linux-kernel - Re: [PATCH 1/3] kvm: memslots: track id_to_index changes during the insertion sort

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Message-ID: <20141114133500.GA10593@potion.brq.redhat.com>
Date:	Fri, 14 Nov 2014 14:35:00 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	yoshikawa_takuya_b1@....ntt.co.jp, imammedo@...hat.com
Subject: Re: [PATCH 1/3] kvm: memslots: track id_to_index changes during the
 insertion sort

2014-11-14 12:12+0100, Paolo Bonzini:
> This completes the optimization from the previous patch, by
> removing the KVM_MEM_SLOTS_NUM-iteration loop from insert_memslot.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  virt/kvm/kvm_main.c | 39 +++++++++++++++++++--------------------
>  1 file changed, 19 insertions(+), 20 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index c0c2202e6c4f..c8ff99cc0ccb 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -677,31 +677,30 @@ static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
>  static void insert_memslot(struct kvm_memslots *slots,
>  			   struct kvm_memory_slot *new)
>  {
> -	int i = slots->id_to_index[new->id];
> -	struct kvm_memory_slot *old = id_to_memslot(slots, new->id);
> +	int id = new->id;
> +	int i = slots->id_to_index[id];
>  	struct kvm_memory_slot *mslots = slots->memslots;
>  
> -	if (new->npages == old->npages) {
> -		*old = *new;
> -		return;
> -	}
> -
> -	while (1) {
> -		if (i < (KVM_MEM_SLOTS_NUM - 1) &&
> -			new->npages < mslots[i + 1].npages) {
> -			mslots[i] = mslots[i + 1];
> -			i++;
> -		} else if (i > 0 && new->npages > mslots[i - 1].npages) {
> -			mslots[i] = mslots[i - 1];
> -			i--;
> -		} else {
> -			mslots[i] = *new;
> -			break;
> +	WARN_ON(mslots[i].id != id);
> +	if (new->npages != mslots[i].npages) {
> +		while (1) {
> +			if (i < (KVM_MEM_SLOTS_NUM - 1) &&
> +    			    new->npages < mslots[i + 1].npages) {
  (^^^^ whitespace error)
> +				mslots[i] = mslots[i + 1];
> +				slots->id_to_index[mslots[i].id] = i;
> +				i++;
> +			} else if (i > 0 &&
> +				   new->npages > mslots[i - 1].npages) {
> +				mslots[i] = mslots[i - 1];
> +				slots->id_to_index[mslots[i].id] = i;
> +				i--;
> +			} else
> +				break;

We are replacing in a sorted array, so the the direction of our
traversal doesn't change, (and we could lose one tab level here,)

	if (new->npages < mslots[i].npages) {
		while (i < (KVM_MEM_SLOTS_NUM - 1) &&
		       new->npages < mslots[i + 1].npages) {
			mslots[i] = mslots[i + 1];
			slots->id_to_index[mslots[i].id] = i;
			i++;
		}
	else if (new->npages > mslots[i].npages)
		while (i > 0 &&
		       new->npages > mslots[i - 1].npages) {
			mslots[i] = mslots[i - 1];
			slots->id_to_index[mslots[i].id] = i;
			i--;
		}

(I guess you don't want me to abstract these two loops further :)

If the probability of slots with same npages was high, we could also
move just the last one from each group, but I think that the current
algorithm is already faster than we need.

(We'll have to change it into an interval tree, or something, if the
 number of slots rises anyway.)
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