Date:   Wed, 5 Jan 2022 15:33:26 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "David S. Miller" <davem@davemloft.net>,
        Ard Biesheuvel <ardb@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Al Viro <viro@zeniv.linux.org.uk>, llvm@lists.linux.dev
Subject: Re: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree
 -v1: Eliminate the Linux kernel's "Dependency Hell"
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On Wed, Jan 05, 2022 at 01:40:32AM +0100, Ingo Molnar wrote:
> 
> * Nathan Chancellor <nathan@kernel.org> wrote:
> 
> > Unfortunately, while the kernel now builds, it does not boot in QEMU. I 
> > tried to checkout at 9006a48618cc0cacd3f59ff053e6509a9af5cc18 to see if I 
> > could reproduce that breakage there but the build errors out at that 
> > change (I do see notes of bisection breakage in some of the commits) so I 
> > assume that is expected.
> 
> Yeah, there's a breakage window on ARM64, I'll track down that 
> bisectability bug.
> 
> Decoupling thread_info and task_struct incrementally, so that it bisects 
> cleanly on all architectures, was always a big challenge. :-/
> 
> > There is no output, even with earlycon, so it seems like something is 
> > going wrong in early boot code. I am not very familiar with the SCS code 
> > so I will see if I can debug this with gdb later (I'll try to see if it 
> > is reproducible with GCC as well; as Nick mentions, there is support 
> > being added to it and I don't mind building from source).
> 
> Just to make sure: with SCS disabled the same kernel boots fine?

Correct (thank you for making sure, I have definitely not tested that
before...).

$ make -skj"$(nproc)" ARCH=arm64 LLVM=1 O=.build/arm64 defconfig Image.gz

$ boot-qemu.sh -a arm64 -k .build/arm64 -t 30s
...
[    0.000000] Linux version 5.16.0-rc8-798083-g1755441e323b (nathan@archlinux-ax161) (ClangBuiltLinux clang version 14.0.0 (https://github.com/llvm/llvm-project 4602f4169a21e75b82261ba1599046b157d1d021), LLD 14.0.0) #1 SMP PREEMPT Wed Jan 5 21:51:29 UTC 2022
...

$ make -skj"$(nproc)" ARCH=arm64 LLVM=1 O=.build/arm64.scs defconfig

$ scripts/config --file .build/arm64.scs/.config -e SHADOW_CALL_STACK

$ make -skj"$(nproc)" ARCH=arm64 LLVM=1 O=.build/arm64.scs olddefconfig Image.gz
...
qemu-system-aarch64: terminating on signal 15 from pid 690472 (timeout)
+ RET=124
+ set +x

Going back to v5.16-rc8, everything works fine.

$ boot-qemu.sh -a arm64 -k .build/arm64 -t 30s
...
[    0.000000] Linux version 5.16.0-rc8-795784-gc9e6606c7fe9 (nathan@archlinux-ax161) (ClangBuiltLinux clang version 14.0.0 (https://github.com/llvm/llvm-project 4602f4169a21e75b82261ba1599046b157d1d021), LLD 14.0.0) #1 SMP PREEMPT Wed Jan 5 22:27:39 UTC 2022
...

I don't think I will have time to look at this today but I will try
tomorrow. Having the bisectability bug fixed would help narrow things
down but I am almost certain it is something up with the new per_task
infrastructure but I'll have to dig around and see if I can understand
that first.

Cheers,
Nathan