Re: [Linux PPC] Disable PREEMPT

["Michal Suchánek" <msuchanek@suse.de>]

On Fri, Sep 04, 2026 at 02:34:21PM +0530, Shrikanth Hegde wrote:
> 
> 
> On 9/4/26 1:05 PM, Michal Suchánek wrote:
> > On Thu, Sep 03, 2026 at 10:52:59PM +0530, Shrikanth Hegde wrote:
> > > 
> > > > > If possible run against current upstream and share the results.
> > > > 
> > > > https://github.com/openSUSE/kernel-source/blob/6824496d1801f73def615dca8794202eeb7b0d86/config/ppc64le/default
> > > > 
> > > > [  472.091531][ T6181] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: kvmhv_run_single_vcpu+0x19d4/0x1b50 [kvm_hv]
> > > > [  472.091598][ T6181] CPU: 29 UID: 107 PID: 6181 Comm: CPU 112/KVM Not tainted 7.2.2-5.g6824496-default #1 PREEMPT(full) openSUSE Tumbleweed (unreleased)  61871a5f06863b4006f5ef27cd9f18e8a7a2edad
> > > > [  472.091612][ T6181] Hardware name: IBM,9824-42A Power11 (architected) 0x820200 0xf000007 of:IBM,FW1110.20 (OB1110_130) hv:phyp pSeries
> > > > [  472.091624][ T6181] Call Trace:
> > > > [  472.091630][ T6181] [c00000000fdfb680] [c00000000134ce90] dump_stack_lvl+0x84/0xc0 (unreliable)
> > > > [  472.091653][ T6181] [c00000000fdfb6b0] [c00000000022e7c8] vpanic+0x324/0x5e4
> > > > [  472.091666][ T6181] [c00000000fdfb760] [c00000000022eac4] do_panic_on_target_cpu+0x0/0x2c
> > > > [  472.091677][ T6181] [c00000000fdfb780] [c0000000013c1ff8] __stack_chk_fail+0x48/0x60
> > > > [  472.091689][ T6181] [c00000000fdfb7f0] [c00800001aae219c] kvmhv_run_single_vcpu+0x19d4/0x1b50 [kvm_hv]
> > > > [  472.091712][ T6181] [c00000000fdfb940] [c00800001aae24b4] kvmppc_vcpu_run_hv+0x19c/0x12f0 [kvm_hv]
> > > > [  472.091732][ T6181] [c00000000fdfba10] [c00800001aeeed18] kvmppc_vcpu_run+0x30/0x48 [kvm]
> > > > [  472.091779][ T6181] [c00000000fdfba30] [c00800001aee9ef4] kvm_arch_vcpu_ioctl_run+0x35c/0x4a0 [kvm]
> > > > [  472.091813][ T6181] [c00000000fdfbac0] [c00800001aedaac4] kvm_vcpu_ioctl+0x1ac/0xad8 [kvm]
> > > > [  472.091844][ T6181] [c00000000fdfbca0] [c0000000007f1244] sys_ioctl+0x374/0x1060
> > > > [  472.091857][ T6181] [c00000000fdfbdb0] [c00000000002f7f8] system_call_exception+0x188/0x430
> > > > [  472.091871][ T6181] [c00000000fdfbe50] [c00000000000cfdc] system_call_vectored_common+0x15c/0x2ec
> > > > [  472.091886][ T6181] ---- interrupt: 3000 at 0x7fffb5565fac
> > > > [  472.091896][ T6181] NIP:  00007fffb5565fac LR: 00007fffb5565fac CTR: 0000000000000000
> > > > [  472.091904][ T6181] REGS: c00000000fdfbe80 TRAP: 3000   Not tainted  (7.2.2-5.g6824496-default)
> > > > [  472.091911][ T6181] MSR:  800000000280f033 <SF,VEC,VSX,EE,PR,FP,ME,IR,DR,RI,LE>  CR: 42044402  XER: 00000000
> > > > [  472.091938][ T6181] IRQMASK: 0
> > > > [  472.091938][ T6181] GPR00: 0000000000000036 00007fbfa77ed7a0 00007fffb5677100 00000000000000fa
> > > > [  472.091938][ T6181] GPR04: 000000002000ae80 0000000000000000 0000000000000000 0000000000000000
> > > > [  472.091938][ T6181] GPR08: 00000000000000fa 0000000000000000 0000000000000000 0000000000000000
> > > > [  472.091938][ T6181] GPR12: 0000000000000000 00007fbfa77f5ec0 000000014676f000 00007fbfa77ee7c0
> > > > [  472.091938][ T6181] GPR16: 000000014674e8d0 00007fbfa77eeec0 00007fbfa77eeec0 fffffffffffffff7
