netdev - Re: [PATCH bpf-next v2 2/7] bpf: Add fd-based tcx multi-prog infra with link support

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Message-ID: <CAEf4BzbuzNw4gRXSSDoHTwGH82moaSWtaX1nvmUAVx4+OgaEyw@mail.gmail.com>
Date: Thu, 8 Jun 2023 14:24:47 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jamal Hadi Salim <jhs@...atatu.com>
Cc: Daniel Borkmann <daniel@...earbox.net>, ast@...nel.org, andrii@...nel.org, 
	martin.lau@...ux.dev, razor@...ckwall.org, sdf@...gle.com, 
	john.fastabend@...il.com, kuba@...nel.org, dxu@...uu.xyz, joe@...ium.io, 
	toke@...nel.org, davem@...emloft.net, bpf@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 2/7] bpf: Add fd-based tcx multi-prog infra
 with link support

On Thu, Jun 8, 2023 at 12:46 PM Jamal Hadi Salim <jhs@...atatu.com> wrote:
>
> Hi Daniel,
>
> On Thu, Jun 8, 2023 at 6:12 AM Daniel Borkmann <daniel@...earbox.net> wrote:
> >
> > Hi Jamal,
> >
> > On 6/8/23 3:25 AM, Jamal Hadi Salim wrote:
> > [...]
> > > A general question (which i think i asked last time as well): who
> > > decides what comes after/before what prog in this setup? And would
> > > that same entity not have been able to make the same decision using tc
> > > priorities?
> >
> > Back in the first version of the series I initially coded up this option
> > that the tc_run() would basically be a fake 'bpf_prog' and it would have,
> > say, fixed prio 1000. It would get executed via tcx_run() when iterating
> > via bpf_mprog_foreach_prog() where bpf_prog_run() is called, and then users
> > could pick for native BPF prio before or after that. But then the feedback
> > was that sticking to prio is a bad user experience which led to the
> > development of what is in patch 1 of this series (see the details there).
> >
>
> Thanks. I read the commit message in patch 1 and followed the thread
> back including some of the discussion we had and i am still
> disagreeing that this couldnt be solved with a smart priority based
> scheme - but i think we can move on since this is standalone and
> doesnt affect tc.
>
> Daniel - i am still curious in the new scheme of things how would
> cilium vs datadog food fight get resolved without some arbitration
> entity?
>
> > > The idea of protecting programs from being unloaded is very welcome
> > > but feels would have made sense to be a separate patchset (we have
> > > good need for it). Would it be possible to use that feature in tc and
> > > xdp?
> > BPF links are supported for XDP today, just tc BPF is one of the few
> > remainders where it is not the case, hence the work of this series. What
> > XDP lacks today however is multi-prog support. With the bpf_mprog concept
> > that could be addressed with that common/uniform api (and Andrii expressed
> > interest in integrating this also for cgroup progs), so yes, various hook
> > points/program types could benefit from it.
>
> Is there some sample XDP related i could look at?  Let me describe our
> use case: lets say we load an ebpf program foo attached to XDP of a
> netdev  and then something further upstream in the stack is consuming
> the results of that ebpf XDP program. For some reason someone, at some
> point, decides to replace the XDP prog with a different one - and the
> new prog does a very different thing. Could we stop the replacement
> with the link mechanism you describe? i.e the program is still loaded
> but is no longer attached to the netdev.

If you initially attached an XDP program using BPF link api
(LINK_CREATE command in bpf() syscall), then subsequent attachment to
the same interface (of a new link or program with BPF_PROG_ATTACH)
will fail until the current BPF link is detached through closing its
last fd.

That is, until we allow multiple attachments of XDP programs to the
same network interface. But even then, no one will be able to
accidentally replace attached link, unless they have that link FD and
replace underlying BPF program.

>
>
> > >> +struct tcx_entry {
> > >> +       struct bpf_mprog_bundle         bundle;
> > >> +       struct mini_Qdisc __rcu         *miniq;
> > >> +};
> > >> +
> > >
> > > Can you please move miniq to the front? From where i sit this looks:
> > > struct tcx_entry {
> > >          struct bpf_mprog_bundle    bundle
> > > __attribute__((__aligned__(64))); /*     0  3264 */
> > >
> > >          /* XXX last struct has 36 bytes of padding */
> > >
> > >          /* --- cacheline 51 boundary (3264 bytes) --- */
> > >          struct mini_Qdisc *        miniq;                /*  3264     8 */
> > >
> > >          /* size: 3328, cachelines: 52, members: 2 */
> > >          /* padding: 56 */
> > >          /* paddings: 1, sum paddings: 36 */
> > >          /* forced alignments: 1 */
> > > } __attribute__((__aligned__(64)));
> > >
> > > That is a _lot_ of cachelines - at the expense of the status quo
> > > clsact/ingress qdiscs which access miniq.
> >
> > Ah yes, I'll fix this up.
>
> Thanks.
>
> cheers,
> jamal
> > Thanks,
> > Daniel