Re: [PATCH v2] panic: call panic handlers before panic_other_cpus_shutdown()

[Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>]

On Mon, Feb 24, 2025 at 07:59:28PM +0000, Michael Kelley wrote:
> From: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com> 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@linux.microsoft.com> 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?

Can you try on a Standard_D16pls_v5 with the stock ubuntu image and
kernel crash dump (i.e. linux-crashdump) installed and setup?

> 
> Michael