linux-kernel - Re: [PATCH v5 4/8] KVM: Optimize kvm_make_vcpus_request

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Date:   Fri, 3 Sep 2021 16:22:47 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/8] KVM: Optimize kvm_make_vcpus_request_mask() a bit

On Fri, Sep 03, 2021, Vitaly Kuznetsov wrote:
> Iterating over set bits in 'vcpu_bitmap' should be faster than going
> through all vCPUs, especially when just a few bits are set.
> 
> Drop kvm_make_vcpus_request_mask() call from kvm_make_all_cpus_request_except()
> to avoid handling the special case when 'vcpu_bitmap' is NULL, move the
> code to kvm_make_all_cpus_request_except() itself.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@...gle.com>

>  bool kvm_make_vcpus_request_mask(struct kvm *kvm, unsigned int req,
>  				 struct kvm_vcpu *except,
>  				 unsigned long *vcpu_bitmap, cpumask_var_t tmp)
>  {
> -	int i, cpu, me;
>  	struct kvm_vcpu *vcpu;
> +	int i, me;
>  	bool called;

Uber nit, if you're moving "int i, me;" to get reverse fir tree ordering, it
should be moved below "bool called;" as well, which you amusingly did do in the
below function :-)
>  
>  	me = get_cpu();
>  

...

> @@ -316,12 +323,23 @@ bool kvm_make_vcpus_request_mask(struct kvm *kvm, unsigned int req,
>  bool kvm_make_all_cpus_request_except(struct kvm *kvm, unsigned int req,
>  				      struct kvm_vcpu *except)
>  {
> +	struct kvm_vcpu *vcpu;
>  	cpumask_var_t cpus;
>  	bool called;
> +	int i, me;