linux-kernel - Re: [PATCH] ftrace: add an fsync tracer

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Date:	Thu, 6 Nov 2008 18:25:06 -0500
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, alan@...hat.com,
	jbaron@...hat.com
Subject: Re: [PATCH] ftrace: add an fsync tracer

Hi -

On Thu, Nov 06, 2008 at 05:25:29PM -0500, Mathieu Desnoyers wrote:
> [...]
> > Arjan asked for something to send formatted strings to tracing buffers
> > that are consumed by user space.  Whether internally that is done by
> > markers, or by tracepoints converted to markers, or by tracepoints
> > with custom ftrace_printf-y code, is indistinguishable to the user.
> > 
> > Can you be specific in what way any of that is exposing an "API
> > directly to userspace"?
> 
> The tracepoint declaration is linked to the kernel code that is meant to
> be traced. Given this code will change over time, it will be more
> difficult to perform those changes if it is in any way coupled with
> format strings exported to userspace via traced.  [...]

The userspace interface to all the various tracing widgets consists of
the textual formatted trace data stream.  ** IF ** these streams are
to represent a long-term interface, can you elaborate why you believe
it would be harder to generate identical text in the future with
markers rather than another technique with more internal layers?  Can
you work out an example?  

(BTW, is "traced" a typo?)

And as to that ** IF **: is there actually consensus that the format
of tracing data generated by e.g.  ftrace engines are supposed to be
preserved into the future versions, just as if they were a user-space
interface like /proc files?  (That's certainly not my impression.)

- FChE
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