On Tue, Aug 12, 2014 at 11:03:21AM +0530, Amit Shah wrote:
> On (Mon) 11 Aug 2014 [20:45:31], Paul E. McKenney wrote:
[ . . . ]
> > > That is a bit surprising. Is it possible that the system is OOMing
> > > quickly due to grace periods not proceeding? If so, maybe giving the
> > > VM more memory would help.
> >
> > Oh, and it is necessary to build the kernel with CONFIG_RCU_TRACE=y
> > for the rcu_nocb_wake trace events to be enabled in the first place.
> > I am assuming that your kernel was built with CONFIG_MAGIC_SYSRQ=y.
>
> Yes, it is :-) I checked the rcu_nocb_poll cmdline option does indeed
> dump all the ftrace buffers to dmesg.
Good. ;-)
> > If all of that is in place and no joy, is it possible to extract the
> > ftrace buffer from the running/hung guest? It should be in there
> > somewhere! ;-)
>
> I know of only virtio-console doing this (via userspace only,
> though).
As in userspace within the guest? That would not work. The userspace
that the qemu is running in might. There is a way to extract ftrace info
from crash dumps, so one approach would be "sendkey alt-sysrq-c", then
pull the buffer from the resulting dump. For all I know, there might also
be some script that uses the qemu "x" command to get at the ftrace buffer.
Again, I cannot reproduce this, and I have been through the code several
times over the past few days, and am not seeing it. I could start
sending you random diagnostic patches, but it would be much better if
we could get the trace data from the failure.
Thanx, Paul
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