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

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Steven Rostedt (rostedt@goodmis.org) wrote:
> On Wed, 2010-04-28 at 16:44 -0400, Mathieu Desnoyers wrote:
> > * Steven Rostedt (rostedt@goodmis.org) wrote:
> > > From: Steven Rostedt <srostedt@redhat.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.

I fear about "subtly" broken things, where trace data could end up being
incorrectly typed and/or corrupted. I think this BUILD_BUG_ON() will
become very useful.

Thanks,

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com