linux-kernel - Re: tracepoint maintainance models

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Message-ID: <20060918122527.GC3951@redhat.com>
Date:	Mon, 18 Sep 2006 08:25:27 -0400
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Karim Yaghmour <karim@...rsys.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jes Sorensen <jes@....com>,
	Andrew Morton <akpm@...l.org>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>,
	"Martin J. Bligh" <mbligh@...igh.org>
Subject: Re: tracepoint maintainance models

Hi -

mingo wrote:

> [...]
>   static int x;
>   void func(int a)
>        MARK(event, a);
>
> if a dynamic tracer installs a probe into that MARK() spot, it will
> have access to 'a', but it can also have access to 'x'. While a
> static in-source markup for _static tracers_, if it also wanted to
> have the 'x' information, would also have to add 'x' as a parameter:
> [...]

Without heroic measures taken by by a static tracer type of tool, this
is correct.

> For dynamic tracers no such 'parameter preparation' instructions
> would need to be generated by gcc.  (thus for example the runtime
> overhead would be lower for inactive tracepoints)

Any such additional code would be small, plus if properly marked up
with unlikely() and compiled with -freorder-blocks, it would all be
out-of-line.  This small cost could be worth the added benefit of
systemtap being able to probe that point without debugging information
present, and avoiding its slow & deliberate way of accessing
target-side variables like $x.  (The slow & deliberate part comes in
from the need to check any pointer dereferences involved.)

- FChE
-
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