[LITMUS^RT] Running LITMUS-RT on ARM64

[Andrii Anisov]

Hello Björn,


On 29.08.17 00:49, Björn Brandenburg wrote:
> Unlikely. To my understanding, Linux uses cycle counters on ARM64, and these cycle counters are synchronized across cores.
>
> I do not know how Linux’s time-tracking code interacts with Xen’s virtualization of the cycle counters, in either native or para-virtualized mode.
>
> In particular, what happens when a vCPU is preempted?
It is transparent to the guest domain. For a kernel it looks like an 
additional time spent for the task.

>   To the guest kernel, the currently scheduled guest process “is using the vCPU” (even though it is not making progress since the vCPU itself is not scheduled), so most likely it will be charged for that execution time
Yes, it is.

> As a result, rtspin would be getting confused regarding how much of its simulated workload is already complete.
That is the case.

> 1) rtspin works just fine on bare metal as intended. It was never designed for virtualized environments.
>
> 2) rtspin should not be used as a benchmarking tool under virtualization. You need a real workload, or at least something that carries out a known amount of work.
I'll implement one.

> 3) It’s not just rtspin that will be confused — likely all of LITMUS^RT’s budget tracking and enforcement code will not work as expected under virtualization (when given vCPUs with non-100% utilization). This is because the scheduler is not aware of any times during which a vCPU does not receive service, so tasks will be charged for bogus execution time.
This sounds really bad.

Guys, I'll dig into this stuff. Will get back with results.

-- 

*Andrii Anisov*