powerpc 5.10-rcN boot failures with RCU_SCALE_TEST=m

[Paul E. McKenney]

On Fri, Nov 27, 2020 at 01:02:29PM +1100, Daniel Axtens wrote:
> Hi all,
> 
> I'm having some difficulty tracking down a bug.
> 
> Some configurations of the powerpc kernel since somewhere in the 5.10
> merge window fail to boot on some ppc64 systems. They hang while trying
> to bring up SMP. It seems to depend on the RCU_SCALE/PERF_TEST option.
> (It was renamed in the 5.10 merge window.)

Adding Mark Rutland on CC in case his similarly mystifying experience
obtaining a fix for ARM has relevance.  From what I could see, that
was a delayed consequence of the x86/entry rewrite.  It was similarly
difficult to bisect.

							Thanx, Paul

> I can reproduce it as follows with qemu tcg:
> 
> make -j64 pseries_le_defconfig
> scripts/config -m RCU_SCALE_TEST
> scripts/config -m RCU_PERF_TEST
> make -j 64 vmlinux CC="ccache gcc"
> 
> qemu-system-ppc64 -cpu power9 -M pseries -m 1G -nographic -vga none -smp 4 -kernel vmlinux
> 
> ...
> [    0.036284][    T0] Mount-cache hash table entries: 8192 (order: 0, 65536 bytes, linear)
> [    0.036481][    T0] Mountpoint-cache hash table entries: 8192 (order: 0, 65536 bytes, linear)
> [    0.148168][    T1] POWER9 performance monitor hardware support registered
> [    0.151118][    T1] rcu: Hierarchical SRCU implementation.
> [    0.186660][    T1] smp: Bringing up secondary CPUs ...
> <hangs>
> 
> I have no idea why RCU_SCALE/PERF_TEST would be causing this, but that
> seems to be what does it: if I don't set that, the kernel boots fine.
> 
> I've tried to git bisect it, but I keep getting different results:
> always a random merge of a seemingly-unrelated subsystem tree - things
> like armsoc or integrity or input.
> 
> It appears to also depend on the way the kernel is booted. Testing with
> a Canonical kernel, so a slightly different config but including
> RCU_SCALE_TEST=m, I see:
> 
> Power8 host + KVM + grub         -> boots
> Power9 host bare metal (kexec)   -> fails
> Power9 host + KVM + grub         -> fails
> Power9 host + KVM + qemu -kernel -> boots
> qemu TCG + power9 cpu            -> fails
> qemu TCG + power8 cpu            -> fails
> 
> Any ideas?
> 
> Kind regards,
> Daniel
> 
> $ qemu-system-ppc64 -version
> QEMU emulator version 4.2.1 (Debian 1:4.2-3ubuntu6.9)
> 
> $ gcc --version
> gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
> 
> It also happens when compiling with GCC 7 and 10.
> 
>