Re: [tip:sched/core] [sched/fair] 79104becf4: BUG:kernel_NULL_pointer_dereference,address

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 21, 2025 at 01:04:49PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 21, 2025 at 01:14:36PM +0800, kernel test robot wrote:
> > 
> > 
> > Hello,
> > 
> > kernel test robot noticed "BUG:kernel_NULL_pointer_dereference,address" on:
> > 
> > commit: 79104becf42baeeb4a3f2b106f954b9fc7c10a3c ("sched/fair: Forfeit vruntime on yield")
> > https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git sched/core
> > 
> > [test failed on linux-next/master 606da5bb165594c052ee11de79bf05bc38bc1aa6]
> > 
> > in testcase: trinity
> > version: 
> > with following parameters:
> > 
> > 	runtime: 300s
> > 	group: group-04
> > 	nr_groups: 5
> > 
> > 
> > 
> > config: x86_64-randconfig-121-20251020
> > compiler: gcc-13
> > test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G
> > 
> 
> > The kernel config and materials to reproduce are available at:
> > https://download.01.org/0day-ci/archive/20251021/202510211205.1e0f5223-lkp@intel.com
> 
> 
> I'm failing at running the kernel as described in:
> 
>   https://download.01.org/0day-ci/archive/20251021/202510211205.1e0f5223-lkp@intel.com/reproduce
> 
> 
> I then start the test as:
> 
> root@spr:/usr/local/src/lkp-tests# bin/lkp qemu -k /usr/src/linux-2.6/tmp-build/arch/x86/boot/bzImage -m /usr/src/linux-2.6/tmp-build/modules.cgz /usr/src/linux-2.6/tmp-build/job-script
> result_root: /root/.lkp//result/trinity/group-04-5-300s/vm-snb/quantal-i386-core-20190426.cgz/x86_64-randconfig-121-20251020/gcc-13/79104becf42baeeb4a3f2b106f954b9fc7c10a3c/4
> downloading initrds ...
> use local modules: /root/.lkp/cache/modules.cgz
> skip downloading /root/.lkp/cache/osimage/quantal/quantal-i386-core-20190426.cgz
> 153636 blocks
> skip downloading /root/.lkp/cache/osimage/pkg/quantal-i386-core.cgz/trinity-static-i386-x86_64-f93256fb_2019-08-28.cgz
> 48101 blocks
> exec command: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -fsdev local,id=test_dev,path=/root/.lkp//result/trinity/group-04-5-300s/vm-snb/quantal-i386-core-20190426.cgz/x86_64-randconfig-121-20251020/gcc-13/79104becf42baeeb4a3f2b106f954b9fc7c10a3c/4,security_model=none -device virtio-9p-pci,fsdev=test_dev,mount_tag=9p/virtfs_mount -kernel /usr/src/linux-2.6/tmp-build/arch/x86/boot/bzImage -append root=/dev/ram0 RESULT_ROOT=/result/trinity/group-04-5-300s/vm-snb/quantal-i386-core-20190426.cgz/x86_64-randconfig-121-20251020/gcc-13/79104becf42baeeb4a3f2b106f954b9fc7c10a3c/1 BOOT_IMAGE=/pkg/linux/x86_64-randconfig-121-20251020/gcc-13/79104becf42baeeb4a3f2b106f954b9fc7c10a3c/vmlinuz-6.18.0-rc1-00001-g79104becf42b branch=linux-devel/devel-hourly-20251019-154855 job=/lkp/jobs/scheduled/vm-meta-291/trinity-group-04-5-300s-quantal-i386-core-20190426.cgz-x86_64-randconfig-121-20251020-79104becf42b-20251020-124904-8548hu-1.yaml user=lkp ARCH=x86_64 kconfig=x86_64-randconfig-121-20251020 commit=79104becf42baeeb4a3f2b106f954b9fc7c10a3c intremap=posted_msi vmalloc=256M initramfs_async=0 page_owner=on carrier_timeout=60 rcupdate.rcu_self_test=0 max_uptime=7200 LKP_LOCAL_RUN=1 selinux=0 debug apic=debug sysrq_always_enabled rcupdate.rcu_cpu_stall_timeout=100 net.ifnames=0 printk.devkmsg=on panic=-1 softlockup_panic=1 nmi_watchdog=panic oops=panic load_ramdisk=2 prompt_ramdisk=0 drbd.minor_count=8 systemd.log_level=err ignore_loglevel console=tty0 earlyprintk=ttyS0,115200 console=ttyS0,115200 vga=normal rw  ip=dhcp result_service=9p/virtfs_mount -initrd /root/.lkp/cache/final_initrd -smp 2 -m 16384M -no-reboot -device i6300esb -rtc base=localtime -device e1000,netdev=net0 -netdev user,id=net0 -display none -monitor null -serial stdio

> [    5.650807][    T1] Starting init: /bin/sh exists but couldn't execute it (error -8)
> [    5.651481][    T1] Kernel panic - not syncing: No working init found.  Try passing init= option to kernel. See Linux Documentation/admin-guide/init.rst for guidance.
> [    5.652784][    T1] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.18.0-rc2-00182-gbab99e58f91f #35 PREEMPT 
> [    5.653668][    T1] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [    5.654582][    T1] Call Trace:
> [    5.654873][    T1]  <TASK>
> [    5.655137][    T1]  dump_stack_lvl+0x3a/0x70
> [    5.655536][    T1]  dump_stack+0x10/0x20
> [    5.655911][    T1]  vpanic+0x104/0x2c0
> [    5.656256][    T1]  ? rest_init+0x140/0x140
> [    5.656652][    T1]  panic+0x51/0x60
> [    5.656989][    T1]  kernel_init+0x111/0x120
> [    5.657360][    T1]  ret_from_fork+0x25/0x140
> [    5.657741][    T1]  ? rest_init+0x140/0x140
> [    5.658130][    T1]  ret_from_fork_asm+0x11/0x20
> [    5.658563][    T1]  </TASK>
> [    5.658841][    T1] Kernel Offset: disabled
> 

Any clues how I can get this thing to actually run the trinity required
to reproduce instead of panic?