Re: [PATCH bpf-next] bpf, bpftool: Allow probing for CONFIG_HZ from kernel config

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, May 15, 2020 at 5:12 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >
> > libbpf supports kconfig values, so don't have to even probe for that,
> > it will just appear as a constant global variable:
> >
> > extern unsigned long CONFIG_HZ __kconfig;
> >
> > But I assume you want this for iproute2 case, which doesn't support
> > this, right? We really should try to make iproute2 just use libbpf as
>
> It's one but not the only reason. Our golang daemon picks up the json config
> from `bpftool feature -j` and based on that we decide which features our BPF
> datapath will have where the daemons needs to also adapt accordingly. So for
> users running older kernels we need fallback behavior in our daemon, for
> example, in case of missing LPM delete or get_next_key from syscall side the
> LPM map management and/or program regeneration will differ in the agent. In
> case of jiffies, it's also not as trivial from control plane side, e.g.
> existing deployments cannot simply switch from ktime to jiffies during upgrade
> while traffic is live in the datapath given this has upgrade and downgrade
> implications on timeouts. However, we can switch to using it for new deployments
> via helm. In that case, the agent today probes for availability, so it needs
> to know i) whether the underlying kernel supports jiffies64 helper, ii) it needs
> to know the kernel hz value in order to convert timeouts for our CT's GC. This
> is done based on bpftool feature -j from the agent's probe manager. Next, we
> also cannot assume general availability of an existing .config from distro side,
> so in that case we run the probe to determine kernel hz and emit the CONFIG_HZ
> define instead, and if all breaks down we fall back to using ktime in our data
> path. From the macro side, the timeouts all resolve nicely during compilation
> time since everything is passed as a constant here. We have a small helper for
> bpf_jiffies_to_sec() and bpf_sec_to_jiffies() that resolves it whereas `extern
> unsigned long CONFIG_HZ __kconfig` hapens at load time plus relies on the fact
> that config must be available, although the latter could probably be fixed via
> user-callback.

fair enough.
Applied to bpf-next. Thanks