Re: [Xen-devel] Xen 4.8 + Linux 4.9 + Credit2 = can't bootup

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/01/2017 01:52, Konrad Rzeszutek Wilk wrote:
> Hey,
>
> I was trying to bootup on an 30 CPU machine (15 core, SMT).
>
> It works just fine with credit1 (see further down the log)
> but if I try credit2 it ends up hanging during bootup. 
>
> I am a going to naively assume it is due to how the vCPUs are
> exposed (Where they match the physical CPUs under credit1),
> but under credit2 they are different.
>
> The dom0_max_vcpus does not seem to have any affect. When I remove it
> things are still being problematic.
>
> Help!?

This matches the symptoms seen by XenServer when trying to stress 32vcpu
guests under Credit2.  Malcolm did find (based on interpreted iperf
throughput graphs) that Credit2 did seem to preferentially schedule the
lower-number vcpus, rather than scheduling them evenly.

The iperf graphs showed this as having a diminishing graph of vcpu id vs
total traffic passed (which was a straight line under credit1), but
Linux deciding a soft lockup is also very plausible is there is
asynchronous scheduling.

~Andrew

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