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Date: Wed, 1 Nov 2017 15:42:22 -0400 From: Waiman Long <longman@...hat.com> To: Boris Ostrovsky <boris.ostrovsky@...cle.com>, Juergen Gross <jgross@...e.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net> Cc: x86@...nel.org, virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org, xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org, Alok Kataria <akataria@...are.com>, Rusty Russell <rusty@...tcorp.com.au>, Paolo Bonzini <pbonzini@...hat.com>, Radim Krčmář <rkrcmar@...hat.com>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH] x86/paravirt: Add kernel parameter to choose paravirt lock type On 11/01/2017 03:01 PM, Boris Ostrovsky wrote: > On 11/01/2017 12:28 PM, Waiman Long wrote: >> On 11/01/2017 11:51 AM, Juergen Gross wrote: >>> On 01/11/17 16:32, Waiman Long wrote: >>>> Currently, there are 3 different lock types that can be chosen for >>>> the x86 architecture: >>>> >>>> - qspinlock >>>> - pvqspinlock >>>> - unfair lock >>>> >>>> One of the above lock types will be chosen at boot time depending on >>>> a number of different factors. >>>> >>>> Ideally, the hypervisors should be able to pick the best performing >>>> lock type for the current VM configuration. That is not currently >>>> the case as the performance of each lock type are affected by many >>>> different factors like the number of vCPUs in the VM, the amount vCPU >>>> overcommitment, the CPU type and so on. >>>> >>>> Generally speaking, unfair lock performs well for VMs with a small >>>> number of vCPUs. Native qspinlock may perform better than pvqspinlock >>>> if there is vCPU pinning and there is no vCPU over-commitment. >>>> >>>> This patch adds a new kernel parameter to allow administrator to >>>> choose the paravirt spinlock type to be used. VM administrators can >>>> experiment with the different lock types and choose one that can best >>>> suit their need, if they want to. Hypervisor developers can also use >>>> that to experiment with different lock types so that they can come >>>> up with a better algorithm to pick the best lock type. >>>> >>>> The hypervisor paravirt spinlock code will override this new parameter >>>> in determining if pvqspinlock should be used. The parameter, however, >>>> will override Xen's xen_nopvspin in term of disabling unfair lock. >>> Hmm, I'm not sure we need pvlock_type _and_ xen_nopvspin. What do others >>> think? >> I don't think we need xen_nopvspin, but I don't want to remove that >> without agreement from the community. > I also don't think xen_nopvspin will be needed after pvlock_type is > introduced. > > -boris Another reason that I didn't try to remove xen_nopvspin is backward compatibility concern. One way to handle it is to depreciate it and treat it as an alias to pvlock_type=queued. Cheers, Longman
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