From mboxrd@z Thu Jan 1 00:00:00 1970 From: david@redhat.com (David Hildenbrand) Date: Thu, 17 Aug 2017 12:31:25 +0200 Subject: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array In-Reply-To: <20bd04bf-9d0d-ac95-212e-88530184fbf2@redhat.com> References: <20170816194037.9460-1-rkrcmar@redhat.com> <20170817093612.024cc4bc.cohuck@redhat.com> <20170817112829.7795820a.cohuck@redhat.com> <69d47131-2405-b2be-60d7-e40fbd17592a@redhat.com> <6609138c-6a29-5106-39a8-3219ce47f01a@redhat.com> <6b2816ca-d8c0-0819-efdb-c30fd3d1a0e9@redhat.com> <20bd04bf-9d0d-ac95-212e-88530184fbf2@redhat.com> Message-ID: <2d5b9eaa-0996-94b3-4823-3d2ba8018611@redhat.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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 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