RE: kdump always hangs in rcu_barrier() -> wait_for_completion()

[Dexuan Cui <decui@microsoft.com>]

> From: Paul E. McKenney <paulmck@kernel.org>
> Sent: Thursday, November 26, 2020 1:42 PM
> 
> > > Another possibility is that rcu_state.gp_kthread is non-NULL, but that
> > > something else is preventing RCU grace periods from completing, but in
> >
> > It looks like somehow the scheduling is not working here: in rcu_barrier()
> > , if I replace the wait_for_completion() with
> > wait_for_completion_timeout(&rcu_state.barrier_completion, 30*HZ), the
> > issue persists.
> 
> Have you tried using sysreq-t to see what the various tasks are doing?

Will try it.

BTW, this is a "Generation 2" VM on Hyper-V, meaning sysrq only starts to
work after the Hyper-V para-virtualized keyboard driver loads... So, at this
early point, sysrq is not working. :-( I'll have to hack the code and use a 
virtual NMI interrupt to force the sysrq handler to be called.
 
> Having interrupts disabled on all CPUs would have the effect of disabling
> the RCU CPU stall warnings.
> 							Thanx, Paul

I'm sure the interrupts are not disabled. Here the VM only has 1 virtual CPU,
and when the hang issue happens the virtual serial console is still responding
when I press Enter (it prints a new line) or Ctrl+C (it prints ^C).

Here the VM does not use the "legacy timers" (PIT, Local APIC timer, etc.) at all.
Instead, the VM uses the Hyper-V para-virtualized timers. It looks the Hyper-V
timer never fires in the kdump kernel when the hang issue happens. I'm 
looking into this... I suspect this hang issue may only be specific to Hyper-V.

Thanks,
-- Dexuan