kbuild: Build kernel module BTFs if BTF is enabled and pahole supports it

Detect if pahole supports split BTF generation, and generate BTF for each selected kernel module, if it does. This is exposed to Makefiles and C code as CONFIG_DEBUG_INFO_BTF_MODULES flag. Kernel module BTF has to be re-generated if either vmlinux's BTF changes or module's .ko changes. To achieve that, I needed a helper similar to if_changed, but that would allow to filter out vmlinux from the list of updated dependencies for .ko building. I've put it next to the only place that uses and needs it, but it might be a better idea to just add it along the other if_changed variants into scripts/Kbuild.include. Each kernel module's BTF deduplication is pretty fast, as it does only incremental BTF deduplication on top of already deduplicated vmlinux BTF. To show the added build time, I've first ran make only just built kernel (to establish the baseline) and then forced only BTF re-generation, without regenerating .ko files. The build was performed with -j60 parallelization on 56-core machine. The final time also includes bzImage building, so it's not a pure BTF overhead. $ time make -j60 ... make -j60 27.65s user 10.96s system 782% cpu 4.933 total $ touch ~/linux-build/default/vmlinux && time make -j60 ... make -j60 123.69s user 27.85s system 1566% cpu 9.675 total So 4.6 seconds real time, with noticeable part spent in compressed vmlinux and bzImage building. To show size savings, I've built my kernel configuration with about 700 kernel modules with full BTF per each kernel module (without deduplicating against vmlinux) and with split BTF against deduplicated vmlinux (approach in this patch). Below are top 10 modules with biggest BTF sizes. And total size of BTF data across all kernel modules. It shows that split BTF "compresses" 115MB down to 5MB total. And the biggest kernel modules get a downsize from 500-570KB down to 200-300KB. FULL BTF ======== $ for f in $(find . -name '*.ko'); do size -A -d $f | grep BTF | awk '{print $2}'; done | awk '{ s += $1 } END { print s }' 115710691 $ for f in $(find . -name '*.ko'); do printf "%s %d\n" $f $(size -A -d $f | grep BTF | awk '{print $2}'); done | sort -nr -k2 | head -n10 ./drivers/gpu/drm/i915/i915.ko 570570 ./drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko 520240 ./drivers/gpu/drm/radeon/radeon.ko 503849 ./drivers/infiniband/hw/mlx5/mlx5_ib.ko 491777 ./fs/xfs/xfs.ko 411544 ./drivers/net/ethernet/intel/i40e/i40e.ko 403904 ./drivers/net/ethernet/broadcom/bnx2x/bnx2x.ko 398754 ./drivers/infiniband/core/ib_core.ko 397224 ./fs/cifs/cifs.ko 386249 ./fs/nfsd/nfsd.ko 379738 SPLIT BTF ========= $ for f in $(find . -name '*.ko'); do size -A -d $f | grep BTF | awk '{print $2}'; done | awk '{ s += $1 } END { print s }' 5194047 $ for f in $(find . -name '*.ko'); do printf "%s %d\n" $f $(size -A -d $f | grep BTF | awk '{print $2}'); done | sort -nr -k2 | head -n10 ./drivers/gpu/drm/i915/i915.ko 293206 ./drivers/gpu/drm/radeon/radeon.ko 282103 ./fs/xfs/xfs.ko 222150 ./drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko 198503 ./drivers/infiniband/hw/mlx5/mlx5_ib.ko 198356 ./drivers/net/ethernet/broadcom/bnx2x/bnx2x.ko 113444 ./fs/cifs/cifs.ko 109379 ./arch/x86/kvm/kvm.ko 100225 ./drivers/gpu/drm/drm.ko 94827 ./drivers/infiniband/core/ib_core.ko 91188

ANAndrii Nakryiko committed 6 years ago