linux-kernel - Re: [PATCH 09/10][RFC] tracing: Remove duplicate id information in event structure

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Date:	Wed, 28 Apr 2010 20:04:11 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 09/10][RFC] tracing: Remove duplicate id information in
 event structure

On Wed, 2010-04-28 at 17:06 -0400, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
> > From: Steven Rostedt <srostedt@...hat.com>
> > 
> > Now that the trace_event structure is embedded in the ftrace_event_call
> > structure, there is no need for the ftrace_event_call id field.
> > The id field is the same as the trace_event type field.
> > 
> > Removing the id and re-arranging the structure brings down the tracepoint
> > footprint by another 5K.
> 
> I might have missed it, but how exactly is the event type allocated
> uniquely ? Is it barely a duplicate of the call "id" field ?

It is allocated in kernel/trace/trace_events.c.

The code there scans the "_ftrace_events" section to find all the events
that are created, and it assigns the events a unique id. Currently the
id is just copied from the trace_event field to the ftrace_event_call
data. The two are the same.

-- Steve


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