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

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Date:   Sun, 22 Jul 2018 21:50:35 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Waiman Long <longman@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        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>,
        the arch/x86 maintainers <x86@...nel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        LKML <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 Sun, 22 Jul 2018, Davidlohr Bueso wrote:

>On Mon, 23 Jul 2018, Wanpeng Li wrote:
>
>>On Fri, 20 Jul 2018 at 06:03, Waiman Long <longman@...hat.com> wrote:
>>>
>>>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
>
>s/veriable
> variable
>
>>>>> 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.
>
>How does this differ then from xen, then? I mean, same principle applies.
>
>>
>>So this is not needed in kvm tree?
>>https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?h=queue&id=3a792199004ec335346cc607d62600a399a7ee02
>
>Hmm I would think that my patch would be more appropiate as it actually does
>what the comment says.

Both would be needed actually yes, but also disabling the virt_spin_lock_key
would be more robust imo.

Thanks,
Davidlohr