linux-kernel - Re: [patch-early-RFC 00/10] LTTng architecture dependent instrumentation

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Message-ID: <20071206101112.GB17299@elte.hu>
Date:	Thu, 6 Dec 2007 11:11:12 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [patch-early-RFC 00/10] LTTng architecture dependent
	instrumentation

hi Mathieu,

* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> Hi,
> 
> Here is the architecture dependent instrumentation for LTTng. [...]

A fundamental observation about markers, and i raised this point many 
many months ago already, so it might sound repetitive, but i'm unsure 
wether it's addressed. Documentation/markers.txt still says:

| * Purpose of markers
|
| A marker placed in code provides a hook to call a function (probe) 
| that you can provide at runtime. A marker can be "on" (a probe is 
| connected to it) or "off" (no probe is attached). When a marker is 
| "off" it has no effect, except for adding a tiny time penalty 
| (checking a condition for a branch) and space penalty (adding a few 
| bytes for the function call at the end of the instrumented function 
| and adds a data structure in a separate section).

could you please eliminate the checking of the flag, and insert a pure 
NOP sequence by default (no extra branches), which is then patched in 
with a function call instruction sequence, when the trace point is 
turned on? (on architectures that have code patching infrastructure - 
such as x86)

	Ingo
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