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Subject: Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU
To:     Wanpeng Li <kernellwp@gmail.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>
Cc:     Davidlohr Bueso <dave@stgolabs.net>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        xen-devel <xen-devel@lists.xenproject.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
References: <1532036397-19449-1-git-send-email-longman@redhat.com>
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 <CANRm+CyHfvBKVKNk+pKF+s6+1seZ8c+NyjHuepT2Mf+4Cdd-2w@mail.gmail.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <29f7dd77-267a-0ba7-f4e8-6ef3b3de167c@redhat.com>
Date:   Mon, 23 Jul 2018 09:33:51 -0400
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On 07/22/2018 11:31 PM, Wanpeng Li wrote:
> On Fri, 20 Jul 2018 at 06:03, Waiman Long <longman@redhat.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
>>>> 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.
> So this is not needed in kvm tree?
> https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?h=queue&id=3a792199004ec335346cc607d62600a399a7ee02
>
> Regards,
> Wanpeng Li

With only 1 vCPU, the slowpath will not be executed. It will be a
deadlock if it happens. So we don't need to explicitly disable the
static key here.

-Longman