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

[Alexei Starovoitov <ast@fb.com>]

On 3/28/18 10:04 AM, Steven Rostedt wrote:
> 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;

above math has to be build time constant, so warn_on likely
won't work.
imo the whole thing is too fragile and obscure.
I suggest to compress this 8 bytes * num_of_tracepoints later.
Especially would be good to do it in one way for
bpf_raw_event_map, ftrace and other places.

> 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
>