> > > > [  472.091938][ T6181] GPR20: 00007fffb71210d0 0000000000000001 00007fbfa77eeec0 0000000000000000
> > > > [  472.091938][ T6181] GPR24: 00007fbfa77ed8e8 0000000105971428 000000002000ae80 0000000105f77a70
> > > > [  472.091938][ T6181] GPR28: 0000000000000000 0000000000000000 000000002000ae80 000000014674f000
> > > > [  472.092020][ T6181] NIP [00007fffb5565fac] 0x7fffb5565fac
> > > > [  472.092027][ T6181] LR [00007fffb5565fac] 0x7fffb5565fac
> > > > [  472.092033][ T6181] ---- interrupt: 3000
> > > > [  472.098256][ T6181] pstore: backend (nvram) writing error (-1)
> > > > 
> > > > This is the host, cannot run the kernel as guest because it fails to boot most
> > > > of the time inside KVM.
> > > > 
> > > > > 
> > > > > > Nonethless, there are quite a few platforms. Originally no preemption
> > > > > > was the only option, and that's the reason why many people run that.
> > > > > > It's the conservative, known working option. And that's the reason a lot
> > > > > > of platfrom code does not get tested with more aggressive preemtion
> > > > > > models, and never gets fixed to work with them.
> > > > > > 
> > > > > 
> > > > > Full preemption has been there for many years!.
> > > > 
> > > > Possible for years, forced only recently.
> > > > 
> > > > > Lazy is not that aggressive compared to that.
> > > > > 
> > > > > > Simply disabling the no preemtion option does not make the platform code
> > > > > > ready.
> > > > > > 
> > > > > 
> > > > > Let's understand your crash case. Let's see where it is going wrong. I am suspecting
> > > > > it is some wrong usage of preemption api rather than arch can't support preemption.
> > > > 
> > > > Very likely some wrong use of the preemption API by the arch code, or no
> > > > use where it should have been used. It did not matter so long as people
> > > > could run their no preempt configs and ignore the problem.
> > > > 
> > > > Thanks
> > > 
> > > 
> > > Friendly LLM analysis says preemption is enabled too early and before
> > > completing kvmppc_handle_exit_hv() / kvmppc_handle_nested_exit()
> > > 
> > > Below is ONLY a speculation and completely UNTESTED.
> > > Maybe worth a try.
> > 
> > The patch is munged by the e-mail client, and it causes immediate
> > voluntary preemprion in rcu critical section and hard lockup on starting
> > a KVM VM.
> > 
> > Also it would be sort of bad news if it worked because that would be
> > specific to book3s KVM HV and would not help with the KVM HV from the
> > original report which likely is not book3s, nor with KVM PR.
> > 
> 
> Thanks for trying. We will try a local repro and look into it why stack is
> getting corrupted.

There is also stack corruption when idling. At this point the machine is
stuck half-dead and would not go through with the panic, it requires
dump from the hyprvisor side to capture this.

Thanks

Michal

[542489.550547] [      T0] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: __sprint_symbol+0x1e4/0x1f0
[542489.557809] [      T0] ------------[ cut here ]------------
[542489.557814] [      T0] Voluntary context switch within RCU read-side critical section!
