Re: [PATCH 02/10][RFC] tracing: Let tracepoints have data passed to tracepoint callbacks

[Li Zefan <lizf@cn.fujitsu.com>]

Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@redhat.com>
> 
> This patch allows data to be passed to the tracepoint callbacks
> if the tracepoint was created to do so.
> 
> If a tracepoint is defined with:
> 
> DECLARE_TRACE_DATA(name, proto, args)
> 
> Then a registered function can also register data to be passed
> to the tracepoint as such:
> 
>   DECLARE_TRACE_DATA(mytracepoint, TP_PROTO(int status), TP_ARGS(status));
> 
>   /* In the C file */
> 
>   DEFINE_TRACE(mytracepoint, TP_PROTO(int status), TP_ARGS(status));
> 
>   [...]
> 
>        trace_mytacepoint(status);
> 
>   /* In a file registering this tracepoint */
> 
>   int my_callback(int status, void *data)
>   {
> 	struct my_struct my_data = data;
> 	[...]
>   }
> 
>   [...]
> 	my_data = kmalloc(sizeof(*my_data), GFP_KERNEL);
> 	init_my_data(my_data);
> 	register_trace_mytracepoint_data(my_callback, my_data);
> 
> The same callback can also be registered to the same tracepoint as long
> as the data registered is the same. Note, the data must also be used
> to unregister the callback:
> 
> 	unregister_trace_mytracepoint_data(my_callback, my_data);
> 
> Because of the data parameter, tracepoints declared this way can not have
> no args. That is:
> 
>   DECLARE_TRACE_DATA(mytracepoint, TP_PROTO(void), TP_ARGS());
> 
> will cause an error, but the original DECLARE_TRACE still allows for this.
> 
> The DECLARE_TRACE_DATA() will be used by TRACE_EVENT() so that it
> can reuse code and bring the size of the tracepoint footprint down.
> This means that TRACE_EVENT()s must have at least one argument defined.

We have to define at least on argument in TRACE_EVENT() even without
this patch, otherwise it'll cause compile error while expanding the
macros.

> This should not be a problem since we should never have a static
> tracepoint in the kernel that simply says "Look I'm here!".
> 

We do have such a tracepoint. ;)

That is trace_power_end, and it uses a dummy argument merely for
passing compilation.