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

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Message-ID: <fd7acd49-9d37-4495-79cd-250e3b54cac2@redhat.com>
Date:   Tue, 27 Jun 2017 14:07:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Zhang <yang.zhang.wz@...il.com>, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com
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Subject: Re: [PATCH 2/2] x86/idle: use dynamic halt poll



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.

Paolo