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

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Date:   Tue, 27 Jun 2017 19:22:57 +0800
From:   Yang Zhang <yang.zhang.wz@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.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 2017/6/23 11:58, Yang Zhang wrote:
> On 2017/6/22 19:51, Paolo Bonzini wrote:
>>
>>
>> On 22/06/2017 13:22, root wrote:
>>>  ==============================================================
>>>
>>> +poll_grow: (X86 only)
>>> +
>>> +This parameter is multiplied in the grow_poll_ns() to increase the
>>> poll time.
>>> +By default, the values is 2.
>>> +
>>> +==============================================================
>>> +poll_shrink: (X86 only)
>>> +
>>> +This parameter is divided in the shrink_poll_ns() to reduce the poll
>>> time.
>>> +By default, the values is 2.
>>
>> Even before starting the debate on whether this is a good idea or a bad
>> idea, KVM reduces the polling value to the minimum (10 us) by default
>
> I noticed it. It looks like the logic inside KVM is more reasonable. I
> will do more testing to compare the two.
>
>> when polling fails.  Also, it shouldn't be bound to
>> CONFIG_HYPERVISOR_GUEST, since there's nothing specific to virtual
>> machines here.
>
> Yes. The original idea to use CONFIG_HYPERVISOR_GUEST because this
> mechanism will only helpful inside VM. But as Thomas mentioned on other
> thread it is wrong to use it since most distribution kernel will set it
> to yes and still affect the bare metal. I will integrate it with
> paravirtualizaion part as you suggested in below.
>
>>
>> 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.

Hi Paolo

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?

-- 
Yang
Alibaba Cloud Computing