kernel-5.10 bpf and tracing

shadowplay

Hi all,
I'm trying to build grapheneOS (and the kernel) from sources on lynx, and I succeded.
But /sys/kernel/tracing/ is empty and I was interested in developing a tracing tool using bpf.
I've compared the gki_defconfig with the AOSP one and I do not see relevant differences (beside maybe LOCKDOWN, which I tried to disable). Importing the AOSP gki_defconfig is doable only adding CONFIG_NET_CLS_TCINDEX=y (which anyway do not seem relevant).
I've also tried to build a kernel with CONFIG_FUNCTION_TRACER enabled but in that case the kernel fail to boot (I've no additional feedback).
So, I was wondering, is that expected having an empty /sys/kernel/tracing/ or I'm doing something wrong? And if that's expected, how can I fix it?

Foggy

Did you build the kernel with all the BPF config flags enabled?
https://github.com/iovisor/bcc/blob/master/INSTALL.md#kernel-configuration

shadowplay

Hi Foggy,
thank you for the reply.
Actually I didn't touch them (but yes, they are enabled). BPF seems to be ok, in /sys/fs/bpf/ I've maps and everything.
Just no event seems to be triggered (so maps are filled with zeros) and /sys/kernel/tracing (where I expected to find traceable events).