FYI, we noticed the following commit (built with gcc-7):

commit: b0f464c9c862c0c912ac3fe1edf8a157517157d0 ("sched: Add task_struct pointer to sched_class::set_curr_task")
https://github.com/digitalocean/linux-coresched coresched

in testcase: locktorture
with following parameters:

	runtime: 300s
	test: cpuhotplug

test-description: This torture test consists of creating a number of kernel threads which acquire the lock and hold it for specific amount of time, thus simulating different critical region behaviors.
test-url: https://www.kernel.org/doc/Documentation/locking/locktorture.txt


on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 2G

caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):


+------------------------------------------------------+------------+------------+
|                                                      | 4f53939e5f | b0f464c9c8 |
+------------------------------------------------------+------------+------------+
| boot_successes                                       | 5          | 10         |
| boot_failures                                        | 1          | 16         |
| BUG:kernel_reboot-without-warning_in_test_stage      | 1          |            |
| WARNING:at_kernel/sched/sched.h:#pick_next_task_fair | 0          | 16         |
| RIP:pick_next_task_fair                              | 0          | 16         |
| WARNING:at_kernel/sched/sched.h:#sched_cpu_dying     | 0          | 16         |
| RIP:sched_cpu_dying                                  | 0          | 16         |
+------------------------------------------------------+------------+------------+


If you fix the issue, kindly add following tag
Reported-by: kernel test robot <lkp@intel.com>


[  276.263169] WARNING: CPU: 0 PID: 11 at kernel/sched/sched.h:1719 pick_next_task_fair+0x754/0x780
[  276.265858] Modules linked in: locktorture torture ppdev snd_pcm crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel snd_timer snd soundcore bochs_drm pcspkr joydev ttm serio_raw drm_kms_helper drm ata_generic pata_acpi i2c_piix4 parport_pc floppy parport qemu_fw_cfg
[  276.272254] CPU: 0 PID: 11 Comm: migration/0 Not tainted 5.1.0-10881-gb0f464c #1
[  276.274218] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[  276.276415] RIP: 0010:pick_next_task_fair+0x754/0x780
[  276.277773] Code: 48 0f af 85 18 01 00 00 48 83 c1 01 48 f7 f1 48 89 83 10 0a 00 00 e9 6a fa ff ff bf 02 00 00 00 e8 e1 33 ff ff e9 08 fa ff ff <0f> 0b e9 1d fb ff ff 80 3d e7 50 61 01 00 0f 85 30 fc ff ff e8 83
[  276.282716] RSP: 0018:ffffbab3c0377d08 EFLAGS: 00010006
[  276.284174] RAX: ffffffff90011dc0 RBX: ffffa0316be2aec0 RCX: ffffffff9064d0c0
[  276.286075] RDX: ffffbab3c0377d90 RSI: 0000000000000001 RDI: ffffa0316be2aec0
[  276.287968] RBP: ffffbab3c0377dd0 R08: 000000405290c768 R09: 0000000000000005
[  276.289876] R10: ffffbab3c0377d48 R11: 000000000000138e R12: ffffffff9064d0c0
[  276.291768] R13: ffffffff90011fc0 R14: ffffbab3c0377d90 R15: 0000000000000000
[  276.293658] FS:  0000000000000000(0000) GS:ffffa0316be00000(0000) knlGS:0000000000000000
[  276.295803] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  276.297325] CR2: 00005608c15f8d60 CR3: 000000007efe0000 CR4: 00000000000006f0
[  276.299238] Call Trace:
[  276.301855]  sched_cpu_dying+0x29a/0x3c0
[  276.302925]  ? sched_cpu_starting+0xd0/0xd0
[  276.304034]  cpuhp_invoke_callback+0x86/0x5d0
[  276.305221]  ? cpu_disable_common+0x1cf/0x1f0
[  276.306375]  take_cpu_down+0x60/0xa0
[  276.307330]  multi_cpu_stop+0x68/0xe0
[  276.308308]  ? cpu_stopper_thread+0x100/0x100
[  276.309469]  cpu_stopper_thread+0x94/0x100
[  276.310591]  ? smpboot_thread_fn+0x2f/0x1e0
[  276.311725]  ? smpboot_thread_fn+0x74/0x1e0
[  276.312843]  ? smpboot_thread_fn+0x14e/0x1e0
[  276.313987]  smpboot_thread_fn+0x149/0x1e0
[  276.315077]  ? sort_range+0x20/0x20
[  276.316034]  kthread+0x11e/0x140
[  276.316903]  ? kthread_park+0x90/0x90
[  276.317889]  ret_from_fork+0x35/0x40
[  276.318851] ---[ end trace 4938a64101275eb2 ]---


To reproduce:

        # build kernel
	cd linux
	cp config-5.1.0-10881-gb0f464c .config
	make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 olddefconfig
	make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 prepare
	make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 modules_prepare
	make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 SHELL=/bin/bash
	make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 bzImage


        git clone https://github.com/intel/lkp-tests.git
        cd lkp-tests
        find lib/ | cpio -o -H newc --quiet | gzip > modules.cgz
	bin/lkp qemu -k <bzImage> -m modules.cgz job-script # job-script is attached in this email




Thanks,
lkp