Re: [PATCH RFC v3 6/9] KVM: rework kvm_vcpu_on_spin loop

From: David Hildenbrand <david@redhat.com>
To: "Radim Krčmář" <rkrcmar@redhat.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	linux-mips@linux-mips.org, kvm-ppc@vger.kernel.org,
	linux-s390@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Christoffer Dall <cdall@linaro.org>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Cornelia Huck <cohuck@redhat.com>,
	James Hogan <james.hogan@imgtec.com>,
	Paul Mackerras <paulus@ozlabs.org>,
	Alexander Graf <agraf@suse.com>
Subject: Re: [PATCH RFC v3 6/9] KVM: rework kvm_vcpu_on_spin loop
Date: Tue, 22 Aug 2017 16:06:57 +0200	[thread overview]
Message-ID: <debc181e-6517-a8da-7a19-909fda5e4505@redhat.com> (raw)
In-Reply-To: <20170821203530.9266-7-rkrcmar@redhat.com>

On 21.08.2017 22:35, Radim Krčmář wrote:
> The original code managed to obfuscate a straightforward idea:
> start iterating from the selected index and reset the index to 0 when
> reaching the end of online vcpus, then iterate until reaching the index
> that we started at.
> 
> The resulting code is a bit better, IMO.  (Still horrible, though.)

I think I prefer dropping this patch and maybe _after_ we have the list
implementation in place, simply start walking the list from
last_boosted_vcpu? (store a pointer instead of an index then, of course)

If I understand correctly, this would then be simply, one walk from
last_boosted_vcpu until we hit last_boosted_vcpu again.

> 
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
>  include/linux/kvm_host.h | 13 +++++++++++++
>  virt/kvm/kvm_main.c      | 47 ++++++++++++++++++-----------------------------
>  2 files changed, 31 insertions(+), 29 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index abd5cb1feb9e..cfb3c0efdd51 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -498,6 +498,19 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
>  	     (vcpup = kvm_get_vcpu(kvm, idx)) != NULL; \
>  	     idx++)
>  
> +#define kvm_for_each_vcpu_from(idx, vcpup, from, kvm) \
> +	for (idx = from, vcpup = kvm_get_vcpu(kvm, idx); \
> +	     vcpup; \
> +	     ({ \
> +		idx++; \
> +		if (idx >= atomic_read(&kvm->online_vcpus)) \
> +			idx = 0; \
> +		if (idx == from) \
> +			vcpup = NULL; \
> +		else \
> +			vcpup = kvm_get_vcpu(kvm, idx); \
> +	      }))
> +
>  static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
>  {
>  	struct kvm_vcpu *vcpu = NULL;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d89261d0d8c6..33a15e176927 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2333,8 +2333,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>  	struct kvm_vcpu *vcpu;
>  	int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
>  	int yielded = 0;
> -	int try = 3;
> -	int pass;
> +	int try = 2;
>  	int i;
>  
>  	kvm_vcpu_set_in_spin_loop(me, true);
> @@ -2345,34 +2344,24 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>  	 * VCPU is holding the lock that we need and will release it.
>  	 * We approximate round-robin by starting at the last boosted VCPU.
>  	 */
> -	for (pass = 0; pass < 2 && !yielded && try; pass++) {
> -		kvm_for_each_vcpu(i, vcpu, kvm) {
> -			if (!pass && i <= last_boosted_vcpu) {
> -				i = last_boosted_vcpu;
> -				continue;
> -			} else if (pass && i > last_boosted_vcpu)
> -				break;
> -			if (!ACCESS_ONCE(vcpu->preempted))
> -				continue;
> -			if (vcpu == me)
> -				continue;
> -			if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
> -				continue;
> -			if (yield_to_kernel_mode && !kvm_arch_vcpu_in_kernel(vcpu))
> -				continue;
> -			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
> -				continue;
> +	kvm_for_each_vcpu_from(i, vcpu, last_boosted_vcpu, kvm) {
> +		if (!ACCESS_ONCE(vcpu->preempted))
> +			continue;
> +		if (vcpu == me)
> +			continue;
> +		if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
> +			continue;
> +		if (yield_to_kernel_mode && !kvm_arch_vcpu_in_kernel(vcpu))
> +			continue;
> +		if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
> +			continue;
>  
> -			yielded = kvm_vcpu_yield_to(vcpu);
> -			if (yielded > 0) {
> -				kvm->last_boosted_vcpu = i;
> -				break;
> -			} else if (yielded < 0) {
> -				try--;
> -				if (!try)
> -					break;
> -			}
> -		}
> +		yielded = kvm_vcpu_yield_to(vcpu);
> +		if (yielded > 0) {
> +			kvm->last_boosted_vcpu = i;
> +			break;
> +		} else if (yielded < 0 && !try--)
> +			break;
>  	}
>  	kvm_vcpu_set_in_spin_loop(me, false);
>  
> 

-- 

Thanks,

David