Messages in this thread | | | From | Wanpeng Li <> | Date | Mon, 23 Jul 2018 11:31:13 +0800 | Subject | Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU |
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On Fri, 20 Jul 2018 at 06:03, Waiman Long <longman@redhat.com> wrote: > > On 07/19/2018 05:54 PM, Davidlohr Bueso wrote: > > On Thu, 19 Jul 2018, Waiman Long wrote: > > > >> On a VM with only 1 vCPU, the locking fast paths will always be > >> successful. In this case, there is no need to use the the PV qspinlock > >> code which has higher overhead on the unlock side than the native > >> qspinlock code. > >> > >> The xen_pvspin veriable is also turned off in this 1 vCPU case to > >> eliminate unneeded pvqspinlock initialization in xen_init_lock_cpu() > >> which is run after xen_init_spinlocks(). > > > > Wouldn't kvm also want this? > > > > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c > > index a37bda38d205..95aceb692010 100644 > > --- a/arch/x86/kernel/kvm.c > > +++ b/arch/x86/kernel/kvm.c > > @@ -457,7 +457,8 @@ static void __init sev_map_percpu_data(void) > > static void __init kvm_smp_prepare_cpus(unsigned int max_cpus) > > { > > native_smp_prepare_cpus(max_cpus); > > - if (kvm_para_has_hint(KVM_HINTS_REALTIME)) > > + if (num_possible_cpus() == 1 || > > + kvm_para_has_hint(KVM_HINTS_REALTIME)) > > static_branch_disable(&virt_spin_lock_key); > > } > > That doesn't really matter as the slowpath will never get executed in > the 1 vCPU case.
So this is not needed in kvm tree? https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?h=queue&id=3a792199004ec335346cc607d62600a399a7ee02
Regards, Wanpeng Li
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