Re: [External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support to record and check the accessed args

梦龙董
2024-03-15T08:01:07Z

On Thu, Mar 14, 2024 at 8:27 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue, Mar 12, 2024 at 6:53 PM 梦龙董 <dongmenglong.8@bytedance.com> wrote:
[......]
> > What does "a hundred attachments max" means? Can't I
> > trace thousands of kernel functions with a bpf program of
> > tracing multi-link?
>
> I mean what time does it take to attach one program
> to 100 fentry-s ?
> What is the time for 1k and for 10k ?
>
> The kprobe multi test attaches to pretty much all funcs in
> /sys/kernel/tracing/available_filter_functions
> and it's fast enough to run in test_progs on every commit in bpf CI.
> See get_syms() in prog_tests/kprobe_multi_test.c
>
> Can this new multi fentry do that?
> and at what speed?
> The answer will decide how applicable this api is going to be.
> Generating different trampolines for every attach point
> is an approach as well. Pls benchmark it too.

I see. Creating plenty of trampolines does take a lot of time,
and I'll do testing on it.

>
> > >
> > > Let's step back.
[......]
>
> For one trampoline to handle all attach points we might
> need some arch support, but we can start simple.
> Make btf_func_model with MAX_BPF_FUNC_REG_ARGS
> by calling btf_distill_func_proto() with func==NULL.
> And use that to build a trampoline.
>
> The challenge is how to use minimal number of trampolines
> when bpf_progA is attached for func1, func2, func3
> and bpf_progB is attached to func3, func4, func5.
> We'd still need 3 trampolines:
> for func[12] to call bpf_progA,
> for func3 to call bpf_progA and bpf_progB,
> for func[45] to call bpf_progB.
>
> Jiri was trying to solve it in the past. His slides from LPC:
> https://lpc.events/event/16/contributions/1350/attachments/1033/1983/plumbers.pdf
>
> Pls study them and his prior patchsets to avoid stepping on the same rakes.