Re: [PATCH] tracing: Add task_prctl_unknown tracepoint

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 6 Nov 2024 19:12:42 +0100
Marco Elver <elver@google.com> wrote:

> No other events should be traced. This is the test program I've used:
> 
> 	#include <sys/prctl.h>
> 	#include <unistd.h>
> 
> 	int main(int argc, char *argv[])
> 	{
> 	  prctl(1234, 101, 102, 103, 104);
> 	  if (argc > 1)
> 	    usleep(1000);
> 	  return 0;
> 	}
> 
> Kernel config is x86_64 default + CONFIG_FUNCTION_TRACER=y +
> CONFIG_FTRACE_SYSCALLS=y. For the test, once booted all I do is:
> 
> 	% echo 1 > /sys/kernel/debug/tracing/events/task/task_prctl_unknown/enable
> 	% cat /sys/kernel/debug/tracing/trace_pipe
> 	... wait for output ...
> 
> That's pretty much it. I've attached my kernel config just in case I
> missed something.

OK, it's because you are using trace_pipe (which by the way should not be
used for anything serious). The read of trace_pipe flushes the buffer
before the task is scheduled out and the comm saved, so it prints the
"<...>". If you instead do the cat of trace_pipe *after* running the
command, you'll see the comm.

So this is just because you are using the obsolete trace_pipe.

-- Steve