> On Jul 8, 2019, at 3:35 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > >> On Mon, 8 Jul 2019, Pingfan Liu wrote: >>> On Mon, Jul 8, 2019 at 3:44 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: >>> >>>> On Fri, 5 Jul 2019, Pingfan Liu wrote: >>>> >>>> I hit a bug on an AMD machine, with kexec -l nr_cpus=4 option. nr_cpus option >>>> is used to speed up kdump process, so it is not a rare case. >>> >>> But fundamentally wrong, really. >>> >>> The rest of the CPUs are in a half baken state and any broadcast event, >>> e.g. MCE or a stray IPI, will result in a undiagnosable crash. >> Very appreciate if you can pay more word on it? I tried to figure out >> your point, but fail. >> >> For "a half baked state", I think you concern about LAPIC state, and I >> expand this point like the following: > > It's not only the APIC state. It's the state of the CPUs in general. > >> For IPI: when capture kernel BSP is up, the rest cpus are still loop >> inside crash_nmi_callback(), so there is no way to eject new IPI from >> these cpu. Also we disable_local_APIC(), which effectively prevent the >> LAPIC from responding to IPI, except NMI/INIT/SIPI, which will not >> occur in crash case. > > Fair enough for the IPI case. > >> For MCE, I am not sure whether it can broadcast or not between cpus, >> but as my understanding, it can not. Then is it a problem? > > It can and it does. > > That's the whole point why we bring up all CPUs in the 'nosmt' case and > shut the siblings down again after setting CR4.MCE. Actually that's in fact > a 'let's hope no MCE hits before that happened' approach, but that's all we > can do. > > If we don't do that then the MCE broadcast can hit a CPU which has some > firmware initialized state. The result can be a full system lockup, triple > fault etc. > > So when the MCE hits a CPU which is still in the crashed kernel lala state, > then all hell breaks lose. > >> From another view point, is there any difference between nr_cpus=1 and >> nr_cpus> 1 in crashing case? If stray IPI raises issue to nr_cpus>1, >> it does for nr_cpus=1. > > Anything less than the actual number of present CPUs is problematic except > you use the 'let's hope nothing happens' approach. We could add an option > to stop the bringup at the early online state similar to what we do for > 'nosmt'. > > How about we change nr_cpus to do that instead so we never have to have this conversation again?
