On Thu, Oct 29, 2020 at 5:33 PM Song Liu <songliubraving@xxxxxx> wrote: > > > > > On Oct 28, 2020, at 5:58 PM, Andrii Nakryiko <andrii@xxxxxxxxxx> wrote: > > > > This patch set adds support for generating and deduplicating split BTF. This > > is an enhancement to the BTF, which allows to designate one BTF as the "base > > BTF" (e.g., vmlinux BTF), and one or more other BTFs as "split BTF" (e.g., > > kernel module BTF), which are building upon and extending base BTF with extra > > types and strings. > > > > Once loaded, split BTF appears as a single unified BTF superset of base BTF, > > with continuous and transparent numbering scheme. This allows all the existing > > users of BTF to work correctly and stay agnostic to the base/split BTFs > > composition. The only difference is in how to instantiate split BTF: it > > requires base BTF to be alread instantiated and passed to btf__new_xxx_split() > > or btf__parse_xxx_split() "constructors" explicitly. > > > > This split approach is necessary if we are to have a reasonably-sized kernel > > module BTFs. By deduping each kernel module's BTF individually, resulting > > module BTFs contain copies of a lot of kernel types that are already present > > in vmlinux BTF. Even those single copies result in a big BTF size bloat. On my > > kernel configuration with 700 modules built, non-split BTF approach results in > > 115MBs of BTFs across all modules. With split BTF deduplication approach, > > total size is down to 5.2MBs total, which is on part with vmlinux BTF (at > > around 4MBs). This seems reasonable and practical. As to why we'd need kernel > > module BTFs, that should be pretty obvious to anyone using BPF at this point, > > as it allows all the BTF-powered features to be used with kernel modules: > > tp_btf, fentry/fexit/fmod_ret, lsm, bpf_iter, etc. > > Some high level questions. Do we plan to use split BTF for in-tree modules > (those built together with the kernel) or out-of-tree modules (those built > separately)? If it is for in-tree modules, is it possible to build split BTF > into vmlinux BTF? It will be possible to use for both in-tree and out-of-tree. For in-tree, this will be integrated into the kernel build process. For out-of-tree, whoever builds their kernel module will need to invoke pahole -J with an extra flag pointing to the right vmlinux image (I haven't looked into the exact details of this integration, maybe there are already scripts in Linux repo that out-of-tree modules have to use, in such case we can add this integration there). Merging all in-tree modules' BTFs into vmlinux's BTF defeats the purpose of the split BTF and will just increase the size of vmlinux BTF unnecessarily. > > Thanks, > Song > > [...]
