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

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Nov 26, 2020 at 10:59:19PM +0000, Dexuan Cui wrote:
> > 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.

Whatever works!

> > 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.

Fair enough, given that timers not working can also suppress RCU CPU
stall warnings.  ;-)

							Thanx, Paul