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

[Quentin Monnet <quentin@isovalent.com>]

2020-05-13 09:58 UTC+0200 ~ Daniel Borkmann <daniel@iogearbox.net>
> In Cilium we've recently switched to make use of bpf_jiffies64() for
> parts of our tc and XDP datapath since bpf_ktime_get_ns() is more
> expensive and high-precision is not needed for our timeouts we have
> anyway. Our agent has a probe manager which picks up the json of
> bpftool's feature probe and we also use the macro output in our C
> programs e.g. to have workarounds when helpers are not available on
> older kernels.
> 
> Extend the kernel config info dump to also include the kernel's
> CONFIG_HZ, and rework the probe_kernel_image_config() for allowing a
> macro dump such that CONFIG_HZ can be propagated to BPF C code as a
> simple define if available via config. Latter allows to have _compile-
> time_ resolution of jiffies <-> sec conversion in our code since all
> are propagated as known constants.
> 
> Given we cannot generally assume availability of kconfig everywhere,
> we also have a kernel hz probe [0] as a fallback. Potentially, bpftool
> could have an integrated probe fallback as well, although to derive it,
> we might need to place it under 'bpftool feature probe full' or similar
> given it would slow down the probing process overall. Yet 'full' doesn't
> fit either for us since we don't want to pollute the kernel log with
> warning messages from bpf_probe_write_user() and bpf_trace_printk() on
> agent startup; I've left it out for the time being.
> 
>   [0] https://github.com/cilium/cilium/blob/master/bpf/cilium-probe-kernel-hz.c
> 
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Martin KaFai Lau <kafai@fb.com>

Looks good to me, thanks!

I think at the time the "bpftool feature probe" was added we didn't
settle on a particular format for dumping the CONFIG_* as part as the C
macro output, but other than that I can see no specific reason why not
to have them, so we could even list them all and avoid the macro_dump
bool. But I'm fine either way, other CONFIG_* can still be added to C
macro output at a later time if someone needs them anyway.

Regarding a fallback for the jiffies, not sure what would be best. I
agree with you for the "full" keyword, so we would need another word I
suppose. But adding new keyword for fallbacks for probing features not
directly related to BPF might be going a bit beyond bpftool's scope? I
don't know. Anyway, for the current patch:

Reviewed-by: Quentin Monnet <quentin@isovalent.com>