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Date: Tue, 12 Aug 2014 09:06:21 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Amit Shah <amit.shah@...hat.com> Cc: linux-kernel@...r.kernel.org, riel@...hat.com, mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com, sbw@....edu Subject: Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB kthread wakeups 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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