linux-kernel - Re: [PATCH] xen/spinlock: Don't use pvqspinlock if only 1 vCPU

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Date:   Thu, 19 Jul 2018 15:59:12 -0400
From:   Waiman Long <longman@...hat.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] xen/spinlock: Don't use pvqspinlock if only 1 vCPU

On 07/19/2018 03:18 PM, Boris Ostrovsky wrote:
> 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@...hat.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

The other pvqspinlock initialization has no runtime impact other than
allocating a bit of extra memory. Anyway, I will revise the patch to
disable xen_pvspin under such condition.

-Longman