Currently, during BPF object loading, load_module_btfs() unconditionally
iterates over all module BTFs and loads each one. This introduces
unnecessary overhead when a BPF program only needs BTFs from a specific,
small subset of modules. In environments with hundreds of modules,
loading all module BTFs can measurably increase the loading time.
This series introduces an optional ".kmod_btfs" ELF section, allowing BPF
programs to declare the modules whose BTFs should be loaded by
load_module_btfs(). libbpf uses this information to skip unrelated module
BTFs and stops the iteration early once all declared modules are found.
Without this section, the existing behavior remains unchanged.
Performance impact (<skel>__open_and_load() time):
Modules loaded | Without .kmod_btfs | With .kmod_btfs | Speedup
---------------|--------------------|-----------------|--------
1 | 35.0 ms | 35.0 ms | Baseline
10 | 36.5 ms | 35.1 ms | +3.8%
100 | 46.2 ms | 35.9 ms | +22.3%
300 | 65.2 ms | 38.0 ms | +41.7%
There are use cases where BPF programs are loaded on demand rather than
as a one-time setup. In our Android testing, the impact is even more
pronounced: the total BPF loading time exceeds 300 ms with 93 module
BTFs, with loading module BTFs accounting for around 69% of the total
loading time. Preloading all BPF programs is not suitable in this case,
as unused programs should remain unloaded to minimize memory usage.
The existing module qualification in SEC(), such as
SEC("fentry/mymod:foo"), is only supported for BPF program types with
explicit attach targets. It only guides find_kernel_btf_id() to search
for the BTF ID in the specified module after load_module_btfs(), and
does not reduce the number of module BTFs loaded by load_module_btfs().
Changelog:
v2:
- Addressed issues with allocation error handling, multiple .kmod_btfs
sections, the transient stack pointer in is_kmod_btf_needed(), and
selftest dependency and comment formatting. (sashiko-bot)
- Removed KMODS_BTF_LOADED and KMODS_BTF_UNLOADED and simplified the
related logic.
v1:
- Link: https://lore.kernel.org/bpf/[email protected]/
Fuyu Zhao (2):
libbpf: support selective kernel module BTF loading via .kmod_btfs
section
selftests/bpf: add tests for selective kmod BTF loading
tools/lib/bpf/bpf_helpers.h | 14 +++
tools/lib/bpf/libbpf.c | 106 ++++++++++++++++++
.../selftests/bpf/prog_tests/kmod_btfs.c | 52 +++++++++
tools/testing/selftests/bpf/progs/kmod_btfs.c | 14 +++
.../selftests/bpf/progs/kmod_btfs_mix.c | 15 +++
.../selftests/bpf/progs/kmod_btfs_nonexist.c | 18 +++
6 files changed, 219 insertions(+)
create mode 100644 tools/testing/selftests/bpf/prog_tests/kmod_btfs.c
create mode 100644 tools/testing/selftests/bpf/progs/kmod_btfs.c
create mode 100644 tools/testing/selftests/bpf/progs/kmod_btfs_mix.c
create mode 100644 tools/testing/selftests/bpf/progs/kmod_btfs_nonexist.c
--
2.34.1
