netdev - Re: [PATCH v3 bpf-next 0/4] Add support for cgroup bpf

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Date:   Mon, 30 Mar 2020 17:32:22 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrey Ignatov <rdna@...com>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 0/4] Add support for cgroup bpf_link

On Mon, Mar 30, 2020 at 05:43:52PM -0600, David Ahern wrote:
> On 3/30/20 4:50 PM, Alexei Starovoitov wrote:
> > On Mon, Mar 30, 2020 at 1:46 PM David Ahern <dsahern@...il.com> wrote:
> >> release. As it stands it is a half-baked feature.
> > 
> > speaking of half-baked.
> > I think as it stands (even without link_query) it's already extremely
> > useful addition and doesn't take anything away from existing cgroup-bpf
> > and doesn't hinder observability. 'bpftool cgroup' works just fine.
> > So I've applied the set.
> > 
> > Even if it was half-baked it would still be applie-able.
> > Many features are developed over the course of multiple
> > kernel releases. Example: your nexthops, mptcp, bpf-lsm.
> > 
> 
> nexthops were not - refactoring in 1 release and the entire feature went
> in to 5.4. Large features / patch sets often must be spread across
> kernel versions because it is not humanly possible to send and review
> the patches.
> 
> This is not a large feature, and there is no reason for CREATE/UPDATE -
> a mere 4 patch set - to go in without something as essential as the
> QUERY for observability.

As I said 'bpftool cgroup' covers it. Observability is not reduced in any way.
Also there is Documentation/bpf/drgn.rst
Kernel is an open book. Just learn this simple tool and everything will
be observable. Not only bpf objects, but the rest of the kernel too.
imo the use case for LINK_QUERY makes sense for xdp only.
There is one program there and it's a dispatcher program that libdispatcher
will be iteratively updating via freplace. For cgroup bpf_links I cannot
think of a reason why apps would need to query it.