Re: [RFCv3 00/19] x86/ftrace/bpf: Add batch support for direct/tracing attach

Andrii Nakryiko
2021-07-06T20:27:01Z

On Sun, Jun 20, 2021 at 11:50 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Sat, Jun 19, 2021 at 11:33 AM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > On Thu, Jun 17, 2021 at 01:29:45PM -0700, Andrii Nakryiko wrote:
> > > On Sat, Jun 5, 2021 at 4:12 AM Jiri Olsa <jolsa@kernel.org> wrote:
> > > >
> > > > hi,
> > > > saga continues.. ;-) previous post is in here [1]
> > > >
> > > > After another discussion with Steven, he mentioned that if we fix
> > > > the ftrace graph problem with direct functions, he'd be open to
> > > > add batch interface for direct ftrace functions.
> > > >
> > > > He already had prove of concept fix for that, which I took and broke
> > > > up into several changes. I added the ftrace direct batch interface
> > > > and bpf new interface on top of that.
> > > >
> > > > It's not so many patches after all, so I thought having them all
> > > > together will help the review, because they are all connected.
> > > > However I can break this up into separate patchsets if necessary.
> > > >
> > > > This patchset contains:
> > > >
> > > > 1) patches (1-4) that fix the ftrace graph tracing over the function
> > > > with direct trampolines attached
> > > > 2) patches (5-8) that add batch interface for ftrace direct function
> > > > register/unregister/modify
> > > > 3) patches (9-19) that add support to attach BPF program to multiple
> > > > functions
> > > >
> > > > In nutshell:
> > > >
> > > > Ad 1) moves the graph tracing setup before the direct trampoline
> > > > prepares the stack, so they don't clash
> > > >
> > > > Ad 2) uses ftrace_ops interface to register direct function with
> > > > all functions in ftrace_ops filter.
> > > >
> > > > Ad 3) creates special program and trampoline type to allow attachment
> > > > of multiple functions to single program.
> > > >
> > > > There're more detailed desriptions in related changelogs.
> > > >
> > > > I have working bpftrace multi attachment code on top this. I briefly
> > > > checked retsnoop and I think it could use the new API as well.
> > >
> > > Ok, so I had a bit of time and enthusiasm to try that with retsnoop.
> > > The ugly code is at [0] if you'd like to see what kind of changes I
> > > needed to make to use this (it won't work if you check it out because
> > > it needs your libbpf changes synced into submodule, which I only did
> > > locally). But here are some learnings from that experiment both to
> > > emphasize how important it is to make this work and how restrictive
> > > are some of the current limitations.
> > >
> > > First, good news. Using this mass-attach API to attach to almost 1000
> > > kernel functions goes from
> > >
> > > Plain fentry/fexit:
> > > ===================
> > > real 0m27.321s
> > > user 0m0.352s
> > > sys 0m20.919s
> > >
> > > to
> > >
> > > Mass-attach fentry/fexit:
> > > =========================
> > > real 0m2.728s
> > > user 0m0.329s
> > > sys 0m2.380s
> >
> > I did not meassured the bpftrace speedup, because the new code
> > attached instantly ;-)
> >
> > >
> > > It's a 10x speed up. And a good chunk of those 2.7 seconds is in some
> > > preparatory steps not related to fentry/fexit stuff.
> > >
> > > It's not exactly apples-to-apples, though, because the limitations you
> > > have right now prevents attaching both fentry and fexit programs to
> > > the same set of kernel functions. This makes it pretty useless for a
> >
> > hum, you could do link_update with fexit program on the link fd,
> > like in the selftest, right?
>
> Hm... I didn't realize we can attach two different prog FDs to the
> same link, honestly (and was too lazy to look through selftests
> again). I can try that later. But it's actually quite a
> counter-intuitive API (I honestly assumed that link_update can be used
> to add more BTF IDs, but not change prog_fd). Previously bpf_link was
> always associated with single BPF prog FD. It would be good to keep
> that property in the final version, but we can get back to that later.

Ok, I'm back from PTO and as a warm-up did a two-line change to make
retsnoop work end-to-end using this bpf_link_update() approach. See
[0]. I still think it's a completely confusing API to do
bpf_link_update() to have both fexit and fentry, but it worked for
this experiment.

BTW, adding ~900 fexit attachments is barely noticeable, which is
great, means that attachment is instantaneous.

real 0m2.739s
user 0m0.351s
sys 0m2.370s

[0] https://github.com/anakryiko/retsnoop/commit/c915d729d6e98f83601e432e61cb1bdf476ceefb

>
> >
> > > lot of cases, in particular for retsnoop. So I haven't really tested
> > > retsnoop end-to-end, I only verified that I do see fentries triggered,
> > > but can't have matching fexits. So the speed-up might be smaller due
> > > to additional fexit mass-attach (once that is allowed), but it's still
> > > a massive difference. So we absolutely need to get this optimization
> > > in.
> > >
> > > Few more thoughts, if you'd like to plan some more work ahead ;)
> > >
> > > 1. We need similar mass-attach functionality for kprobe/kretprobe, as
> > > there are use cases where kprobe are more useful than fentry (e.g., >6
> > > args funcs, or funcs with input arguments that are not supported by
> > > BPF verifier, like struct-by-value). It's not clear how to best
> > > represent this, given currently we attach kprobe through perf_event,
> > > but we'll need to think about this for sure.
> >
> > I'm fighting with the '2 trampolines concept' at the moment, but the
> > mass attach for kprobes seems interesting ;-) will check
> >
> > >
> > > 2. To make mass-attach fentry/fexit useful for practical purposes, it
> > > would be really great to have an ability to fetch traced function's
> > > IP. I.e., if we fentry/fexit func kern_func_abc, bpf_get_func_ip()
> > > would return IP of that functions that matches the one in
> > > /proc/kallsyms. Right now I do very brittle hacks to do that.
> >
> > so I hoped that we could store ip always in ctx-8 and have
> > the bpf_get_func_ip helper to access that, but the BPF_PROG
> > macro does not pass ctx value to the program, just args
> >
> > we could perhaps somehow store the ctx in BPF_PROG before calling
> > the bpf program, but I did not get to try that yet
> >
> > >
> > > So all-in-all, super excited about this, but I hope all those issues
> > > are addressed to make retsnoop possible and fast.
> > >
> > > [0] https://github.com/anakryiko/retsnoop/commit/8a07bc4d8c47d025f755c108f92f0583e3fda6d8
> >
> > thanks for checking on this,
> > jirka
> >