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Subject: Re: [PATCH] ftrace: add an fsync tracer
From: Peter Zijlstra <peterz@infradead.org>
To: "Frank Ch. Eigler" <fche@redhat.com>
Cc: Arjan van de Ven <arjan@infradead.org>,
       Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       mingo@elte.hu, alan@redhat.com, jbaron@redhat.com,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
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Date: Thu, 06 Nov 2008 21:29:03 +0100
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On Thu, 2008-11-06 at 15:19 -0500, Frank Ch. Eigler wrote:
> Arjan van de Ven <arjan@infradead.org> writes:
> 
> > [...]
> > what is the real need is
> > 1) Have a trace point in the source
> > 2) Associate a "formatting function" with that point 
> >    (which basically transforms the trace parameters to, say, a string)
> > 3) A way to turn the trace point on/off.
> 
> For 1 and 2, it may be worth considering a plain trace_mark() in
> do_sync().  The complication that makes this uglier than a one-liner
> is d_path()'s buffer and error handling.
> 
>     {
>     char *buffer = kzalloc (4096, GFP_KERNEL);
>     trace_mark(fsync, "Process %s is calling fsync on %s\n",
>                        current->comm,
>                        ({char *err = d_path (...);
>                          IS_ERR(err) ? "?" : err;}));
>     kfree (buffer);
>     }
> 
> With a bit of extension on the marker front, the allocation could be
> made conditional on the marker being enabled.
> 
> 
> For 3, the kernel could merge a backend that connects arbitrary
> markers to an ftrace (or whatever) buffer.  Several compact prototypes
> for the latter exist.


I prefer we keep using trace points but do what jason has been proposing
for a while, which is add a format and arg list to the trace point
definition.

Something like

DEFINE_TRACE_FMT(sched_switch,
	TPPROTO(struct rq *rq, struct task_struct *prev,
		 struct task_struct *next),
	TPARGS(rq, prev, next),
	TPFMT("%d to %d\n", prev->pid, next->pid));

Which would be similar to attaching a trace_mark() to the trace point
and can in these cases save a lot of lines of code.

Both lttng and the ftrace event tracer can use these default text
strings.