> On Oct 29, 2020, at 7:33 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > 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). Thanks for the explanation. > > 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. Is the purpose of split BTF to save memory used by module BTF? In the example above, I guess part of those 5.2MB will be loaded at run time, so the actual saving is less than 5.2MB. 5.2MB is really small for a decent system, e.g. ~0.03% of my laptop's main memory. Did I miss anything here? Song
