On 07/19/2018 09:48 AM, 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.
>
> Signed-off-by: Waiman Long <long...@redhat.com>
> ---
> arch/x86/xen/spinlock.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index cd97a62..38f47ae 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -130,7 +130,8 @@ void xen_uninit_lock_cpu(int cpu)
> void __init xen_init_spinlocks(void)
> {
>
> - if (!xen_pvspin) {
> + /* Don't need to use pvqspinlock code if there is only 1 vCPU. */
> + if (!xen_pvspin || num_possible_cpus() == 1) {
> printk(KERN_DEBUG "xen: PV spinlocks disabled\n");
> return;
> }
I think we need to set xen_pvspin to false for such configurations.
Notice that xen_init_lock_cpu() will try to perform some additional
pvspinlock initializations.
-boris
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