Messages in this thread |  | | Date | Thu, 14 Nov 2013 23:48:12 -0500 | From | Steven Rostedt <> | Subject | Why have another variable deciding a tracepoint? |
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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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