Re: [Xen-devel] [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU

[Davidlohr Bueso <dave@xxxxxxxxxxxx>]

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@xxxxxxxxxx> 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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