Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB kthread wakeups

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Aug 12, 2014 at 01:11:26AM +0530, Amit Shah wrote:
> On (Mon) 11 Aug 2014 [09:28:07], Paul E. McKenney wrote:
> > On Mon, Aug 11, 2014 at 12:43:08PM +0530, Amit Shah wrote:
> > > On (Fri) 08 Aug 2014 [14:46:48], Paul E. McKenney wrote:
> > > > On Fri, Aug 08, 2014 at 02:43:47PM -0700, Paul E. McKenney wrote:
> > > > > On Sat, Aug 09, 2014 at 12:04:24AM +0530, Amit Shah wrote:
> > > > > > On (Fri) 08 Aug 2014 [11:18:35], Paul E. McKenney wrote:
> > > > > 
> > > > > [ . . . ]
> > > > > 
> > > > > > > Hmmm... What happens if you boot a7d7a143d0b4cb1914705884ca5c25e322dba693
> > > > > > > with the kernel parameter "acpi=off"?
> > > > > > 
> > > > > > That doesn't change anything - still hangs.
> > > > > > 
> > > > > > I intend to look at this more on Monday, though - turning in for
> > > > > > today.  In the meantime, if there's anything else you'd like me to
> > > > > > try, please let me know.
> > > > > 
> > > > > OK, given that I still cannot reproduce it, I do need your help with
> > > > > the diagnostics.  And so what sorts of diagnostics work for you in
> > > > > the hung state?  Are you able to dump ftrace buffers?
> > > > > 
> > > > > If you are able to dump ftrace buffers, please enable rcu:rcu_nocb_wake
> > > > > and send me the resulting trace.
> > > > 
> > > > And another random kernel boot parameter to try is rcu_nocb_poll.
> > > 
> > > Right, this gets the boot going again:
> > 
> > OK, that likely indicates a lost wakeup.  The event tracing enabled by
> > "rcu:rcu_nocb_wake" should help track those down.  Last time, it was qemu
> > losing the wakeups, but maybe it is RCU this time.  ;-)
> 
> The guest goes dead pretty early; is there a trick to enabling and
> getting these traces out of the guest that I don't know of that
> doesn't involve being booted into userspace?  I can perhaps try
> getting the trace output out from a virtio-serial channel; but even
> that driver isn't probed yet when the lockup happens.

First boot with the kernel parameter "trace_event=rcu:rcu_nocb_wake".
Then when the system hangs, do "sendkey alt-sysrq-z" at the "(qemu)"
prompt to dump the ftrace buffer.  This hopefully dumps the trace buffer
to dmesg.

In addition "sendkey alt-sysrq-t" at the "(qemu)" prompt dumps all tasks'
stacks, which would also likely be useful information.

							Thanx, Paul