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

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Date: Fri, 15 Mar 2024 16:00:55 +0800
From: 梦龙董 <dongmenglong.8@...edance.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...ux.dev>, Eddy Z <eddyz87@...il.com>, 
	Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, 
	John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Sven Schnelle <svens@...ux.ibm.com>, "David S. Miller" <davem@...emloft.net>, 
	David Ahern <dsahern@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	X86 ML <x86@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Quentin Monnet <quentin@...valent.com>, 
	bpf <bpf@...r.kernel.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, LKML <linux-kernel@...r.kernel.org>, 
	linux-riscv <linux-riscv@...ts.infradead.org>, linux-s390 <linux-s390@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, linux-trace-kernel@...r.kernel.org, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, linux-stm32@...md-mailman.stormreply.com
Subject: Re: [External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support
 to record and check the accessed args

On Thu, Mar 14, 2024 at 8:27 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Mar 12, 2024 at 6:53 PM 梦龙董 <dongmenglong.8@...edance.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.