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

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Message-ID: <20060915235707.GB29929@elte.hu>
Date:	Sat, 16 Sep 2006 01:57:07 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Nicholas Miell <nmiell@...cast.net>
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


* 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! ;)

	Ingo
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