Re: [PATCH 0/2 v2] Syscalls tracing

[Ingo Molnar <mingo@elte.hu>]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> And actually I don't think two copy_from_user will really change a 
> lot the tracing throughput.

Correct. It's already in the CPU cache so it is a performance 
non-issue and essentially for free. Copy avoidance is only an issue 
when touchig cache-cold data.

( Yes, a few cycles could be shaven off but the beauty of 
  all-encompassing non-source-code-invasive syscall tracing covering 
  hundreds of syscalls straight away trumps those concerns. )

> The idea would be now to join the syscalls metadata with such 
> quick handlers. We will have to think about how to join these in a 
> proper way.

We could allow per syscall tracepoints via the attribute table. The 
call signature could be a standard:

   long sys_call(unsinged long arg1, unsigned long arg2,
		 unsigned long arg3, unsigned long arg4,
		 unsigned long arg5, unsigned long arg6);

This would allow interested plugins/tools to install a system call 
specific callback. (we might allow two tracepoints - one before and 
one after the syscall)

The registration API could be driven by the name or by the syscall 
index - NR_sys_open or so.

	Ingo