linux-kernel - Re: [PATCH v2 33/40] tracing: Add hist trigger support for variable reference aliases

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Date:   Fri, 08 Sep 2017 14:41:42 -0500
From:   Tom Zanussi <tom.zanussi@...ux.intel.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     tglx@...utronix.de, mhiramat@...nel.org, namhyung@...nel.org,
        vedang.patel@...el.com, bigeasy@...utronix.de,
        joel.opensrc@...il.com, joelaf@...gle.com,
        mathieu.desnoyers@...icios.com, baohong.liu@...el.com,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v2 33/40] tracing: Add hist trigger support for variable
 reference aliases

On Fri, 2017-09-08 at 15:09 -0400, Steven Rostedt wrote:
> On Tue,  5 Sep 2017 16:57:45 -0500
> Tom Zanussi <tom.zanussi@...ux.intel.com> wrote:
> 
> > Add support for alias=$somevar where alias can be used as
> > onmatch($alias).
> 
> This change log is very lacking. What exactly is the purpose of alias?
> 
> Sounds like the variable is going undercover.
> 

This was a feature added at the request of a user, who wanted more
flexibility in making naming clearer in certain cases e.g.:

# echo 'hist:keys=next_pid:new_lat=$common_timestamp.usecs' > /sys/kernel/debug/tracing/events/sched/sched_switch/trigger

# echo 'hist:keys=pid:latency=$new_lat:onmatch(sched.sched_switch).wake2($latency,pid)' > /sys/kernel/debug/tracing/events/synthetic/wake1/trigger

I'll add a better description and exampe to the change log for this.

Tom