linux-kernel - Re: Very slow clang kernel config ..

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Message-ID: <CAKwvOdk0nxxUATg2jEKgx4HutXCMXcW92SX3DT+uCTgqBwQHBg@mail.gmail.com>
Date:   Fri, 30 Apr 2021 18:48:11 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Stellard <tstellar@...hat.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Fangrui Song <maskray@...gle.com>,
        Serge Guelton <sguelton@...hat.com>,
        Sylvestre Ledru <sylvestre@...illa.com>
Subject: Re: Very slow clang kernel config ..

On Fri, Apr 30, 2021 at 6:22 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, Apr 30, 2021 at 5:25 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > Ah, no, sorry, these are the runtime link editor/loader. So probably
> > spending quite some time resolving symbols in large binaries.
>
> Yeah. Appended is the profile I see when I profile that "make
> oldconfig", so about 45% of all time seems to be spent in just symbol
> lookup and relocation.
>
> And a fair amount of time just creating and tearing down that huge
> executable (with a lot of copy-on-write overhead too), with the kernel
> side of that being another 15%. The cost of that is likely also fairly
> directly linked to all the dynamic linking costs, which brings in all
> that data.
>
> Just to compare, btw, this is the symbol lookup overhead for the gcc case:
>
>    1.43%  ld-2.33.so             do_lookup_x
>    0.96%  ld-2.33.so             _dl_relocate_object
>    0.69%  ld-2.33.so             _dl_lookup_symbol_x
>
> so it really does seem to be something very odd going on with the clang binary.
>
> Maybe the Fedora binary is built some odd way, but it's likely just
> the default clang build.
>
>              Linus
>
> ----
>   23.59%  ld-2.33.so          _dl_lookup_symbol_x
>   11.41%  ld-2.33.so          _dl_relocate_object
>    9.95%  ld-2.33.so          do_lookup_x
>    4.00%  [kernel.vmlinux]    copy_page
>    3.98%  [kernel.vmlinux]    next_uptodate_page
>    3.05%  [kernel.vmlinux]    zap_pte_range
>    1.81%  [kernel.vmlinux]    clear_page_rep
>    1.68%  [kernel.vmlinux]    asm_exc_page_fault
>    1.33%  ld-2.33.so          strcmp
>    1.33%  ld-2.33.so          check_match

47.61% spent in symbol table lookup. Nice. (Not counting probably a
fair amount of the libc calls below).

>    0.92%  libLLVM-12.so       llvm::StringMapImpl::LookupBucketFor

^ wait a minute; notice how in your profile the `Shared Object` is
attributed to `libLLVM-12.so` while mine is `clang-13`?  Clang can be
built as either having libllvm statically linked or dynamically; see
the cmake variables
LLVM_BUILD_LLVM_DYLIB:BOOL
LLVM_LINK_LLVM_DYLIB:BOOL
BUILD_SHARED_LIBS:BOOL
https://llvm.org/docs/CMake.html

I think those are frowned upon; useful for cutting down on developers
iteration speed due to not having to relink llvm when developing
clang. But shipping that in production? I just checked and it doesn't
look like we do that for AOSP's build of LLVM.

Tom, is one of the above intentionally set for clang builds on Fedora?
I'm guessing it's intentional that there are packages for
libLLVM-12.so and libclang-cpp.so.12, perhaps they have other
dependents?

>    0.83%  [kernel.vmlinux]    rmqueue_bulk
>    0.77%  conf                yylex
>    0.75%  libc-2.33.so        __gconv_transform_utf8_internal
>    0.74%  libc-2.33.so        _int_malloc
>    0.69%  libc-2.33.so        __strlen_avx2
>    0.62%  [kernel.vmlinux]    pagecache_get_page
>    0.58%  [kernel.vmlinux]    page_remove_rmap
>    0.56%  [kernel.vmlinux]    __handle_mm_fault
>    0.54%  [kernel.vmlinux]    filemap_map_pages
>    0.54%  libc-2.33.so        __strcmp_avx2
>    0.54%  [kernel.vmlinux]    __free_one_page
>    0.52%  [kernel.vmlinux]    release_pages
-- 
Thanks,
~Nick Desaulniers