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Tue, 15 Sep 2026 09:09:00 +0000 (GMT) Received: from [9.124.211.238] (unknown [9.124.211.238]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Tue, 15 Sep 2026 09:09:00 +0000 (GMT) Message-ID: Date: Tue, 15 Sep 2026 14:38:59 +0530 Precedence: bulk X-Mailing-List: kvm-ppc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [Linux PPC] Disable PREEMPT To: =?UTF-8?Q?Michal_Such=C3=A1nek?= , Narayana Murty N Cc: "Christophe Leroy (CS GROUP)" , Michael Ellerman , harshpb@linux.ibm.com, Gautam Menghani , amachhiw@linux.ibm.com, Christian Zigotzky , linuxppc-dev , "kvm-ppc@vger.kernel.org" , Madhavan Srinivasan , Peter Zijlstra , "R.T.Dickinson" , hypexed@yahoo.com.au, mad skateman , Christian Zigotzky , Samir M , Venkat Rao Bagalkote References: <5ed38c3e-4aaa-4a91-9eda-e31d09b44abc@linux.ibm.com> <4eef1f0b-0c02-4377-b950-43c3b120865a@linux.ibm.com> <393117bc-094e-4c47-b061-f255f5bff937@linux.ibm.com> <25cf64ab-2a32-4d4c-a0c7-2e9007c276d4@linux.ibm.com> Content-Language: en-US From: Shrikanth Hegde In-Reply-To: Content-Type: text/plain; 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On 9/15/26 1:05 PM, Michal Suchánek wrote: > On Tue, Sep 15, 2026 at 11:17:23AM +0530, Narayana Murty N wrote: >> >> >> On 14/09/26 4:28 PM, Michal Suchánek wrote: >>> On Fri, Sep 11, 2026 at 10:39:10PM +0530, Shrikanth Hegde wrote: >>>> Hi Michal, >>>> >>>> On 9/10/26 5:46 PM, Michal Suchánek wrote: >>>>> On Thu, Sep 10, 2026 at 01:29:08PM +0200, Michal Suchánek wrote: >>>>>> On Thu, Sep 10, 2026 at 04:11:20PM +0530, Shrikanth Hegde wrote: >>>>>>> >>>>>>> >>>>>>> On 9/10/26 2:29 PM, Michal Suchánek wrote: >>>>>>>> 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 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. >>>>>>>> >>>> >>>> As we discussed offlist, samir helped to run a similar test on his machine, and he didn't >>>> run into issue so far. we will try more. >>>> >>>> I am just wondering what different in your case? >>>> By any chance we are running into below one? Can you check your gcc version? >>>> >>>> https://lore.kernel.org/all/CAABZP2z=xu+07-y5fqFLidZz1VpSgrSwXa1mFHPb=b3Ezr3OtA@mail.gmail.com/ >>>> >>>> Maybe CONFIG_DEBUG_PREEMPT worth a try to if it shows up anything. >>> >>> Does not change anything, the kernel crashes all the same. >>> >>> Nonetheless, I noticed that a tool that does some BPF tracing is >>> running, and without it the problem is not reproducible. >>> >>> Thanks >>> >>> Michal >> Thanks Michal for the update. >> >> Interesting finding! Since the crash is only reproducible when the BPF >> tracing tool is running, could you share the following details? >> >> Which BPF tracing tool are you using (e.g., bpftrace, perf, bcc-based tool, >> a custom tool)? > > A custom tool, likely a version of this: > https://build.opensuse.org/package/show/security:sensor/velociraptor > >> What is the specific BPF program or filter being attached — >> particularly which kernel function(s) or tracepoints it hooks into (e.g., is >> it attaching a kprobe/uprobe/tracepoint anywhere inside >> kvmhv_run_single_vcpu or related KVM HV paths)? >> What is the BPF program type — kprobe, tracepoint, perf_event, fentry/fexit, >> etc.? > > It would be likely these programs: > https://github.com/SUSE/linux-security-sensor/tree/sensor-base-0.76.7/vql/linux/bpf > > Thanks > > Michal It seems easy to re-create. The key is running bcc/bpf program in parallel to kernel build. For example, these two in parallal, leads to stack corruption panic. Stacktrace does differ sometimes. ./funccount rcu* -d 100 make -j 32 Boom. Will try to debug this further. [ 3979.381003][ T0] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: prb_reserve+0x498/0x4a0 [ 3979.385530][ C114] pstore: backend (nvram) writing error (-1) [ 3980.385590][ C114] Oops: System Reset, sig: 6 [#1] [ 3980.385605][ C114] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries [ 3980.385611][ C114] Modules linked in: bonding pseries_rng rng_core vmx_crypto fuse ibmvfc ibmveth dm_mirror dm_region_hash dm_log autofs4 [ 3980.385630][ C114] CPU: 114 UID: 0 PID: 0 Comm: swapper/114 Not tainted 7.3.0-rc1-00026-gd3d9a20b4eb9 #23 PREEMPT(full) [ 3980.385634][ C114] Hardware name: IBM,9043-MRU Power11 (architected) 0x820200 0xf000007 of:IBM,FW1120.00 (RF1120_183) hv:phyp pSeries [ 3980.385636][ C114] NIP: c0000000001ae714 LR: c000000001552b08 CTR: 0000000000000000 [ 3980.385638][ C114] REGS: c0000007ffc2fd60 TRAP: 0100 Not tainted (7.3.0-rc1-00026-gd3d9a20b4eb9) [ 3980.385641][ C114] MSR: 800000000298b033 CR: 22000202 XER: 20040006 [ 3980.385649][ C114] CFAR: 000000000000011c IRQMASK: 3 [ 3980.385649][ C114] GPR00: 0000000000000000 c00000040030fd50 c000000001be8100 0000000000000000 [ 3980.385649][ C114] GPR04: 0000000000000001 000000000000003a 0000000000000000 0000010000000000 [ 3980.385649][ C114] GPR08: ffffffffffffffbf 0000000000000000 ffffffffffffffff 0000000000000000 [ 3980.385649][ C114] GPR12: 0000000000000000 c0000007fffdeb00 0000000000000000 000000002ef01f40 [ 3980.385649][ C114] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 [ 3980.385649][ C114] GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000001 [ 3980.385649][ C114] GPR24: 0000000000000001 0000000000000000 0000039e85974a0e c0000000029ab5a8 [ 3980.385649][ C114] GPR28: c0000007fe8f9a40 0000000000000001 c0000000022020a8 c0000000022020b0 [ 3980.385675][ C114] NIP [c0000000001ae714] plpar_hcall_norets_notrace+0x18/0x2c [ 3980.385684][ C114] LR [c000000001552b08] check_and_cede_processor+0x48/0x60 [ 3980.385692][ C114] Call Trace: [ 3980.385693][ C114] [c00000040030fd50] [0000000042000202] 0x42000202 (unreliable) [ 3980.385703][ C114] [c00000040030fdb0] [c000000001552d30] shared_cede_loop+0x70/0x170 [ 3980.385708][ C114] [c00000040030fdf0] [c000000001552170] cpuidle_enter_state+0x300/0x748 [ 3980.385712][ C114] [c00000040030fe90] [c0000000011394f0] cpuidle_enter+0x50/0x80 [ 3980.385718][ C114] [c00000040030fed0] [c0000000002a5a78] call_cpuidle+0x48/0x90 [ 3980.385722][ C114] [c00000040030fef0] [c0000000002acb0c] do_idle+0x2cc/0x470 [ 3980.385725][ C114] [c00000040030ff60] [c0000000002acf7c] cpu_startup_entry+0x4c/0x50 [ 3980.385729][ C114] [c00000040030ff90] [c00000000005be80] start_secondary+0x290/0x2a0 [ 3980.385733][ C114] [c00000040030ffe0] [c00000000000e258] start_secondary_prolog+0x10/0x14 [ 3980.385737][ C114] Code: 3d220063 39295780 7c634a78 7c630074 7863d182 4e800020 3c4c01a4 38429a04 7c421378 7c000026 90010008 44000022 <38800000> 988d0931 80010008 7c0ff120