linux-kernel - Why have another variable deciding a tracepoint?

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Message-ID: <20131114234812.3bf7bce9@gandalf.local.home>
Date:	Thu, 14 Nov 2013 23:48:12 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Terje Bergstrom <tbergstrom@...dia.com>,
	Arto Merilainen <amerilainen@...dia.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	Erik Faye-Lund <kusmabite@...il.com>
Subject: Why have another variable deciding a tracepoint?

I've been reviewing different users of tracepoints and I stumbled
across this:

drivers/gpu/host1x/cdma.c: host1x_cdma_push()

	if (host1x_debug_trace_cmdbuf)
		trace_host1x_cdma_push(dev_name(cdma_to_channel(cdma)->dev),
				       op1, op2);

That host1x_debug_trace_cmdbuf is a variable that gets set by another
debugfs file "trace_cmdbuf" that is custom to this driver.

Why?

The tracepoint host1x_cdma_push is already controlled by either ftrace
or perf. If it gets enabled by perf or ftrace, it still wont be traced
unless we also enable this trace_cmdbuf. Is there some reason for this?
I can't figure it out from the change log: 6236451d83a720 ("gpu:
host1x: Add debug support").

As tracepoints uses jump labels, there is no branch cost associated
with them. That is, they are either a direct jump, or a nop (in most
cases a nop). But here you added the overhead of a conditional branch
depending on this variable.

If this is truly needed, then use TRACE_EVENT_CONDITION() for that
tracepoint.

/me is baffled

-- Steve

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