linux-kernel - Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV

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Date:   Mon, 27 Nov 2017 08:43:39 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Eduardo Valentin <eduval@...zon.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Matt Wilson <msw@...zon.com>, Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>, kvm <kvm@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Jan H . Schoenherr" <jschoenh@...zon.de>,
        Anthony Liguori <aliguori@...zon.com>
Subject: Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set

Hi Eduardo,
2017-11-16 12:54 GMT+08:00 Eduardo Valentin <eduval@...zon.com>:
> Hey Radim,
>
> On Thu, Nov 09, 2017 at 03:17:33PM +0100, Radim Krčmář wrote:
>
> <cut>
>
>>
>> This is what I'm doubting, because the patch is adding about two
>> thousand cycles to every spinlock-taken path.
>> Doesn't this patch yield better results?
>>
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 3df743b60c80..d9225e48c11a 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -676,6 +676,12 @@ void __init kvm_spinlock_init(void)
>>  {
>>       if (!kvm_para_available())
>>               return;
>> +
>> +     if (kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED)) {
>> +             static_branch_disable(&virt_spin_lock_key);
>> +             return;
>> +     }
>> +
>
> Yes, the above suggestion is a much better approach. The code has probably changed from the time I wrote the first version. I will refresh with the above suggestion.

Do you mind to send a new version since the merge window is closed?

Regards,
Wanpeng Li

>
>
>>       /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
>>       if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
>>               return;
>>
>> >                                                              However, the key aspect
>> > here is this patch gives a way for the host to instruct the guest to use qspinlock.
>> > Even with Longman's patch which allows guest to select the spinlock implementation,
>> > there should still be the auto-select mode. In such mode, PV_DEDICATED should
>> > allow the host to get the guest to use qspinlock, without, the guest will fallback
>> > to tas when PV_UNHALT == 0.
>>
>> I agree that a flag can be useful for certains setups.
>
> Cool!
>
>>
>
> --
> All the best,
> Eduardo Valentin