Re: [PATCH 2/2] x86/idle: use dynamic halt poll

[Wanpeng Li]

2017-06-27 20:07 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>
>
> On 27/06/2017 13:22, Yang Zhang wrote:
>>>>
>>>> Regarding the good/bad idea part, KVM's polling is made much more
>>>> acceptable by single_task_running().  At least you need to integrate it
>>>> with paravirtualization.  If the VM is scheduled out, you shrink the
>>>> polling period.  There is already vcpu_is_preempted for this, it is used
>>>> by mutexes.
>>>
>>> I have considered single_task_running() before. But since there is no
>>> such paravirtual interface currently and i am not sure whether it is a
>>> information leak from host if introducing such interface, so i didn't do
>>> it. Do you mean vcpu_is_preempted can do the same thing? I check the
>>> code and seems it only tells whether the VCPU is scheduled out or not
>>> which cannot satisfy the needs.
>>
>> Can you help to answer my confusion? I have double checked the code, but
>> still not get your point. Do you think it is necessary to introduce an
>> paravirtual interface to expose single_task_running() to guest?
>
> I think vcpu_is_preempted is a good enough replacement.

For example, vcpu->arch.st.steal.preempted is 0 when the vCPU is sched
in and vmentry, then several tasks are enqueued on the same pCPU and
waiting on cfs red-black tree, the guest should avoid to poll in this
scenario, however, vcpu_is_preempted returns false and guest decides
to poll.

Regards,
Wanpeng Li