Messages in this thread | | | Date | Tue, 12 Aug 2014 09:06:21 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB kthread wakeups |
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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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