linux-kernel - Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

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Date:	Fri, 15 Sep 2006 17:41:22 -0700
From:	Nicholas Miell <nmiell@...cast.net>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Roman Zippel <zippel@...ux-m68k.org>,
	Thomas Gleixner <tglx@...utronix.de>, karim@...rsys.com,
	Andrew Morton <akpm@...l.org>,
	Paul Mundt <lethal@...ux-sh.org>, Jes Sorensen <jes@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Tom Zanussi <zanussi@...ibm.com>, ltt-dev@...fik.org,
	Michel Dagenais <michel.dagenais@...ymtl.ca>
Subject: Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

On Sat, 2006-09-16 at 01:57 +0200, Ingo Molnar wrote:
> * Nicholas Miell <nmiell@...cast.net> wrote:
> 
> > You're going to want to be able to trace every function in the kernel, 
> > which means they'd all need a __trace -- and in that case, a 
> > -fpad-functions-for-tracing gcc option would make more sense then 
> > per-function attributes.
> 
> the __trace attribute would be a _specific_ replacement for a _specific_ 
> static markup at the entry of a function. So no, we would not want to 
> add __trace to _every_ function in the kernel: only those which get 
> commonly traced. And note that SystemTap can trace the rest too, just 
> with slighly higher overhead.
> 
> In that sense __trace is not an enabling infrastructure, it's a 
> performance tuning infrastructure.
> 
> > The option could also insert NOPs before RETs, not just before the 
> > prologue so that function returns are equally easy to trace. (It might 
> > also inhibit tail calls, assuming being able to trace all function 
> > returns is more important than that optimization.)
> 
> yeah. __trace_entry and __trace_exit [or both] attributes. Makes sense.
> 
> > And SystemTap can already hook into sock_sendmsg() (or any other 
> > function) and examine it's arguments -- all of this GCC extension talk 
> > is just performance enhancement.
> 
> yes, yes, yes, exactly!!! Finally someone reads my mails and understands 
> my points. There's hope! ;)

I'm not sure that I do, actually.

You seem to be opposed to all static probe markers in general, but I
think that they'd be useful for big abstract things like "new thread
created" (which would encompass fork/vfork/clone and probably consist of
a single marker in do_fork) or for similar things that happen all over
the kernel (for example, I imagine that all filesystems would want to
use the same set of probe names just to make I/O tracing easier for
userspace).




-- 
Nicholas Miell <nmiell@...cast.net>

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