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

From	Wanpeng Li <kernellwp@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 2/2] x86/idle: use dynamic halt poll
Date	2017-06-27 14:30 +0200
Message-ID	<tWXMZ-pJ-17@gated-at.bofh.it> (permalink)
References	(1 earlier) <tV8tb-3Zd-7@gated-at.bofh.it> <tV8We-4b0-15@gated-at.bofh.it> <tVnVf-5pm-9@gated-at.bofh.it> <tWWQV-8f0-9@gated-at.bofh.it> <tWXtD-hF-3@gated-at.bofh.it>
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2017-06-27 20:07 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>
>
> 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

Thread

[PATCH 0/2] x86/idle: add halt poll support root <yang.zhang.wz@gmail.com> - 2017-06-22 13:30 +0200
  [PATCH 1/2] x86/idle: add halt poll for halt idle root <yang.zhang.wz@gmail.com> - 2017-06-22 13:30 +0200
    Re: [PATCH 1/2] x86/idle: add halt poll for halt idle Thomas Gleixner <tglx@linutronix.de> - 2017-06-22 16:30 +0200
      Re: [PATCH 1/2] x86/idle: add halt poll for halt idle Yang Zhang <yang.zhang.wz@gmail.com> - 2017-06-23 06:10 +0200
  [PATCH 2/2] x86/idle: use dynamic halt poll root <yang.zhang.wz@gmail.com> - 2017-06-22 13:30 +0200
    Re: [PATCH 2/2] x86/idle: use dynamic halt poll Paolo Bonzini <pbonzini@redhat.com> - 2017-06-22 14:00 +0200
      Re: [PATCH 2/2] x86/idle: use dynamic halt poll Yang Zhang <yang.zhang.wz@gmail.com> - 2017-06-23 06:00 +0200
        Re: [PATCH 2/2] x86/idle: use dynamic halt poll Yang Zhang <yang.zhang.wz@gmail.com> - 2017-06-27 13:30 +0200
          Re: [PATCH 2/2] x86/idle: use dynamic halt poll Paolo Bonzini <pbonzini@redhat.com> - 2017-06-27 14:10 +0200
            Re: [PATCH 2/2] x86/idle: use dynamic halt poll Wanpeng Li <kernellwp@gmail.com> - 2017-06-27 14:30 +0200
              Re: [PATCH 2/2] x86/idle: use dynamic halt poll Paolo Bonzini <pbonzini@redhat.com> - 2017-06-27 14:30 +0200
                Re: [PATCH 2/2] x86/idle: use dynamic halt poll Radim Krčmář <rkrcmar@redhat.com> - 2017-06-27 15:50 +0200
                Re: [PATCH 2/2] x86/idle: use dynamic halt poll Paolo Bonzini <pbonzini@redhat.com> - 2017-06-27 16:00 +0200
                Re: [PATCH 2/2] x86/idle: use dynamic halt poll Paolo Bonzini <pbonzini@redhat.com> - 2017-06-27 16:30 +0200
                Re: [PATCH 2/2] x86/idle: use dynamic halt poll Radim Krčmář <rkrcmar@redhat.com> - 2017-06-27 16:30 +0200
                Re: [PATCH 2/2] x86/idle: use dynamic halt poll Yang Zhang <yang.zhang.wz@gmail.com> - 2017-07-03 11:30 +0200
                Re: [PATCH 2/2] x86/idle: use dynamic halt poll Thomas Gleixner <tglx@linutronix.de> - 2017-07-03 12:10 +0200
    Re: [PATCH 2/2] x86/idle: use dynamic halt poll Thomas Gleixner <tglx@linutronix.de> - 2017-06-22 16:40 +0200
      Re: [PATCH 2/2] x86/idle: use dynamic halt poll Yang Zhang <yang.zhang.wz@gmail.com> - 2017-06-23 06:10 +0200
    Re: [PATCH 2/2] x86/idle: use dynamic halt poll kbuild test robot <lkp@intel.com> - 2017-06-23 00:50 +0200
  Re: [PATCH 0/2] x86/idle: add halt poll support Yang Zhang <yang.zhang.wz@gmail.com> - 2017-06-22 13:40 +0200
  Re: [PATCH 0/2] x86/idle: add halt poll support Wanpeng Li <kernellwp@gmail.com> - 2017-06-22 14:00 +0200
    Re: [PATCH 0/2] x86/idle: add halt poll support Yang Zhang <yang.zhang.wz@gmail.com> - 2017-06-23 06:10 +0200
      Re: [PATCH 0/2] x86/idle: add halt poll support Wanpeng Li <kernellwp@gmail.com> - 2017-06-23 06:40 +0200
        Re: [PATCH 0/2] x86/idle: add halt poll support Yang Zhang <yang.zhang.wz@gmail.com> - 2017-06-23 08:50 +0200
          Re: [PATCH 0/2] x86/idle: add halt poll support Radim Krčmář <rkrcmar@redhat.com> - 2017-06-27 16:10 +0200

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