linux-kernel - Re: [PATCH v3 3/4] KVM: Optimize kvm_make_vcpus_request

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Message-ID: <87fsuwi3io.fsf@vitty.brq.redhat.com>
Date:   Thu, 26 Aug 2021 18:17:51 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/4] KVM: Optimize kvm_make_vcpus_request_mask() a bit

Sean Christopherson <seanjc@...gle.com> writes:

> On Thu, Aug 26, 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.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>
> ...
>
>> +	if (vcpu_bitmap) {
>> +		for_each_set_bit(i, vcpu_bitmap, KVM_MAX_VCPUS) {
>> +			vcpu = kvm_get_vcpu(kvm, i);
>> +			if (!vcpu || vcpu == except)
>> +				continue;
>> +			kvm_make_vcpu_request(kvm, vcpu, req, tmp, me);
>> +		}
>> +	} else {
>> +		kvm_for_each_vcpu(i, vcpu, kvm) {
>> +			if (vcpu == except)
>> +				continue;
>> +			kvm_make_vcpu_request(kvm, vcpu, req, tmp, me);
>>  		}
>>  	}
>
> Rather than feed kvm_make_all_cpus_request_except() into kvm_make_vcpus_request_mask(),
> I think it would be better to move the kvm_for_each_vcpu() path into
> kvm_make_all_cpus_request_except() (see bottom of the mail).  That eliminates the
> silliness of calling a "mask" function without a mask, and also allows a follow-up
> patch to drop @except from kvm_make_vcpus_request_mask(), which is truly nonsensical
> as the caller can and should simply not set that vCPU in the mask. 

Both make perfect sense, thanks! v4 is being prepared.

-- 
Vitaly