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Re: [Xen-devel] Xen unstability on HP Moonshot m400

On Sat, 2015-03-21 at 13:34 +0100, Christoffer Dall wrote:
> Hi,
> I have been experiencing a problematic crash running Xen on m400 over
> the last few days.  I already spoke to Ian and Stefano about this, but
> thought I'd summarize what I've seen so far and loop in a wider
> audience.
> The basic setup is this:
>  - Two m400 nodes, one running Linux bare-metal, the other running
> Xen.
>  - The Xen node runs Dom0 and 1 DomU
>  - The m400 has a Mellanox Connectx-3 PCIe 10G ethernet card with two
> parts on it
>  - Dom0 uses NAT forwarding from Dom0's eth0 (which is connected to
> the internet) and regular bridging to eth1 which is connected to a
> private VLAN to the bare-metal node
>  - Dom0 and DomU are configured with 14GB of ram, 4 cpus each
>  - DomU runs apache2 serving the GCC manual (see
> https://github.com/chazy/kvmperf/blob/master/cmdline_tests/apache_install.sh)
> The bare-metal node runs apache bench, like this: "ab -n 100000 -c 100
> http://secure-web.cisco.com/1r5tZ8-7RF8gHRANwFdizEZzgeMsjxVO0yKbYiV4zy7LeiUfYBXMkFq7FGW_SZ1x-VxdzyK-ErDsOUiQ9z2x-Ny7XkL_loHP8ene_BuNFscGyWmQ3r6CtXAYaZCY4xRmmPT1uJOsZDLMu7j-LfCOGmQDSdBwgW7QYukI2bCtTrXM/http%3A%2F%2F10.10.1.120%2Fgcc%2Findex.html";
> ( is the DomU IP address of the bridged interface to eth1)
> What happens now is that the entire Xen node goes down.  I see various
> errors in the kernel log, some examples:
> http://pastebin.ubuntu.com/10642148/
> http://pastebin.ubuntu.com/10642177/
> http://pastebin.ubuntu.com/10642181/
> http://pastebin.ubuntu.com/10635573/
> All Linux kernels are 3.18 plus some tweaks for the m400 cartridge:
> https://github.com/columbia/linux-kvm-arm/tree/columbia-armvirt-3.18

Is it worth adding
 to your kernel? It isn't Xen specific but it's perhaps possible that Xen opens 
the window wider.

How confident are you in
(although I suppose you aren't running in ACPI mode if you are running

If we think the issue might be to do with coherency of foreign mappings
undergoing i/o from dom0 and we've already ruled out disk (by using a
loopback mounted rootfs) then it might be worth bodging netback to
always copy too.

Adding a call to skb_orphan_frags right before the netif_receive_skb in
drivers/net/xen-netback/netback.c:xenvif_tx_submit is a simple but
rather inefficient way of doing that (so I hope it doesn't perturb the

Stefano (who is more familiar with the Linux swiotlb side of things than
me) is travelling this week so he'll be on West coast time, not sure
when he gets off a plane nor if he's on email anyway (he's at ELC + this
ARM ACPI thing)


> config: columbia_armvirt_defconfig (from the same tree:
> https://github.com/columbia/linux-kvm-arm/blob/columbia-armvirt-3.18/arch/arm64/configs/columbia_armvirt_defconfig)
> I have also tried applying a set of swiotlb fixes provided by Stefano
> to both the Dom0 and DomU kernel, like this:
> https://github.com/columbia/linux-kvm-arm/commits/columbia-armvirt-3.18-with-xen-fixes
> With these patches I sometime also saw this error in the kernel log
> (but not always):
> http://pastebin.ubuntu.com/10635062/
> Other data points of interest:
>  - Bare-metal serving apache doesn't exhibit this behavior
>  - KVM guests with bridged networking on identical hardware/setup with
> the same kernels also don't exhibit this behavior
>  - Other physical identical nodes exhibit the same behavior
>  - Just running Dom0 serving apache without running DomU doesn't
> appear to exhibit this behavior
>  - Running apache on Dom0 and benchmarking the system using Dom0's ip
> address but running DomU idle in the background causes this behavior
> (http://pastebin.ubuntu.com/10642311/), but the system seems to stay
> alive (at least for much longer)!
> Stefano suggested that this could be related DMA cache coherency, but
> I'm not sure how to investigate that further.
> This is a somewhat urgent issue for us at Columbia so I would
> appreciate any feedback and/or ideas and will be happy to try out any
> debugging steps to get to the bottom of this.
> Thanks,
> -Christoffer

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