linux-kernel - Re: [PATCH] tracing/markers: make markers select tracepoints

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Message-ID: <20090223171032.GA2566@redhat.com>
Date:	Mon, 23 Feb 2009 12:10:32 -0500
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, zippel@...ux-m68k.org,
	linux-kbuild@...r.kernel.org
Subject: Re: [PATCH] tracing/markers: make markers select tracepoints

On Mon, Feb 23, 2009 at 05:22:39PM +0100, Peter Zijlstra wrote:
> [...]
> > > Not so. In both cases the regular stuff (NMI trace, OOPS,
> > > function/graph/sched trace, etc) is not enough and you wish to
> > > augment its output.
> > 
> > Sorry, I don't see how that relates.  If the general function tracing
> > widgetry is insufficient for some subsystem/purpose, some sort of
> > static instrumentation is needed.  Whether that instrumentation is
> > done by markers (with a thin glue to ftrace) or by tracepoints (with a
> > thick glue to ftrace) doesn't change the need for "augmentation".
> 
> I'm not arguing against static instrumentation per-se (although
> expanding the coverage of dynamic/automatic instrumentation is much more
> profitable IMHO).

Much prior discussion (incl. at the kernel summit) indicates that we
need both.

> What I'm arguing is that trace_mark()s one distinguishing feature over
> tracepoints is only suited for quick debug like work.

I see where you're coming from, but one may also caricaturize the
other alternative as requiring make-work glue code to pack & unpack
all the same inforation.


> Furthermore, trace_mark() exposes that crap like an ABI, now suppose
> some distro goes and declares that stable for some daft reason,
> imagine the poor sod having to fix something littered with
> trace_mark().

The impression that this is somehow different with tracepoints is
mistaken.  Tracepoints are *exactly* as "ABI-like" as markers.


> [...]  presenting that information in big bloated blobs is beyond
> that scope.

Do you have some specific bloated blobs in mind?  It's not as if the
rendered text is necessarily much bigger than a struct containing all
the same parameters.  Consider all the fields rounded up to 4 or 8
bytes each.


- FChE
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