2020-05-13 09:58 UTC+0200 ~ Daniel Borkmann <daniel@xxxxxxxxxxxxx> > 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@xxxxxxxxxxxxx> > Cc: Martin KaFai Lau <kafai@xxxxxx> 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@xxxxxxxxxxxxx>
