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To: Arjan van de Ven <arjan@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
       Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
       mingo@elte.hu, alan@redhat.com, jbaron@redhat.com
Subject: Re: [PATCH] ftrace: add an fsync tracer
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From: fche@redhat.com (Frank Ch. Eigler)
Date: Thu, 06 Nov 2008 15:19:48 -0500
In-Reply-To: <20081106094558.50d94bcc@infradead.org> (Arjan van de Ven's message of "Thu, 6 Nov 2008 09:45:58 -0800")
Message-ID: <y0mej1ofvyz.fsf@ton.toronto.redhat.com>
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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.


- FChE