On Thu, Sep 17, 2026 at 05:06:27PM +0100, Lorenzo Stoakes (ARM) wrote: > During a kernel build objtool is used to decode vmlinux.o's instructions > and resolve every jump and call destination. > > This forms a large part of the work objtool does during the build process, > and it is all done in serial. > > Decode these in parallel at a function granularity to speed things up, but > limit this to invocations that pass --link, and only where the there is 8 > MiB or more text to justify it. > > In practice this limits this to processing vmlinux.o in the kernel build > and modules are processed as they were before. > > Only the instruction hash is shared between the threads and nothing is ever > removed from it, so an insertion is a compare-and-swap on the bucket head. > > Threading is limited to decoding and the jump pass, so the gain flattens > out at 16 threads and any further threads were found to only add overhead. > > When performing an allmodconfig build, the clang invocation of objtool when > processing vmlinux.o took 5.93s on 1 thread, 4.56s on 8, 4.51s on > 16 and 4.63s on 128. > > Therefore cap the thread count at 16 or the number of CPUs, whichever is > fewer. > > The output of objtool before and after this change was confirmed to be > byte-for-byte identical for x86_64 defconfig and allmodconfig with gcc and > clang, and for a loongarch defconfig, where objtool runs on every object. > > On a 128-thread machine, objtool on the clang allmodconfig vmlinux.o goes > from 5.2s to 4.4s (6.5s to 4.4s together with the previous two patches), > and on defconfig from 1.99s to 1.38s. > > objtool on vmlinux.o is on the serial tail of every build that links > vmlinux, no-op builds are unchanged. > > Whole build, 128-thread Threadripper 9980X, best of N runs: > > before after delta > ------------------------------- > x86 defconfig, touch mm/vma.c, gcc 8.2s 7.7s -0.55s (-7%) > x86 defconfig, touch mm/vma.c, clang 7.3s 6.9s -0.44s (-6%) > x86 defconfig, clean, gcc 28.6s 28.2s -0.47s (-2%) > x86 defconfig, clean, clang 29.1s 28.7s -0.45s (-2%) > x86 allmodconfig, touch mm/vma.c, gcc 25.7s 23.7s -2.0s (-8%) > x86 allmodconfig, touch mm/vma.c, clang 24.1s 22.5s -1.6s (-7%) > > Assisted-by: LLM > Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]> > --- > tools/objtool/Makefile | 2 +- > tools/objtool/check.c | 717 > ++++++++++++++++++++++++++++++++++++------------ > tools/objtool/objtool.c | 14 +- > 3 files changed, 546 insertions(+), 187 deletions(-)
This is an interesting patch, but I'm not really convinced it's worth the pain. -- Josh
