Re: [PATCH v3 17/20] objtool: decode instructions and resolve branch targets in parallel

[Josh Poimboeuf]

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) <ljs@xxxxxxxxxx>
> ---
>  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