linux-kernel - Re: [PATCH v4 8/8] trace: Add alternative synthetic event trace action syntax

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Message-ID: <1537298203.11628.8.camel@kernel.org>
Date:   Tue, 18 Sep 2018 14:16:43 -0500
From:   Tom Zanussi <zanussi@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     rostedt@...dmis.org, tglx@...utronix.de, namhyung@...nel.org,
        vedang.patel@...el.com, bigeasy@...utronix.de,
        joel@...lfernandes.org, mathieu.desnoyers@...icios.com,
        julia@...com, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v4 8/8] trace: Add alternative synthetic event trace
 action syntax

Hi Masami,

On Wed, 2018-09-19 at 03:54 +0900, Masami Hiramatsu wrote:
> Hi Tom,
> 
> On Mon, 10 Sep 2018 14:10:46 -0500
> Tom Zanussi <zanussi@...nel.org> wrote:
> 
> > From: Tom Zanussi <tom.zanussi@...ux.intel.com>
> > 
> > Add a 'trace(synthetic_event_name, params)' alternative to
> > synthetic_event_name(params).
> > 
> > Currently, the syntax used for generating synthetic events is to
> > invoke synthetic_event_name(params) i.e. use the synthetic event
> > name
> > as a function call.
> > 
> > Users requested a new form that more explicitly shows that the
> > synthetic event is in effect being traced.  In this version, a new
> > 'trace()' keyword is used, and the synthetic event name is passed
> > in
> > as the first argument.
> 
> Hmm, what is the advantage of adding this new form?
> 

There's no real advantage other than user preference - Namhyung thought
that since the event-name-as-function-call actions are all defined as
ACTION_TRACE, there should also be an explicit 'trace' action.

So I added it as alternative syntax - the event-name-as-function-call
form remains unchanged.

By the way, I also have a patch implementing your alternative syntax
change, where if you have only one handler, you can do away with the
explicit action.handler form e.g.

  # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-ts0: \
    onmax($wakeup_lat): \
    save(next_prio,next_comm,prev_pid,prev_prio,prev_comm):snapshot() \
    if next_comm=="cyclictest"' >> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger

It just wasn't ready to go in with the v4 patches yet, so I decided
to submit it later as a follow-on.

Tom


> Thanks,
>