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

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Date:   Thu, 6 Jan 2022 16:20:05 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
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, 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.
The speed of attach/detach has to be fast.
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.