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From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Subject: Re: [PATCH 2/2] x86/idle: use dynamic halt poll
Date: Tue, 27 Jun 2017 15:40:44 +0200
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Cc: Wanpeng Li <kernellwp@gmail.com>,
        Yang Zhang <yang.zhang.wz@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>, tony.luck@intel.com,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>, mchehab@kernel.org,
        Andrew Morton <akpm@linux-foundation.org>, krzk@kernel.org,
        jpoimboe@redhat.com, Andy Lutomirski <luto@kernel.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Thomas Garnier <thgarnie@google.com>,
        Robert Gerst <rgerst@gmail.com>,
        Mathias Krause <minipli@googlemail.com>,
        douly.fnst@cn.fujitsu.com, Nicolai Stange <nicstange@gmail.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, dvlasenk@redhat.com,
To: Paolo Bonzini <pbonzini@redhat.com>
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2017-06-27 14:28+0200, Paolo Bonzini:
> On 27/06/2017 14:23, Wanpeng Li wrote:
>>>>> 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.
> 
> ... which is not necessarily _wrong_.  It's just a different heuristic.

Right, it's just harder to use than host's single_task_running() -- the
VCPU calling vcpu_is_preempted() is never preempted, so we have to look
at other VCPUs that are not halted, but still preempted.

If we see some ratio of preempted VCPUs (> 0?), then we stop polling and
yield to the host.  Working under the assumption that there is work for
this PCPU if other VCPUs have stuff to do.  The downside is that it
misses information about host's topology, so it would be hard to make it
work well.