linux-kernel - Re: [PATCH v5 01/13] KVM: x86: Cache total page count to avoid traversing the memslot array

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Date:   Wed, 20 Oct 2021 20:40:19 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 01/13] KVM: x86: Cache total page count to avoid
 traversing the memslot array

On 20.10.2021 00:31, Sean Christopherson wrote:
> On Tue, Oct 19, 2021, Sean Christopherson wrote:
>> On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
>>> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
>>>
>>> There is no point in recalculating from scratch the total number of pages
>>> in all memslots each time a memslot is created or deleted.
>>>
>>> Just cache the value and update it accordingly on each such operation so
>>> the code doesn't need to traverse the whole memslot array each time.
>>>
>>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
>>> ---
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 28ef14155726..65fdf27b9423 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -11609,9 +11609,23 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>>>   				const struct kvm_memory_slot *new,
>>>   				enum kvm_mr_change change)
>>>   {
>>> -	if (!kvm->arch.n_requested_mmu_pages)
>>> -		kvm_mmu_change_mmu_pages(kvm,
>>> -				kvm_mmu_calculate_default_mmu_pages(kvm));
>>> +	if (change == KVM_MR_CREATE)
>>> +		kvm->arch.n_memslots_pages += new->npages;
>>> +	else if (change == KVM_MR_DELETE) {
>>> +		WARN_ON(kvm->arch.n_memslots_pages < old->npages);
>>> +		kvm->arch.n_memslots_pages -= old->npages;
>>> +	}
>>> +
>>> +	if (!kvm->arch.n_requested_mmu_pages) {
>>
>> Hmm, once n_requested_mmu_pages is set it can't be unset.  That means this can be
>> further optimized to skip avoid taking mmu_lock on flags-only changes (and
>> memslot movement).  E.g.
>>
>> 	if (!kvm->arch.n_requested_mmu_pages &&
>> 	    (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
>>
>> 	}
>>
>> It's a little risky, but kvm_vm_ioctl_set_nr_mmu_pages() would need to be modified
>> to allow clearing n_requested_mmu_pages and it already takes slots_lock, so IMO
>> it's ok to force kvm_vm_ioctl_set_nr_mmu_pages() to recalculate pages if it wants
>> to allow reverting back to the default.
> 
> Doh, and then I read patch 2...
> 
> I would swap the order of patch 2 and patch 1, that way the optimization patch is
> super simple, and you don't end up reworking a bunch of code that was added in the
> immediately preceding patch.  E.g. as a first patch
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 28ef14155726..f3b1aed08566 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11609,7 +11609,8 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>                                  const struct kvm_memory_slot *new,
>                                  enum kvm_mr_change change)
>   {
> -       if (!kvm->arch.n_requested_mmu_pages)
> +       if (!kvm->arch.n_requested_mmu_pages &&
> +           (change == KVM_MR_CREATE || change == KVM_MR_DELETE))
>                  kvm_mmu_change_mmu_pages(kvm,
>                                  kvm_mmu_calculate_default_mmu_pages(kvm));
> 
> 
> 

Will do.

Thanks,
Maciej