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

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Message-ID: <2d5b9eaa-0996-94b3-4823-3d2ba8018611@redhat.com>
Date:   Thu, 17 Aug 2017 12:31:25 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Paolo Bonzini <pbonzini@...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 12:23, Paolo Bonzini wrote:
> 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.
> 

Thanks, now I get it, then of course srcu is the right thing to do.

> Paolo
> 


-- 

Thanks,

David