[542489.557822] [      T0] WARNING: [] kernel/rcu/tree_plugin.h:332 at rcu_note_context_switch+0x6a0/0x7b0, CPU#59: swapper/59/0
[542489.557837] [      T0] Modules linked in: vhost_net vhost vhost_iotlb tap rpadlpar_io rpaphp kvm_hv kvm mptcp_diag xsk_diag tcp_diag udp_diag raw_diag inet_diag unix_diag af_packet_diag netlink_diag xt_conntrack xt_MASQUERADE xfrm_user xt_set ip_set xt_addrtype nft_compat nf_conntrack_netbios_ns nf_conntrack_broadcast nft_masq nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct bonding tls nft_chain_nat nf_nat nf_conntrack tun nf_defrag_ipv6 nf_defrag_ipv4 rfkill af_packet binfmt_misc xfs aes_gcm_p10_crypto crypto_simd ibmveth cryptd xts vmx_crypto rtc_generic fuse nvme_fabrics ip_tables x_tables ibmvscsi scsi_transport_srp sd_mod scsi_dh_emc scsi_dh_rdac scsi_dh_alua sg scsi_mod scsi_common nvme nvme_core nvme_keyring nvme_auth pseries_wdt btrfs xor libblake2b raid6_pq dm_mirror dm_region_hash dm_log dm_mod br_netfilter bridge stp llc nf_tables
[542489.557944] [      T0] CPU: 59 UID: 0 PID: 0 Comm: swapper/59 Tainted: G      D W           7.2.2-5.g6824496-default #1 PREEMPT(full) openSUSE Tumbleweed (unreleased)  61871a5f06863b4006f5ef27cd9f18e8a7a2edad
[542489.557957] [      T0] Tainted: [D]=DIE, [W]=WARN
[542489.557961] [      T0] Hardware name: IBM,9824-42A Power11 (architected) 0x820200 0xf000007 of:IBM,FW1110.20 (OB1110_130) hv:phyp pSeries
[542489.557967] [      T0] NIP:  c00000000035d700 LR: c00000000035d6fc CTR: 0000000000000000
[542489.557974] [      T0] REGS: c000001de047f990 TRAP: 0700   Tainted: G      D W            (7.2.2-5.g6824496-default)
[542489.557980] [      T0] MSR:  800000000282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 44002284  XER: 0000000e
[542489.557999] [      T0] CFAR: c00000000022d7a8 IRQMASK: 1 
                           GPR00: c00000000035d6fc c000001de047fc30 c00000000205ad00 000000000000003f 
                           GPR04: 3ffffffffffc02ef c000001de047f950 c000001de047f910 0000003ff0e20000 
                           GPR08: 0000000000000027 0000000000000001 c000001de0191300 0000000044002284 
                           GPR12: c000003ff1614890 c000003fffff9f00 0000000000000000 000000002eef9860 
                           GPR16: 0000000000000000 c000003ff40dcc80 0000003ff0e20000 c000000000058010 
                           GPR20: c000000000058160 c0000000032bcc80 c0000000032bcc80 c000000000057f80 
                           GPR24: ffffffffffffffff 000000000000003b c000001de0191300 c000000003bea6e0 
                           GPR28: c000000000058010 ffffffffffffffff 0000000000000000 c000003ff40de100 
[542489.558066] [      T0] NIP [c00000000035d700] rcu_note_context_switch+0x6a0/0x7b0
[542489.558072] [      T0] LR [c00000000035d6fc] rcu_note_context_switch+0x69c/0x7b0
[542489.558078] [      T0] Call Trace:
[542489.558082] [      T0] [c000001de047fc30] [c00000000035d6fc] rcu_note_context_switch+0x69c/0x7b0 (unreliable)
[542489.558092] [      T0] [c000001de047fcd0] [c0000000013c4fc0] __schedule+0xf0/0x1d10
[542489.558108] [      T0] [c000001de047fe20] [c0000000013c6eb0] schedule_idle+0x30/0x60
[542489.558121] [      T0] [c000001de047fe40] [c0000000002caed8] do_idle+0x258/0x470
[542489.558133] [      T0] [c000001de047feb0] [c0000000002cb3b4] cpu_startup_entry+0x54/0x60
[542489.558145] [      T0] [c000001de047fee0] [c00000000005a720] start_secondary+0x860/0xd70
