On 07/19/2018 05:39 PM, 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().
>
> Signed-off-by: Waiman Long <long...@redhat.com>
Reviewed-by: Boris Ostrovsky <boris.ostrov...@oracle.com>
> ---
> arch/x86/xen/spinlock.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index cd97a62..973f10e 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -130,6 +130,10 @@ void xen_uninit_lock_cpu(int cpu)
> void __init xen_init_spinlocks(void)
> {
>
> + /* Don't need to use pvqspinlock code if there is only 1 vCPU. */
> + if (num_possible_cpus() == 1)
> + xen_pvspin = false;
> +
> if (!xen_pvspin) {
> printk(KERN_DEBUG "xen: PV spinlocks disabled\n");
> return;
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