On Thu, Mar 6, 2025 at 11:39 AM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > On Wed, Mar 5, 2025 at 6:59 PM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote: > > > > I'm not sure if it works. However, indirect call is also used > > in function graph, so we still have better performance. Isn't it? > > > > Let me have a look at the code of the function graph first :/ > > Menglong, > > Function graph infra isn't going to help. > "call foo" isn't a problem either. > > But we have to step back. > per-function metadata is an optimization and feels like > we're doing a premature optimization here without collecting > performance numbers first. > > Let's implement multi-fentry with generic get_metadata_by_ip() first. > get_metadata_by_ip() will be a hashtable in such a case and > then we can compare its performance when it's implemented as > a direct lookup from ip-4 (this patch) vs hash table > (that does 'ip' to 'metadata' lookup). Hi, Alexei You are right, I should do such a performance comparison. > > If/when we decide to do this per-function metadata we can also > punt to generic hashtable for cfi, IBT, FineIBT, etc configs. > When mitigations are enabled the performance suffers anyway, > so hashtable lookup vs direct ip-4 lookup won't make much difference. > So we can enable per-function metadata only on non-mitigation configs > when FUNCTION_ALIGNMENT=16. > There will be some number of bytes available before every function > and if we can tell gcc/llvm to leave at least 5 bytes there > the growth of vmlinux .text will be within a noise. Sounds great! It's so different to make the per-function metadata work in all the cases. Especially, we can't implement it in arm64 if CFI_CLANG is enabled. And the fallbacking to the hash table makes it much easier in these cases. > > So let's figure out the design of multi-fenty first with a hashtable > for metadata and decide next steps afterwards. Ok, I'll develop a version for fentry multi-link with both hashtable and function metadata, and do some performance testing. Thank you for your advice :/ Thanks! Menglong Dong
