Re: [PATCH] cpuidle-haltpoll: Enable kvm guest polling when dedicated physical CPUs are available

[Paolo Bonzini]

On 13/08/19 02:55, Wanpeng Li wrote:
>> I think KVM_HINTS_REALTIME is being abused somewhat.
>> It has no clear meaning and used in different locations
>> for different purposes.
> 
> Now it disables pv queued spinlock, pv tlb shootdown, pv sched yield
> which are not expected present in vCPUs are never preempted for an
> unlimited time scenario.

Guest side polling definitely matches the purpose of KVM_HINTS_REALTIME.
 While host-side polling is conditional on single_task_running, this is
obviously not true of guest-side polling.

The alternative would be to enable it only if KVM_FEATURE_POLL_CONTROL
is available, but I prefer Wanpeng's patch.

Paolo

>> For example, i think that using pv queued spinlocks and
>> haltpoll is a desired scenario, which the patch below disallows.
> 
> So even if dedicated pCPU is available, pv queued spinlocks should
> still be chose if something like vhost-kthreads are used instead of
> DPDK/vhost-user. kvm adaptive halt-polling will compete with
> vhost-kthreads, however, poll in guest unaware other runnable tasks in
> the host which will defeat vhost-kthreads.
> 
> Regards,
> Wanpeng Li
>