Re: [PATCH v2] sched/debug: Add new tracepoint to track cpu_capacity

[Alexei Starovoitov]

On Mon, Jan 4, 2021 at 10:29 AM Qais Yousef <qais.yousef@xxxxxxx> wrote:
>
> On 09/08/20 09:19, Phil Auld wrote:
> > Hi Quais,
> >
> > On Mon, Sep 07, 2020 at 12:02:24PM +0100 Qais Yousef wrote:
> > > On 09/02/20 09:54, Phil Auld wrote:
> > > > >
> > > > > I think this decoupling is not necessary. The natural place for those
> > > > > scheduler trace_event based on trace_points extension files is
> > > > > kernel/sched/ and here the internal sched.h can just be included.
> > > > >
> > > > > If someone really wants to build this as an out-of-tree module there is
> > > > > an easy way to make kernel/sched/sched.h visible.
> > > > >
> > > >
> > > > It's not so much that we really _want_ to do this in an external module.
> > > > But we aren't adding more trace events and my (limited) knowledge of
> > > > BPF let me to the conclusion that its raw tracepoint functionality
> > > > requires full events.  I didn't see any other way to do it.
> > >
> > > I did have a patch that allowed that. It might be worth trying to upstream it.
> > > It just required a new macro which could be problematic.
> > >
> > > https://github.com/qais-yousef/linux/commit/fb9fea29edb8af327e6b2bf3bc41469a8e66df8b
> > >
> > > With the above I could attach using bpf::RAW_TRACEPOINT mechanism.
> > >
> >
> > Yeah, that could work. I meant there was no way to do it with what was there :)
> >
> > In our initial attempts at using BPF to get at nr_running (which I was not
> > involved in and don't have all the details...) there were issues being able to
> > keep up and losing events.  That may have been an implementation issue, but
> > using the module and trace-cmd doesn't have that problem. Hopefully you don't
> > see that using RAW_TRACEPOINTs.
>
> So I have a proper patch for that now, that actually turned out to be really
> tiny once you untangle exactly what is missing.
>
> Peter, bpf programs aren't considered ABIs AFAIK, do you have concerns about
> that?
>
> Thanks
>
> --
> Qais Yousef
>
> -->8--
>
> From cf81de8c7db03d62730939aa902579339e2fc859 Mon Sep 17 00:00:00 2001
> From: Qais Yousef <qais.yousef@xxxxxxx>
> Date: Wed, 30 Dec 2020 22:20:34 +0000
> Subject: [PATCH] trace: bpf: Allow bpf to attach to bare tracepoints
>
> Some subsystems only have bare tracepoints (a tracepoint with no
> associated trace event) to avoid the problem of trace events being an
> ABI that can't be changed.
>
> From bpf presepective, bare tracepoints are what it calls
> RAW_TRACEPOINT().
>
> Since bpf assumed there's 1:1 mapping, it relied on hooking to
> DEFINE_EVENT() macro to create bpf mapping of the tracepoints. Since
> bare tracepoints use DECLARE_TRACE() to create the tracepoint, bpf had
> no knowledge about their existence.
>
> By teaching bpf_probe.h to parse DECLARE_TRACE() in a similar fashion to
> DEFINE_EVENT(), bpf can find and attach to the new raw tracepoints.
>
> Enabling that comes with the contract that changes to raw tracepoints
> don't constitute a regression if they break existing bpf programs.
> We need the ability to continue to morph and modify these raw
> tracepoints without worrying about any ABI.
>
> Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx>
> ---
>  include/trace/bpf_probe.h | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h
> index cd74bffed5c6..a23be89119aa 100644
> --- a/include/trace/bpf_probe.h
> +++ b/include/trace/bpf_probe.h
> @@ -55,8 +55,7 @@
>  /* tracepoints with more than 12 arguments will hit build error */
>  #define CAST_TO_U64(...) CONCATENATE(__CAST, COUNT_ARGS(__VA_ARGS__))(__VA_ARGS__)
>
> -#undef DECLARE_EVENT_CLASS
> -#define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print) \
> +#define __BPF_DECLARE_TRACE(call, proto, args)                         \
>  static notrace void                                                    \
>  __bpf_trace_##call(void *__data, proto)                                        \
>  {                                                                      \
> @@ -64,6 +63,10 @@ __bpf_trace_##call(void *__data, proto)                                      \
>         CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(prog, CAST_TO_U64(args));  \
>  }
>
> +#undef DECLARE_EVENT_CLASS
> +#define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print) \
> +       __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))
> +
>  /*
>   * This part is compiled out, it is only here as a build time check
>   * to make sure that if the tracepoint handling changes, the
> @@ -111,6 +114,11 @@ __DEFINE_EVENT(template, call, PARAMS(proto), PARAMS(args), size)
>  #define DEFINE_EVENT_PRINT(template, name, proto, args, print) \
>         DEFINE_EVENT(template, name, PARAMS(proto), PARAMS(args))
>
> +#undef DECLARE_TRACE
> +#define DECLARE_TRACE(call, proto, args)                               \
> +       __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))          \
> +       __DEFINE_EVENT(call, call, PARAMS(proto), PARAMS(args), 0)
> +
>  #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)

The patch looks fine to me.
Please add a few things:
- selftests to make sure it gets routinely tested with bpf CI.
- add a doc with contents from commit log.
The "Does bpf make things into an abi ?" question keeps coming back
over and over again.
Everytime we have the same answer that No, bpf cannot bake things into abi.
I think once it's spelled out somewhere in Documentation/ it would be easier to
repeat this message.
Also please tag future patches to bpf-next tree to make sure things
keep being tested.

Thanks