trace_printk - printf formatting in the ftrace buffer
__trace_printk is an internal function for trace_printk and the ip is passed in via the trace_printk macro.
This function allows a kernel developer to debug fast path sections that printk is not appropriate for. By scattering in various printk like tracing in the code, a developer can quickly see where problems are occurring.
This is intended as a debugging tool for the developer only. Please refrain from leaving trace_printks scattered around in your code. (Extra memory is used for special buffers that are allocated when trace_printk is used)
A little optization trick is done here. If there's only one argument, there's no need to scan the string for printf formats. The trace_puts will suffice. But how can we take advantage of using trace_puts when trace_printk has only one argument? By stringifying the args and checking the size we can tell whether or not there are args. __stringify((__VA_ARGS__)) will turn into “()\0” with a size of 3 when there are no args, anything else will be bigger. All we need to do is define a string to this, and then take its size and compare to 3. If it's bigger, use do_trace_printk otherwise, optimize it to trace_puts. Then just let gcc optimize the rest.
|January 2017||Kernel Hackers Manual 4.8.|