linux-kernel - Re: [PATCH 04/10][RFC] tracing: Remove per event trace registering

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Date:	Wed, 28 Apr 2010 20:00:18 -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 04/10][RFC] tracing: Remove per event trace registering

On Wed, 2010-04-28 at 16:44 -0400, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
> > From: Steven Rostedt <srostedt@...hat.com>
> > 
> > This patch removes the register functions of TRACE_EVENT() to enable
> > and disable tracepoints. The registering of a event is now down
> > directly in the trace_events.c file. The tracepoint_probe_register()
> > is now called directly.
> > 
> > The prototypes are no longer type checked, but this should not be
> > an issue since the tracepoints are created automatically by the
> > macros. If a prototype is incorrect in the TRACE_EVENT() macro, then
> > other macros will catch it.
> > 
> > The trace_event_class structure now holds the probes to be called
> > by the callbacks. This removes needing to have each event have
> > a separate pointer for the probe.
> > 
> > To handle kprobes and syscalls, since they register probes in a
> > different manner, a "reg" field is added to the ftrace_event_class
> > structure. If the "reg" field is assigned, then it will be called for
> > enabling and disabling of the probe for either ftrace or perf. To let
> > the reg function know what is happening, a new enum (trace_reg) is
> > created that has the type of control that is needed.
> > 
> > With this new rework, the 82 kernel events and 616 syscall events
> > has their footprint dramatically lowered:
> > 
> >    text	   data	    bss	    dec	    hex	filename
> > 5788186	1337252	9351592	16477030	 fb6b66	vmlinux.orig
> > 5792282	1333796	9351592	16477670	 fb6de6	vmlinux.class
> > 5793448	1333780	9351592	16478820	 fb7264	vmlinux.tracepoint
> > 5796926	1337748	9351592	16486266	 fb8f7a	vmlinux.data
> > 5774316	1306580	9351592	16432488	 fabd68	vmlinux.regs
> > 
> > The size went from 16477030 to 16432488, that's a total of 44K
> > in savings. With tracepoints being continuously added, this is
> > critical that the footprint becomes minimal.
> 
> Have you tried doing a BUILD_BUG_ON() on __typeof__() mismatch between
> the type of the callback generated by TRACE_EVENT() and the expected
> type ?  This might help catching tricky preprocessor macro errors early.

Well, we could, but if it is broken once, it is broken everywhere.

-- Steve


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