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

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Date:	Thu, 06 Nov 2008 15:19:48 -0500
From:	fche@...hat.com (Frank Ch. Eigler)
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, alan@...hat.com,
	jbaron@...hat.com
Subject: Re: [PATCH] ftrace: add an fsync tracer

Arjan van de Ven <arjan@...radead.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.

- FChE
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