Message-ID: <20220617200553.kg7jmkvwdp7yqfkm@google.com>
Date:   Fri, 17 Jun 2022 13:05:53 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Sedat Dilek <sedat.dilek@...il.com>, Jiri Slaby <jslaby@...e.cz>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        clang-built-linux <llvm@...ts.linux.dev>
Subject: Re: [PATCH] kbuild: pass jobserver to cmd_ld_vmlinux.o

On 2022-06-18, Masahiro Yamada wrote:
>(+LLVM list, Fangrui Song)

Thanks for tagging me. I'll clarify some stuff.

>On Fri, Jun 17, 2022 at 7:41 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
>>
>> On Fri, Jun 17, 2022 at 12:35 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
>> >
>> > On Fri, Jun 17, 2022 at 12:53 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
>> > >
>> > > On Thu, Jun 16, 2022 at 4:09 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
>> > > >
>> > > > On Thu, Jun 16, 2022 at 12:45 PM Jiri Slaby <jslaby@...e.cz> wrote:
>> > > > >
>> > > > > Until the link-vmlinux.sh split (cf. the commit below), the linker was
>> > > > > run with jobserver set in MAKEFLAGS. After the split, the command in
>> > > > > Makefile.vmlinux_o is not prefixed by "+" anymore, so this information
>> > > > > is lost.
>> > > > >
>> > > > > Restore it as linkers working in parallel (esp. the LTO ones) make a use
>> > > > > of i
>
>Hi Jiri,
>
>Please let me clarify first.
>
>Here, is it OK to assume you are talking about Clang LTO
>instead of GCC LTO because the latter is not upstreamed ?
>
>
>
>
>
>I tested this patch but I did not see any performance change for Clang LTO.
>
>
>[1] CONFIG_CLANG_LTO_FULL
>
>   lld always runs sequential.
>   It never runs in parallel even if you pass -j option to Make

"lld always runs sequential" is not accurate. There are a number of
parallel linker passes.  ld.lld --threads= defaults to
llvm::hardware_concurrency (similar to
https://en.cppreference.com/w/cpp/thread/thread/hardware_concurrency,
but uses sched_getaffinity to compute the number of available cores).

"lld always runs sequential" is only correct only when --threads=1 is
specified or the system only provides one thread to the lld process.

I think people may be more interested in LTO parallelism here.  Regular
LTO (sometimes called full LTO when there is mixed-thin-and-regular LTO)
supports limited parallelism which applies to code generation, but not
IR-level optimization.  (IR-level optimization has many interprocedural
optimizations passes.  Splitting will make LTO less effective. Code
generation is per function, so parallelism does not regress
optimization.)

>
>[2] CONFIG_CLANG_LTO_THIN
>
>   lld always runs in parallel even if you do not pass -j option
>
>   In my machine, lld always allocated 12 threads.
>   This is irrespective of the Make parallelisms.
>
>
>
>
>One more thing, if a program wants to participate in
>Make's jobserver, it must parse MAKEFLAGS, and extract
>file descriptors to be used to communicate to the jobserver.
>
>As a code example in the kernel tree,
>scripts/jobserver-exec parses "MAKEFLAGS" and "--jobserver".
>
>
>I grepped the lld source code, but it does not contain
>"MAKEFLAGS" or "jobserver".

>masahiro@...ar:~/ref/lld$ git remote  show origin
>* remote origin
>  Fetch URL: https://github.com/llvm-mirror/lld.git
>  Push  URL: https://github.com/llvm-mirror/lld.git
>  HEAD branch: master
>  Remote branches:
>    master     tracked
>    release_36 tracked
>    release_37 tracked
>    release_38 tracked
>    release_39 tracked
>    release_40 tracked
>    release_50 tracked
>    release_60 tracked
>    release_70 tracked
>    release_80 tracked
>    release_90 tracked
>  Local branch configured for 'git pull':
>    master merges with remote master
>  Local ref configured for 'git push':
>    master pushes to master (up to date)
>masahiro@...ar:~/ref/lld$ git grep MAKEFLAGS
>masahiro@...ar:~/ref/lld$ git grep jobserver
>
>
>So, in my research, LLD does not seem to support the jobserver.


Correct. lld does not support GNU make's jobserver.  On the other hand,
I don't think the jobserver implementation supports flexible "give this
target N hardware concurrency". A heavy link target does not necessarily
get more resources than a quick target.

If a make target knows how many hardware concurrency it gets, we can
pass --threads= to lld. LTO easily takes 95+% link time, so LTO
parallelism may needs a dedicated setting. lld has --thinlto-jobs=.

>
>
>
>If you are talking about GCC LTO, yes, the code
>tries to parse "--jobserver-auth=" from the MAKEFLAGS
>environment variable.  [1]
>
>[1]:  https://github.com/gcc-mirror/gcc/blob/releases/gcc-12.1.0/gcc/lto-wrapper.cc#L1341
>
>
>But, as you may know, GCC LTO works in a different way,
>at least, we cannot do it before modpost.
>
>
>-- 
>Best Regards
>Masahiro Yamada
>

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