Messages in this thread | | | Date | Sun, 22 Jul 2018 21:50:35 -0700 | From | Davidlohr Bueso <> | Subject | Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU |
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On Sun, 22 Jul 2018, Davidlohr Bueso wrote:
>On Mon, 23 Jul 2018, Wanpeng Li wrote: > >>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 > >s/veriable > variable > >>>>> 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. > >How does this differ then from xen, then? I mean, same principle applies. > >> >>So this is not needed in kvm tree? >>https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?h=queue&id=3a792199004ec335346cc607d62600a399a7ee02 > >Hmm I would think that my patch would be more appropiate as it actually does >what the comment says.
Both would be needed actually yes, but also disabling the virt_spin_lock_key would be more robust imo.
Thanks, Davidlohr
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