linux-kernel - Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array

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Message-ID: <20bd04bf-9d0d-ac95-212e-88530184fbf2@redhat.com>
Date:   Thu, 17 Aug 2017 12:23:06 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>
Cc:     Alexander Graf <agraf@...e.de>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-mips@...ux-mips.org, linux-arm-kernel@...ts.infradead.org,
        kvm@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Christoffer Dall <cdall@...aro.org>,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array

On 17/08/2017 12:20, David Hildenbrand wrote:
> On 17.08.2017 12:18, Paolo Bonzini wrote:
>> On 17/08/2017 11:55, David Hildenbrand wrote:
>>> On 17.08.2017 11:44, Paolo Bonzini wrote:
>>>> On 17/08/2017 11:28, Cornelia Huck wrote:
>>>>> On Thu, 17 Aug 2017 11:16:59 +0200
>>>>> Paolo Bonzini <pbonzini@...hat.com> wrote:
>>>>>
>>>>>> On 17/08/2017 09:36, Cornelia Huck wrote:
>>>>>>>> What if we just sent a "vcpu move" request to all vcpus with the new 
>>>>>>>> pointer after it moved? That way the vcpu thread itself would be 
>>>>>>>> responsible for the migration to the new memory region. Only if all 
>>>>>>>> vcpus successfully moved, keep rolling (and allow foreign get_vcpu again).
>>>>>>>>
>>>>>>>> That way we should be basically lock-less and scale well. For additional 
>>>>>>>> icing, feel free to increase the vcpu array x2 every time it grows to 
>>>>>>>> not run into the slow path too often.  
>>>>>>>
>>>>>>> I'd prefer the rcu approach: This is a mechanism already understood
>>>>>>> well, no need to come up with a new one that will likely have its own
>>>>>>> share of problems.  
>>>>>>
>>>>>> What Alex is proposing _is_ RCU, except with a homegrown
>>>>>> synchronize_rcu.  Using kvm->srcu seems to be the best of both worlds.
>>>>>
>>>>> I'm worried a bit about the 'homegrown' part, though.
>>>>
>>>> I agree, that's why I'm suggesting SRCU instead.  But it's a trick that
>>>> has its uses.  For example, if you were only doing reads from a work
>>>> queue, flush_work_queue could be used as the "homegrown
>>>> synchronize_rcu".  In KVM you might use kvm_make_all_cpus_request, I guess.
>>>>
>>>>> I also may be misunderstanding what Alex means with "vcpu move"...
>>>>
>>>> My interpretation was "resizing the array" (so it moves in memory).
>>>
>>> Unpopular opinion: Let's keep it simple first (straight rcu) and
>>> optimize later on.
>>
>> RCU vs. SRCU is about correctness, not optimization...
> 
> Guess I am still missing the point why RCU cannot be used here.

Because the body of kvm_foreach_vcpu might sleep.

Paolo