Add build documentation for LTO. Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx> --- Documentation/lto-build | 121 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 121 insertions(+) create mode 100644 Documentation/lto-build diff --git a/Documentation/lto-build b/Documentation/lto-build new file mode 100644 index 0000000..4c8a713 --- /dev/null +++ b/Documentation/lto-build @@ -0,0 +1,121 @@ +Link time optimization (LTO) for the Linux kernel + +This is an experimental feature which still has various problems. + +Link Time Optimization allows the compiler to optimize the complete program +instead of just each file. Link Time Optimization was a new feature in gcc 4.6, +but only really works with gcc 4.7. The kernel LTO build also requires +the Linux binutils (the normal FSF releases do not work at the moment) + +The compiler can inline functions between files and do some other global +optimizations. It will also drop unused functions which can make the kernel +image smaller in some circumstances. The binary gets somewhat larger. +In return the resulting kernels (usually) have better performance. + +Build time and memory consumption at build time will increase. +The build time penalty depends on the size of the vmlinux. Reasonable +sized vmlinux build about twice as long, much larger monolithic kernels +like allyesconfig ~4x as long. Modular kernels are less affected. + +Normal "reasonable" builds work with less than 4GB of RAM, but very large +configurations like allyesconfig may need more memory. The actual +memory needed depends on the available memory (gcc sizes its garbage +collector pools based on that or on the ulimit -m limits) + +Issues: +- Various workarounds in kernel needed for toolchain problems. +- A few kernel features are currently incompatible with LTO, in particular +function tracing, because they require special compiler flags for +specific files, which is not supported in LTO right now. +- The build is faster with LTO_SLIM enabled, but this still triggers +problems in some circumstances (currently disabled) +- Jobserver control for -j does not work correctly for the final +LTO phase. The makefiles hardcodes -j<number of online cpus> + +Configuration: +- Enable CONFIG_LTO_MENU and then disable CONFIG_LTO_DISABLE. +This is mainly to not have allyesconfig default to LTO. +- FUNCTION_TRACER, STACK_TRACER, FUNCTION_GRAPH_TRACER have to disabled +because they are currently incompatible with LTO. +- MODVERSIONS have to be disabled because they are not fixed for LTO +yet. + +Requirements: +- Enough memory: 4GB for a standard build, more for allyesconfig +If you are tight on memory and use tmpfs as /tmp define TMPDIR and +point it to a directory on disk. The peak memory usage +happens single threaded (when lto-wpa merges types), so dialing +back -j options will not help much. + +A 32bit compiler is unlikely to work due to the memory requirements. +You can however build a kernel targetted at 32bit on a 64bit host. + +- Get the Linux binutils from +http://www.kernel.org/pub/linux/devel/binutils/ +Sorry standard binutils releases don't work +The kernel build has to use this linker, so if it is installed +in a non standard location use LD=... on the make line. + +- gcc 4.7 built with plugin ld (--with-plugin-ld) also pointing to the +linker from the Linux binutils and LTO + +If the gcc is not built with this option it may also work to put the correct +binutils linker first in $PATH when building. I haven't tested +this however. + +Example build procedure for the tool chain and kernel. This does not +overwrite the standard compiler toolchain on the system. If you already +have a suitable gcc 4.7+ compiler and linker the toolchain build can +be skipped (note that a distribution gcc 4.7 is not necessarily +correctly configured for LTO) + +Get the Linux binutils from http://www.kernel.org/pub/linux/devel/binutils/ +The standard binutils do not work at this point! + +Unpack binutils + +cd binutils-VERSION (or plain binutils in some versions) +./configure --prefix=/opt/binutils-VERSION --enable-plugins +nice -n20 make -j$(getconf _NPROCESSORS_ONLN) +sudo make install +sudo ln -sf /opt/binutils-VERSION/bin/ld /usr/local/bin/ld-plugin + +Unpack gcc-4.7 + +mkdir obj-gcc +# please don't skip this cd. the build will not work correctly in the +# source dir, you have to use the separate object dir +cd obj-gcc +# make sure to install gmp-devel and mpfr-devel +# and the 32bit glibc package if you have a multilib system +# if mpc-devel is not there get it from +# http://www.multiprecision.org/mpc/download/mpc-0.8.2.tar.gz +# and install in gcc-4.7*/mpc +../gcc-4.7*/configure --prefix=/opt/gcc-4.7 --enable-lto \ +--with-plugin-ld=/usr/local/bin/ld-plugin \ +--disable-nls --enable-languages=c,c++ \ +--disable-libstdcxx-pch +nice -n20 make -j$(getconf _NPROCESSORS_ONLN) +sudo make install-no-fixedincludes +sudo ln -sf /opt/gcc-4.7/bin/gcc /usr/local/bin/gcc47 +sudo ln -sf /opt/gcc-4.7/bin/gcc-ar /usr/local/bin/gcc-ar47 + +# get lto tree in linux-lto + +mkdir obj-lto +cd obj-lto +# copy a suitable kernel config file into .config +make -C ../linux-lto O=$(pwd) oldconfig +./source/scripts/config --disable function_tracer --disable function_graph_tracer \ + --disable stack_tracer --enable lto_menu \ + --disable lto_disable --disable lto_debug --disable lto_slim +export TMPDIR=$(pwd) +# this lowers memory usage with /tmp=tmpfs +# note the special ar is only needed if CONFIG_LTO_SLIM is enabled +# The PATH is that gcc-ar finds a plugin aware ar, if your standard +# binutils doesn't support that. If the standard ar supports --plugin +# it is not needed +PATH=/opt/binutils-VERSION:$PATH nice -n20 make CC=gcc47 LD=ld-plugin AR=gcc-ar47 \ +-j $(getconf _NPROCESSORS_ONLN) + +Andi Kleen -- 1.8.5.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
