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

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Date:   Thu, 19 Jul 2018 18:02:22 -0400
From:   Waiman Long <longman@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     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>, x86@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU

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
>> 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.

-Longman