Re: Regression on vcpu_is_preempted()

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Oct 29, 2022 at 05:15:15PM +0800, Miaohe Lin wrote:
> On 2022/10/29 16:58, Peter Zijlstra wrote:
> > On Fri, Oct 28, 2022 at 04:48:21PM +0800, Miaohe Lin wrote:
> >>   When scheduler tries to select a CPU to run the gc thread,
> >>   available_idle_cpu() will check whether vcpu_is_preempted().  It
> >>   will choose other vcpu to run gc threads when the current vcpu is
> >>   preempted. But the preempted vcpu has no other work to do except
> >>   continuing to do gc. In our guest, there are more vcpus than java gc
> >>   threads. So there could always be some available vcpus when
> >>   scheduler tries to select a idle vcpu (runing on host). This leads
> >>   to lots of cpu migrations and results in regression.
> >>
> >>   I'm not really familiar with this mechanism. Is this a problem that
> >>   needs to be fixed or improved? Or is this just expected behavior?
> >>   Any response would be really appreciated!
> > 
> > This is pretty much expected behaviour. When a vCPU is preempted the
> > guest cannot know it's state or latency. Typically in the overcomitted
> > case another vCPU will be running on the CPU and getting our vCPU thread
> > back will take a considerable amount of time.
> 
> I see. Many thanks for your kindly reply and explanation. :)
> 
> > 
> > If you know you're not over-committed, perhaps you should configure your
> > VM differently.
> 
> Do you have any suggestion about how should I configure my VM when it's not over-committed?

I'm not an expert on VMs, but IIRC when you construct a pinned VM (ie.
1:1 vCPU:CPU relations) this all goes away.