[542489.558154] [      T0] [c000001de047ffe0] [c00000000000e158] start_secondary_prolog+0x10/0x14
[542489.558162] [      T0] Code: f8c900a8 4bfffe6c 60000000 60420000 3c62ff87 39200001 f9410068 3d4201b2 386329d8 992aa087 4becff91 60000000 <0fe00000> e9410068 4bfff9c4 60420000 
[542489.558184] [      T0] ---[ end trace 0000000000000000 ]---
[542521.228724] [     C59] watchdog: CPU 59 Hard LOCKUP
[542521.228726] [     C59] watchdog: CPU 59 TB:3697937465165330, last heartbeat TB:3697921085812082 (31990ms ago)
[542521.228728] [     C59] Modules linked in: vhost_net vhost vhost_iotlb tap rpadlpar_io rpaphp kvm_hv kvm mptcp_diag xsk_diag tcp_diag udp_diag raw_diag inet_diag unix_diag af_packet_diag netlink_diag xt_conntrack xt_MASQUERADE xfrm_user xt_set ip_set xt_addrtype nft_compat nf_conntrack_netbios_ns nf_conntrack_broadcast nft_masq nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct bonding tls nft_chain_nat nf_nat nf_conntrack tun nf_defrag_ipv6 nf_defrag_ipv4 rfkill af_packet binfmt_misc xfs aes_gcm_p10_crypto crypto_simd ibmveth cryptd xts vmx_crypto rtc_generic fuse nvme_fabrics ip_tables x_tables ibmvscsi scsi_transport_srp sd_mod scsi_dh_emc scsi_dh_rdac scsi_dh_alua sg scsi_mod scsi_common nvme nvme_core nvme_keyring nvme_auth pseries_wdt btrfs xor libblake2b raid6_pq dm_mirror dm_region_hash dm_log dm_mod br_netfilter bridge stp llc nf_tables
[542521.228776] [     C59] CPU: 59 UID: 0 PID: 0 Comm: swapper/59 Tainted: G      D W           7.2.2-5.g6824496-default #1 PREEMPT(full) openSUSE Tumbleweed (unreleased)  61871a5f06863b4006f5ef27cd9f18e8a7a2edad
[542521.228780] [     C59] Tainted: [D]=DIE, [W]=WARN
[542521.228781] [     C59] Hardware name: IBM,9824-42A Power11 (architected) 0x820200 0xf000007 of:IBM,FW1110.20 (OB1110_130) hv:phyp pSeries
[542521.228783] [     C59] NIP:  c0000000013d2c94 LR: c0000000013d5330 CTR: c000000003812be8
[542521.228785] [     C59] REGS: c000003ffed2bd60 TRAP: 0100   Tainted: G      D W            (7.2.2-5.g6824496-default)
[542521.228788] [     C59] MSR:  8000000000081033 <SF,ME,IR,DR,RI,LE>  CR: 42002282  XER: 2004000e
[542521.228794] [     C59] CFAR: 000000000000011c IRQMASK: 3 
                           GPR00: c000000000260a58 c000003ffef2fb60 c00000000205ad00 c000001deaab3c00 
                           GPR04: 00000000000001f8 0000000000000000 fffffffffffe0000 0000000000010000 
                           GPR08: 0000000000000001 0000000000000001 000000001c82ef77 fffffffffffffffd 
                           GPR12: c000000003812be0 c000003fffff9f00 0000000000000000 0000000000000000 
                           GPR16: 0000000000000000 0000000000000000 0000000000000001 c000000003bc2b00 
                           GPR20: c000003ff40c8de8 0000000000000001 c000003ff40c8da8 c000003ffef2fe08 
                           GPR24: 0000000000000000 c000000003bea6e0 000000000000003b 0000003ff0e20000 
                           GPR28: 0000000000780000 c0000000032bbe80 c000003ff40dbe80 c000001deaab3c00 
[542521.228820] [     C59] NIP [c0000000013d2c94] queued_spin_lock_slowpath+0x164/0x19a0
[542521.228824] [     C59] LR [c0000000013d5330] _raw_spin_lock+0x80/0xa0
[542521.228827] [     C59] Call Trace:
[542521.228827] [     C59] [c000003ffef2fb60] [0000000000001000] 0x1000 (unreliable)
[542521.228833] [     C59] [c000003ffef2fc60] [c000001de0191300] 0xc000001de0191300
[542521.228836] [     C59] [c000003ffef2fc80] [c000000000260a58] __queue_work+0x2f8/0x740
