Re: [PATCH v7 bpf-next 06/10] tracepoint: compute num_args at build time

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 28 Mar 2018 09:43:56 -0700
Alexei Starovoitov <ast@fb.com> wrote:
> >
> > Given that only eBPF needs this parameter count, we can move
> > it to the struct bpf_raw_event_map newly introduced by Steven,
> > right ? This would reduce bloat of struct tracepoint. For instance,
> > we don't need to keep this count around when eBPF is configured
> > out.  
> 
> Makes sense. That is indeed cleaner. Will respin.
> 
> What I don't like though is 'bloat' argument.
> 'u32 num_args' padded to 8-byte takes exactly the same amount
> of space in 'struct tracepoint' and in 'struct bpf_raw_event_map'
> The number of these structures is the same as well
> and chances that tracepoints are on while bpf is off are slim.
> More so few emails ago you said:
> "I'm perfectly fine with adding the "num_args" stuff. I think it's
> really useful. It's only the for_each_kernel_tracepoint change for
> which I'm trying to understand the rationale."

I don't really care which one it goes in. The padding bloat is the same
for both :-/  But I wonder if we can shrink it by doing a trick that
Josh did in one of his patches. That is, to use a 32bit offset instead
of a direct pointer. Since you are only accessing core kernel
tracepoints.

Thus, we could have

struct bpf_raw_event_map {
	u32			tp_offset;
	u32			num_args;
	void			*bpf_func;
};

and have:

	u64 tp_offset = (u64)tp - (u64)_sdata;

	if (WARN_ON(tp_offset > UINT_MAX)
		return -EINVAL;

	 btp->tp_offset = (u32)tp_offset;

And to get the tp, all you need to do is:

	tp = (struct tracepoint *)(btp->tp_offset + (unsigned long)_sdata);

I've been thinking of doing this for other parts of the tracepoints and
ftrace code.

BTW, thanks for changing your code. I really appreciate it.

-- Steve