linux-kernel - Re: [RFC 00/13] kprobe/bpf: Add support to attach multiple kprobes

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Message-Id: <20220107215508.eaac4e225add48b0705f5f14@kernel.org>
Date:   Fri, 7 Jan 2022 21:55:08 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, lkml <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC 00/13] kprobe/bpf: Add support to attach multiple kprobes

On Thu, 6 Jan 2022 16:20:05 -0800
Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> On Thu, Jan 6, 2022 at 3:52 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> >
> > On Thu, 6 Jan 2022 09:40:17 -0800
> > Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> >
> > > On Thu, Jan 6, 2022 at 5:59 AM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > > >
> > > > That seems to bind your mind. The program type is just a programing
> > > > 'model' of the bpf. You can choose the best implementation to provide
> > > > equal functionality. 'kprobe' in bpf is just a name that you call some
> > > > instrumentations which can probe kernel code.
> > >
> > > No. We're not going to call it "fprobe" or any other name.
> > > From bpf user's pov it's going to be "multi attach kprobe",
> > > because this is how everyone got to know kprobes.
> > > The 99% usage is at the beginning of the funcs.
> > > When users say "kprobe" they don't care how kernel attaches it.
> > > The func entry limitation for "multi attach kprobe" is a no-brainer.
> >
> > Agreed. I think I might mislead you. From the bpf user pov, it always be
> > shown as 'multi attached kprobes (but only for the function entry)'
> > the 'fprobe' is kernel internal API name.
> >
> > > And we need both "multi attach kprobe" and "multi attach kretprobe"
> > > at the same time. It's no go to implement one first and the other
> > > some time later.
> >
> > You can provide the interface to user space, but the kernel implementation
> > is optimized step by step. We can start it with using real multiple
> > kretprobes, and then, switch to 'fprobe' after integrating fgraph
> > callback. :)
> 
> Sounds good to me.
> My point was that users often want to say:
> "profile speed of all foo* functions".
> To perform such a command a tracer would need to
> attach kprobes and kretprobes to all such functions.

Yeah, I know. That is more than 10 years issue since
systemtap. :)

> The speed of attach/detach has to be fast.

Yes, that's why I provided register/unregister_kprobes()
but it sounds not enough (and maybe not optimized enough
because all handlers are same)

> Currently tracers artificially limit the regex just because
> attach/detach is so slow that the user will likely Ctrl-C
> instead of waiting for many seconds.

Ah, OK.
Anyway I also would like to fix that issue. If user wants
only function entry/exit, it should be done by ftrace. But
since the syntax (and user's mind model) it should be done via
'kprobe', so transparently converting such request to ftrace
is needed.

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>