linux-kernel - Re: [PATCH RFC v3 1/9] KVM: s390: optimize detection of started vcpus

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Date:   Tue, 29 Aug 2017 14:42:47 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mips@...ux-mips.org, kvm-ppc@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christoffer Dall <cdall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>,
        Alexander Graf <agraf@...e.com>
Subject: Re: [PATCH RFC v3 1/9] KVM: s390: optimize detection of started
 vcpus

On Tue, 29 Aug 2017 14:05:40 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 29.08.2017 13:23, Cornelia Huck wrote:
> > On Tue, 22 Aug 2017 13:31:27 +0200
> > David Hildenbrand <david@...hat.com> wrote:
> >   
> >> On 21.08.2017 22:35, Radim Krčmář wrote:  
> >>> We can add a variable instead of scanning all online VCPUs to know how
> >>> many are started.  We can't trivially tell which VCPU is the last one,
> >>> though.    
> >>
> >> You could keep the started vcpus in a list. Then you might drop unsigned
> >> started_vcpus;
> >>
> >> No started vcpus: Start pointer NULL
> >> Single started vcpu: Only one element in the list (easy to check)  
> >>> 1 started vcpus: More than one element int he list (easy to check)    
> > 
> > I'm not sure the added complication of keeping a list buys us much
> > here: We only have the "look for the last vcpu not stopped" operation
> > for the 2->1 transition.
> >   
> 
> That is wrong, we also have to know the last remaining (started) VCPU.
> For that, right now we have to iterate over all VCPUs.

Yes, but that's only for the 2->1 transition... and all in all that is
still much better that before.

> 
> There shouldn't be much complexity. We already perform changes under a
> lock, so it is as simple as adding/removing from the list.
> 
> Detecting the transitions boils down to looking at two pointers.

Having a private vcpu list feels a bit wrong... and I don't really see
much benefit.