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Subject: Re: [PATCH] xen/spinlock: Don't use pvqspinlock if only 1 vCPU
To:     Waiman Long <longman@redhat.com>, Juergen Gross <jgross@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Cc:     x86@kernel.org, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
References: <1532008100-26922-1-git-send-email-longman@redhat.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <22f2465a-128e-730f-bc51-5a55ef16a039@oracle.com>
Date:   Thu, 19 Jul 2018 15:18:58 -0400
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Sender: linux-kernel-owner@vger.kernel.org
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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 <longman@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