From: Hamza Mahfooz <hamzamahfooz@xxxxxxxxxxxxxxxxxxx> Sent: Monday, February 24, 2025 6:49 AM > > On Fri, Feb 21, 2025 at 11:01:09PM +0000, Michael Kelley wrote: > > From: Hamza Mahfooz <hamzamahfooz@xxxxxxxxxxxxxxxxxxx> Sent: Friday, February > 21, 2025 1:31 PM > > > > > > Since, the panic handlers may require certain cpus to be online to panic > > > gracefully, we should call them before turning off SMP. Without this > > > re-ordering, on Hyper-V hv_panic_vmbus_unload() times out, because the > > > vmbus channel is bound to VMBUS_CONNECT_CPU and unless the crashing cpu > > > is the same as VMBUS_CONNECT_CPU, VMBUS_CONNECT_CPU will be offlined by > > > crash_smp_send_stop() before the vmbus channel can be deconstructed. > > > > Hamza -- what specifically is the problem with the way vmbus_wait_for_unload() > > works today? That code is aware of the problem that the unload response comes > > only on the VMBUS_CONNECT_CPU, and that cpu may not be able to handle > > the interrupt. So the code polls the message page of each CPU to try to get the > > unload response message. Is there a scenario where that approach isn't working? > > > > It doesn't work on arm64 (if the crashing cpu isn't VMBUS_CONNECT_CPU), it > always ends up at "VMBus UNLOAD did not complete" without fail. It seems > like arm64's crash_smp_send_stop() is more aggressive than x86's. FWIW, I tested on a D16plds_v6 arm64 VM in Azure, running Ubuntu 20.04 with a linux-next20252021 kernel. I caused a panic using "echo c >/proc/sysrq-trigger" using "taskset" to make sure the panic is triggered on a CPU other than CPU 0. I didn't see any problem. The panic code path completely quickly, and there were no messages from vmbus_wait_for_unload(), including none of the periodic "Waiting for unload" messages . I tried initiating the panic on several different CPUs (4, 7, and 15) with the same result. I tested with kdump disabled and with kdump enabled, both with no problems. So I think the current vmbus_wait_for_unload() code works on arm64, as least in some ordinary scenarios. Any key differences in the configuration or test environment when you see the "did not complete" message? Michael
