linux-kernel - Re: [PATCH] x86/paravirt: Add kernel parameter to choose paravirt lock type

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Message-ID: <575a60df-78b8-de6c-f70b-9714fde90025@redhat.com>
Date:   Wed, 1 Nov 2017 15:42:22 -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>, Jonathan Corbet <corbet@....net>
Cc:     x86@...nel.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, Alok Kataria <akataria@...are.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/paravirt: Add kernel parameter to choose paravirt
 lock type

On 11/01/2017 03:01 PM, Boris Ostrovsky wrote:
> On 11/01/2017 12:28 PM, Waiman Long wrote:
>> On 11/01/2017 11:51 AM, Juergen Gross wrote:
>>> On 01/11/17 16:32, Waiman Long wrote:
>>>> Currently, there are 3 different lock types that can be chosen for
>>>> the x86 architecture:
>>>>
>>>>  - qspinlock
>>>>  - pvqspinlock
>>>>  - unfair lock
>>>>
>>>> One of the above lock types will be chosen at boot time depending on
>>>> a number of different factors.
>>>>
>>>> Ideally, the hypervisors should be able to pick the best performing
>>>> lock type for the current VM configuration. That is not currently
>>>> the case as the performance of each lock type are affected by many
>>>> different factors like the number of vCPUs in the VM, the amount vCPU
>>>> overcommitment, the CPU type and so on.
>>>>
>>>> Generally speaking, unfair lock performs well for VMs with a small
>>>> number of vCPUs. Native qspinlock may perform better than pvqspinlock
>>>> if there is vCPU pinning and there is no vCPU over-commitment.
>>>>
>>>> This patch adds a new kernel parameter to allow administrator to
>>>> choose the paravirt spinlock type to be used. VM administrators can
>>>> experiment with the different lock types and choose one that can best
>>>> suit their need, if they want to. Hypervisor developers can also use
>>>> that to experiment with different lock types so that they can come
>>>> up with a better algorithm to pick the best lock type.
>>>>
>>>> The hypervisor paravirt spinlock code will override this new parameter
>>>> in determining if pvqspinlock should be used. The parameter, however,
>>>> will override Xen's xen_nopvspin in term of disabling unfair lock.
>>> Hmm, I'm not sure we need pvlock_type _and_ xen_nopvspin. What do others
>>> think?
>> I don't think we need xen_nopvspin, but I don't want to remove that
>> without agreement from the community.
> I also don't think xen_nopvspin will be needed after pvlock_type is
> introduced.
>
> -boris

Another reason that I didn't try to remove xen_nopvspin is backward 
compatibility concern. One way to handle it is to depreciate it and
treat it as an alias to pvlock_type=queued.

Cheers,
Longman