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

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Message-ID: <20081106215310.GA15163@Krystal>
Date:	Thu, 6 Nov 2008 16:53:11 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>,
	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

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Thu, 2008-11-06 at 15:19 -0500, Frank Ch. Eigler wrote:
> > 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.
> 
> 
> 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.
> 
> 

Argh. No, please. Doing this would end up exposing the inner kernel API
(the tracepoints) directly to userspace.

I've done an addition to the markers which does approximately the same.
It's in the -lttng tree now, I should post it soon.

A new marker type, trace_mark_tp() also takes a tracepoint name as
parameter. When the tracepoint is enabled, it also enables the
associated tracepoint. And it makes sure tracepoints are not exposed to
userspace.

Mathieu



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Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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