161703.3961a96e84314fe31157f308@kernel.org>
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Hi Masami,

On Fri, 2020-10-02 at 16:17 +0900, Masami Hiramatsu wrote:
> Hi Tom,
> 
> On Wed, 30 Sep 2020 13:40:52 -0500
> Tom Zanussi <zanussi@kernel.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?
> 

Yes, I'll add it in the next version.

> > 
> > 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@kernel.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.
> 

Yeah, good to add regardless, will do.

Thanks,

Tom