[542521.228841] [     C59] [c000003ffef2fd50] [c00000000038ceac] call_timer_fn+0x4c/0x260
[542521.228845] [     C59] [c000003ffef2fde0] [c00000000038d308] __run_timers+0x248/0x410
[542521.228849] [     C59] [c000003ffef2fec0] [c00000000038d56c] run_timer_softirq+0x9c/0x130
[542521.228852] [     C59] [c000003ffef2fef0] [c00000000023ab0c] handle_softirqs+0x14c/0x4a0
[542521.228855] [     C59] [c000003ffef2ffe0] [c000000000018418] do_softirq_own_stack+0x38/0x50
[542521.228860] [     C59] [c000001de047f910] [c00000000023a238] __irq_exit_rcu+0x198/0x1c0
[542521.228862] [     C59] [c000001de047f940] [c000000000026acc] timer_interrupt+0x1cc/0x4f0
[542521.228866] [     C59] [c000001de047f9b0] [c000000000009fdc] decrementer_common_virt+0x28c/0x290
[542521.228871] [     C59] ---- interrupt: 900 at plpar_hcall_norets_notrace+0x18/0x2c
[542521.228876] [     C59] NIP:  c0000000001b7634 LR: c0000000013c4898 CTR: 0000000000000000
[542521.228877] [     C59] REGS: c000001de047f9e0 TRAP: 0900   Tainted: G      D W            (7.2.2-5.g6824496-default)
[542521.228879] [     C59] MSR:  800000000280b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 22000282  XER: 2004000e
[542521.228886] [     C59] CFAR: 0000000000000000 IRQMASK: 0 
                           GPR00: 0000000000000000 c000001de047fc80 c00000000205ad00 0000000000000000 
                           GPR04: 0000000000000000 0000000000000039 0000000000000000 0000000000000000 
                           GPR08: ffffffffffffff7f 0000000000000000 ffffffffffffffff 0000000000000000 
                           GPR12: 0000000000000000 c000003fffff9f00 0000000000000000 000000002eef9860 
                           GPR16: 0000000000000000 0000000000000001 0000000000000001 c000000000058010 
                           GPR20: c000000000058160 0000000000000002 c000000000057fb0 c000000000057f80 
                           GPR24: ffffffffffffffff 0001ed6431fbb852 0000000000000000 0000000000000001 
                           GPR28: 0000000000000000 0000000000000000 c0000000032a1878 c0000000032a1870 
[542521.228911] [     C59] NIP [c0000000001b7634] plpar_hcall_norets_notrace+0x18/0x2c
[542521.228915] [     C59] LR [c0000000013c4898] check_and_cede_processor+0x48/0x60
[542521.228918] [     C59] ---- interrupt: 900
[542521.228919] [     C59] [c000001de047fc80] [ffffffffffffffff] 0xffffffffffffffff (unreliable)
[542521.228922] [     C59] [c000001de047fce0] [c0000000013c4944] dedicated_cede_loop+0x94/0x1a0
[542521.228926] [     C59] [c000001de047fd30] [c0000000013c3f00] cpuidle_enter_state+0x300/0x748
[542521.228930] [     C59] [c000001de047fde0] [c000000000f5ff10] cpuidle_enter+0x50/0x80
[542521.228935] [     C59] [c000001de047fe20] [c0000000002ba358] call_cpuidle+0x48/0x90
[542521.228939] [     C59] [c000001de047fe40] [c0000000002caf5c] do_idle+0x2dc/0x470
[542521.228942] [     C59] [c000001de047feb0] [c0000000002cb3b8] cpu_startup_entry+0x58/0x60
[542521.228945] [     C59] [c000001de047fee0] [c00000000005a720] start_secondary+0x860/0xd70
[542521.228948] [     C59] [c000001de047ffe0] [c00000000000e158] start_secondary_prolog+0x10/0x14
[542521.228951] [     C59] Code: 41810e8c 39400000 39000000 3cc0fffe 60000000 813f0000 71290001 41820050 3ce00001 60000000 60420000 7c40003c <71090001> 394a0001 40820028 3d2201ba 
[568119.679467] [     C59] pstore: dump skipped in NMI path because of concurrent dump