RFC: SysRq nice-all-RT-tasks is broken

[Laurent Dufour]

Hi,

It appears that triggering the SysRq nice-all-RT-tasks from the console
while a real task is active is leading to panic the system like this :

 sysrq: SysRq : Nice All RT Tasks
 ------------[ cut here ]------------
 kernel BUG at /build/linux-twbIHf/linux-4.10.0/kernel/sched/core.c:4089!
 Oops: Exception in kernel mode, sig: 5 [#1]
 SMP NR_CPUS=2048
 NUMA
 pSeries
 Modules linked in: rpcsec_gss_krb5 auth_rpcgss nfsv4 nfs lockd grace
fscache pseries_rng vmx_crypto binfmt_misc dm_multipath scsi_dh_rdac
scsi_dh_emc scsi_dh_alua sunrpc ip_tables x_tables autofs4 btrfs xor
raid6_pq ses enclosure scsi_transport_sas crc32c_vpmsum mlx5_core be2net
ipr devlink
 CPU: 100 PID: 0 Comm: swapper/100 Not tainted 4.10.0-8-generic #10-Ubuntu
 task: c0000003b6179c00 task.stack: c0000003b620c000
 NIP: c0000000001044f0 LR: c00000000010e524 CTR: c0000000006d0ec0
 REGS: c00000077fc978d0 TRAP: 0700   Not tainted  (4.10.0-8-generic)
 MSR: 8000000000029033 <SF,EE,ME,IR,DR,RI,LE>
   CR: 28002228  XER: 20000001
 CFAR: c00000000010e520 SOFTE: 0
 GPR00: c00000000010e524 c00000077fc97b50 c00000000143c900 c00000038785fa00
 GPR04: c00000077fc97c00 0000000000000000 0000000000000000 ffffffffffffffff
 GPR08: 0000000000000000 0000000000010000 0000000000000000 0000000000000406
 GPR12: 0000000028002228 c000000007b58400 c0000003b620ff90 000000001ede2d80
 GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
 GPR20: 0000000000000001 0000000000000000 0000000000000000 0000000000000000
 GPR24: 0000000000000000 c00000077a0f8b50 0000000000000063 0000000000000000
 GPR28: c00000038785fa00 c00000077fc97c00 0000000000000000 c00000038785fa00
 NIP [c0000000001044f0] __sched_setscheduler+0x90/0xd30
 LR [c00000000010e524] normalize_rt_tasks+0x184/0x1c0
 Call Trace:
 [c00000077fc97be0] [c00000000010e524] normalize_rt_tasks+0x184/0x1c0
 [c00000077fc97c60] [c0000000006d0ee0] sysrq_handle_unrt+0x20/0x40
 [c00000077fc97c80] [c0000000006d1c98] __handle_sysrq+0xe8/0x280
 [c00000077fc97d20] [c0000000006eab38] hvc_poll+0x1c8/0x360
 [c00000077fc97dc0] [c0000000006ebd74] hvc_handle_interrupt+0x24/0x50
 [c00000077fc97de0] [c0000000001453d0] __handle_irq_event_percpu+0x90/0x2c0
 [c00000077fc97ea0] [c000000000145638] handle_irq_event_percpu+0x38/0x90
 [c00000077fc97ee0] [c0000000001456f4] handle_irq_event+0x64/0xb0
 [c00000077fc97f10] [c00000000014add8] handle_fasteoi_irq+0xe8/0x290
 [c00000077fc97f40] [c000000000143ebc] generic_handle_irq+0x4c/0x80
 [c00000077fc97f60] [c0000000000167b0] __do_irq+0x80/0x1d0
 [c00000077fc97f90] [c000000000029414] call_do_irq+0x14/0x24
 [c0000003b620fa40] [c000000000016994] do_IRQ+0x94/0x140
 [c0000003b620fa80] [c000000000008be4] hardware_interrupt_common+0x114/0x120
 --- interrupt: 501 at plpar_hcall_norets+0x1c/0x28
     LR = check_and_cede_processor+0x34/0x50
 [c0000003b620fd70] [c0000003b620fe10] 0xc0000003b620fe10 (unreliable)
 [c0000003b620fdd0] [c00000000095db50] shared_cede_loop+0x50/0x150
 [c0000003b620fe00] [c00000000095accc] cpuidle_enter_state+0x16c/0x430
 [c0000003b620fe60] [c00000000012c8bc] call_cpuidle+0x4c/0x90
 [c0000003b620fe80] [c00000000012cce0] do_idle+0x2c0/0x330
 [c0000003b620ff00] [c00000000012cfa8] cpu_startup_entry+0x38/0x50
 [c0000003b620ff30] [c000000000044180] start_secondary+0x330/0x370
 [c0000003b620ff90] [c00000000000aa6c] start_secondary_prolog+0x10/0x14
 Instruction dump:
 7c7f1b78 7cb62b78 7cdb3378 2b8a0006 7d5507b4 419e0010 83440014 235a0063
 7f5a07b4 78290464 8129000c 552902ee <0b090000> 2f950000 419c04b8 2f950005
 ---[ end trace 891618fbca78ebe7 ]---

I got it on Power and on X86_64, but I guess it should happen in all
architectures.
Here are the steps to recreate it :
1. Create a RT task : sudo chrt -f 50 /bin/sleep 999999
2. On the console trigger the 'nice-all-RT-tasks' SYS-RQ.

The panic is triggered by the BUG_ON(in_interrupt()) introduced by this
commit:

66e5393a78b3 ("[PATCH] BUG() if setscheduler is called from interrupt
context")

Since SysRq is run from the interrupt context, the panic is expected.

Looking at the code, I'm wondering if the BUG_ON() is still required in
__sched_setscheduler(). But I'm not so confident, so requesting your
advise here.

Thanks,
Laurent.