Re: [Xen-devel] arm: network throughput decreases 5Mbps (Arndale Exynos5250)

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-04-23 at 06:04 +0100, Sengul Thomas wrote:
> Hello,
> 
> While I was doing some tests in Arndale board, the network (iperf)
> throughput suddenly decreases to 5Mbps (from 100Mbps) out of nowhere.
> 
> This typically happens when I generate traffic using iperf (100Mbps)
> while installing some packages using apt-get in dom0.
> Whenever this behavior happens, I can not see any distinguishable logs
> from any places.

How strange. Do you get any packet loss or just slow delivery of the
packets which are flowing?

This is traffic from the dom0 to the outside world, rather than between
virtual machines on the same host, right?

> After some debugging, I found that usb_hcd_irq handler
> (linux/drivers/usb/core/hcd.c)
> is not called and the pending requests are only handled by periodic
> timer (100ms).
> That is the reason of the sharp throughput reduction. I also checked
> that the flag for
> usb_hcd_irq is not disabled in Linux, and saw no reason for not
> calling usb_hcd_irq.

So is the Arndale using a USB Ethernet device?

> This is quite a odd behavior since I can not see this happening when I
> run native linux
> on top of Arndale, so I guess it is somehow related to xen (maybe vgic
> delivery).
> 
> I would like to know if there is any tracing or debugging helpers (for
> vgic and evt_chn)

If this is traffic with the outside world over a physical NIC (USB or
otherwise) then vgic is the one to concentrate on, evtchn would be for
virtual devices only.

> in Xen or Linux to see further what triggers this behavior.
> I'm a bit tired of using printk all over the places :)

If you press the Ctrl-A key three times (assuming you didn't change it
with the consswitch hypervisor command line option) then you should get
some info about the vgic's state from the 'q' debug key.

It would also be useful to examine the physical interrupt to be sure it
was arriving at the hypervisor. On x86 the 'i' key dumps information
about the physical interrupts (see
xen/arch/x86/irq.c:dump_irqs_keyhandler) but AFAIK there is no
equivalent on ARM, apart from a little bit of hardware state printed by
the 'q' key. If you wanted to code up a more comprehensive dump of the
physical GIC state and wire it up to 'i; that would be very useful here
as well as in the future I imagine.

Thanks,
Ian.



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