Re: [PATCH] tracing/function-graph-tracer: tracing only syscalls mode

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Jan 02, 2009 at 08:40:14PM +0100, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > Impact: make more easy the syscalls tracing
> > 
> > This patch extends to syscalls the features which let one to trace
> > only one ore more function (and those they call).
> > This way we can get rid of the workqueues, kernel threads, softirqs from
> > ksoftirqd and only have the syscall path on the trace.
> > 
> > Note that hardirq that interrupt the syscalls are still traced. But we could
> > add an option to disable the interrupt tracing as well in the future.
> > 
> > To use this new feature:
> > 
> > echo function_graph > /debugfs/tracing/current_tracer
> > echo syscalls > /debugfs/tracing/set_graph_function
> 
> nice. I think a key item is missing though:
> 
> > @@ -1397,6 +1398,9 @@ asmregparm long syscall_trace_enter(struct pt_regs *regs)
> >  {
> >  	long ret = 0;
> >  
> > +	/* Tell the function graph tracer we are entering a system call */
> > +	ftrace_graph_syscall_enter();
> 
> We could actually trace the _syscalls_ themselves, a'la strace:


"a la" ? Oh pretty, those two words have been picked from french to english
language :-)
I didn't know.

Yes, these parameters displaying a la strace are in my projects, as well as
the return value of the syscalls (and other functions later).

 
>     munmap(0xb7ff3000, 4096)                = 0
> 
> So instead of the current ftrace output:
> 
>   0)   cc1-30212    |               |  sys32_mmap2() {
>   0)   cc1-30212    |               |    down_write() {
>   0)   cc1-30212    |   0.272 us    |      _spin_lock_irq();
>   0)   cc1-30212    |   0.969 us    |    }
>   0)   cc1-30212    |               |    do_mmap_pgoff() {
> 
> We could get something like:
> 
>   0)   cc1-30212    |               |  sys32_mmap2(0xb7ff3000, 4096) {
>   0)   cc1-30212    |               |    down_write() {
>   0)   cc1-30212    |   0.272 us    |      _spin_lock_irq();
>   0)   cc1-30212    |   0.969 us    |    }
>   0)   cc1-30212    |               |    do_mmap_pgoff() {
>   [...]
>   0)   cc1-30212    | + 22.537 us   |  } = 0
> 
> 
> Or maybe as separate trace entries:
> 
>   0)   cc1-30212    |               |> sys32_mmap2 [0xb7ff3000, 4096]
>   0)   cc1-30212    |               |  sys32_mmap2 {
>   0)   cc1-30212    |               |    down_write() {
>   0)   cc1-30212    |   0.272 us    |      _spin_lock_irq();
>   0)   cc1-30212    |   0.969 us    |    }
>   0)   cc1-30212    |               |    do_mmap_pgoff() {
>   [...]
>   0)   cc1-30212    | + 22.537 us   |  }
>   0)   cc1-30212    |               |< sys32_mmap2 => 0
> 
> For that we'd have to pass in something like the syscall function address 
> (sys_call_table[regs->ax]), and the up to 6 parameters 
> [regs->bx,cx,dx,si,di,bp].
> 
> (and initially we could just print all of them i guess, instead of a 
> variable-width thing)
> 
> We wouldnt do smart decoding of syscall arguments normally - just print 
> the raw arguments with no decoding. (like  strace -e raw=all)
> 
> How does this sound?


That sounds good. I just need a new entry type for this. I can pass all these
registers on insertion and filter them by syscall number on output stage.