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JMIPaG1cSQwb+KKtki77V/Rp6jJuA6braEMWt17UFEO089uatvD4C78UWwohTEey4/CTDLuHEx0 EdXlZQxnVvMyWt2GyBYSsG3BX2XwPO1Uml69SFv4CvYJXcEySQ4n54Dw2RmBrefnbzuiupX54wd 6m6BriTzVxGxIboF7Jq8KZ6YQ/wrlGsKilRhbCbipUYLePBB3yMklvjtGDpljVATSOuLu+RadN2 9F41CWYNT6UIXQOuJ9qVkLfU9XVxOzRga1SZwQ4O3HYXe6E2CAcfS5IRuA3C+sIimmZxnw/I2ey 4uFjgJQpoKA42vS0yNPCdba2hzBu2w== X-Authority-Analysis: v=2.4 cv=OolCCi/t c=1 sm=1 tr=0 ts=68ee648a cx=c_pps a=OOZaFjgC48PWsiFpTAqLcw==:117 a=OOZaFjgC48PWsiFpTAqLcw==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=x6icFKpwvdMA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=yPCof4ZbAAAA:8 a=Rw79ccuu-uNeDIphBfMA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 a=cPQSjfK2_nFv0Q5t_7PE:22 X-Proofpoint-ORIG-GUID: TEOt9EX8z9gZ1UpI7sDh54XdqKRcQhgh On 14/10/2025 12:52, Jiri Olsa wrote: > On Mon, Oct 13, 2025 at 05:12:45PM -0700, Alexei Starovoitov wrote: >> On Mon, Oct 13, 2025 at 12:38 AM Alan Maguire wrote: >>> >>> >>> I was trying to avoid being specific about inlines since the same >>> approach works for function sites with optimized-out parameters and they >>> could be easily added to the representation (and probably should be in a >>> future version of this series). Another "extra" source of info >>> potentially is the (non per-cpu) global variables that Stephen sent >>> patches for a while back and the feeling was it was too big to add to >>> vmlinux BTF proper. >>> >>> But extra is a terrible name. .BTF.aux for auxiliary info perhaps? >> >> aux is too abstract and doesn't convey any meaning. >> How about "BTF.func_info" ? It will cover inlined and optimized funcs. >> >> Thinking more about reuse of struct btf_type for these... >> After sleeping on it it feels a bit awkward today, since if they're >> types they suppose to be in one table with other types, >> searchable and so on, but we actually don't want them there. >> btf_find_*() isn't fast and people are trying to optimize it. >> Also if we teach the kernel to use these loc-s they probably >> should be in a separate table. >> >> global non per-cpu vars fit into current BTF's datasec concept, >> so they can be another kernel module with a different name. >> >> I guess one can argue that LOCSEC is similar to DATASEC. >> Both need their own search tables separate from the main type table. >> >>> >>>> The partially inlined functions were the biggest footgun so far. >>>> Missing fully inlined is painful, but it's not a footgun. >>>> So I think doing "kloc" and usdt-like bpf_loc_arg() completely in >>>> user space is not enough. It's great and, probably, can be supported, >>>> but the kernel should use this "BTF.inline_info" as well to >>>> preserve "backward compatibility" for functions that were >>>> not-inlined in an older kernel and got partially inlined in a new kernel. >>>> >>> >>> That would be great; we'd need to teach the kernel to handle multi-split >>> BTF but I would hope that wouldn't be too tricky. >>> >>>> If we could use kprobe-multi then usdt-like bpf_loc_arg() would >>>> make a lot of sense, but since libbpf has to attach a bunch >>>> of regular kprobes it seems to me the kernel support is more appropriate >>>> for the whole thing. >>> >>> I'm happy with either a userspace or kernel-based approach; the main aim >>> is to provide this functionality in as straightforward a form as >>> possible to tracers/libbpf. I have to confess I didn't follow the whole >>> kprobe multi progress, but at one stage that was more kprobe-based >>> right? Would there be any value in exploring a flavour of kprobe-multi >>> that didn't use fprobe and might work for this sort of use case? As you >>> say if we had that keeping a user-space based approach might be more >>> attractive as an option. >> >> Agree. >> >> Jiri, >> how hard would it be to make multi-kprobe work on arbitrary IPs ? > > multi-kprobe uses fprobe which uses ftrace/fgraph fast api to attach, > but it can do that only on the entry of ftrace-able functions which > have nop5 hooks at the entry > > attaching anywhere else requires standard kprobe and the attach time > (and execution time) will be bad > > would be great if inlined functions kept the nop5/fentry hooks ;-) > but that's probably not that simple > Yeah, if it was doable - and with metadata about inline sites it certainly _seems_ possible - it does seem to work against the reason we inline stuff (saving overheads). Steve mentioned this as a possibility at GNU cauldron too if I remember, so worth discussing of course! I was thinking about something simpler to be honest; a flavour of kprobe multi that used kprobes under the hood in kernel to be suitable for inline sites without any tweaking of the sites. So there is a kprobe performance penalty if you're tracing, but none otherwise. Thanks! Alan