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From: =?UTF-8?B?5qKm6b6Z6JGj?= <dongmenglong.8@bytedance.com>
Date: Thu, 28 Mar 2024 22:43:46 +0800
Message-ID: <CALz3k9jG5Jrqw=BGjt05yMkEF-1u909GbBYrV-02W0dQtm6KQQ@mail.gmail.com>
Subject: Re: [External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support
 to record and check the accessed args
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>, Jiri Olsa <jolsa@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>, Alexei Starovoitov <ast@kernel.org>, 
	Daniel Borkmann <daniel@iogearbox.net>, Martin KaFai Lau <martin.lau@linux.dev>, Eddy Z <eddyz87@gmail.com>, 
	Song Liu <song@kernel.org>, Yonghong Song <yonghong.song@linux.dev>, 
	John Fastabend <john.fastabend@gmail.com>, KP Singh <kpsingh@kernel.org>, 
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, 
	Alexander Gordeev <agordeev@linux.ibm.com>, Christian Borntraeger <borntraeger@linux.ibm.com>, 
	Sven Schnelle <svens@linux.ibm.com>, "David S. Miller" <davem@davemloft.net>, 
	David Ahern <dsahern@kernel.org>, Dave Hansen <dave.hansen@linux.intel.com>, 
	X86 ML <x86@kernel.org>, Steven Rostedt <rostedt@goodmis.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Quentin Monnet <quentin@isovalent.com>, 
	bpf <bpf@vger.kernel.org>, 
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>, LKML <linux-kernel@vger.kernel.org>, 
	linux-riscv <linux-riscv@lists.infradead.org>, linux-s390 <linux-s390@vger.kernel.org>, 
	Network Development <netdev@vger.kernel.org>, linux-trace-kernel@vger.kernel.org, 
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On Fri, Mar 15, 2024 at 4:00=E2=80=AFPM =E6=A2=A6=E9=BE=99=E8=91=A3 <dongme=
nglong.8@bytedance.com> wrote:
>
> On Thu, Mar 14, 2024 at 8:27=E2=80=AFAM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Tue, Mar 12, 2024 at 6:53=E2=80=AFPM =E6=A2=A6=E9=BE=99=E8=91=A3 <do=
ngmenglong.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.
>

I have done a simple benchmark on creating 1000
trampolines. It is slow, quite slow, which consume up to
60s. We can't do it this way.

Now, I have a bad idea. How about we introduce
a "dynamic trampoline"? The basic logic of it can be:

"""
save regs
bpfs =3D trampoline_lookup_ip(ip)
fentry =3D bpfs->fentries
while fentry:
  fentry(ctx)
  fentry =3D fentry->next

call origin
save return value

fexit =3D bpfs->fexits
while fexit:
  fexit(ctx)
  fexit =3D fexit->next

xxxxxx
"""

And we lookup the "bpfs" by the function ip in a hash map
in trampoline_lookup_ip. The type of "bpfs" is:

struct bpf_array {
  struct bpf_prog *fentries;
 struct bpf_prog *fexits;
  struct bpf_prog *modify_returns;
}

When we need to attach the bpf progA to function A/B/C,
we only need to create the bpf_arrayA, bpf_arrayB, bpf_arrayC
and add the progA to them, and insert them to the hash map
"direct_call_bpfs", and attach the "dynamic trampoline" to
A/B/C. If bpf_arrayA exist, just add progA to the tail of
bpf_arrayA->fentries. When we need to attach progB to
B/C, just add progB to bpf_arrayB->fentries and
bpf_arrayB->fentries.

Compared to the trampoline, extra overhead is introduced
by the hash lookuping.

I have not begun to code yet, and I am not sure the overhead is
acceptable. Considering that we also need to do hash lookup
by the function in kprobe_multi, maybe the overhead is
acceptable?

Thanks!
Menglong Dong

> >
> > > >
> > > > 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=3D=3DNULL.
> > 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/pl=
umbers.pdf
> >
> > Pls study them and his prior patchsets to avoid stepping on the same ra=
kes.