linux-kernel - Re: [Patch v2 2/3] iommu: optimize iova_magazine_free

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Date:   Fri, 29 Nov 2019 13:24:41 +0000
From:   John Garry <john.garry@...wei.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        <iommu@...ts.linux-foundation.org>
CC:     <linux-kernel@...r.kernel.org>
Subject: Re: [Patch v2 2/3] iommu: optimize iova_magazine_free_pfns()

On 29/11/2019 00:48, Cong Wang wrote:
> If the maganize is empty, iova_magazine_free_pfns() should

magazine

> be a nop, however it misses the case of mag->size==0. So we
> should just call iova_magazine_empty().
> 
> This should reduce the contention on iovad->iova_rbtree_lock
> a little bit.
> 
> Cc: Joerg Roedel <joro@...tes.org>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>   drivers/iommu/iova.c | 22 +++++++++++-----------
>   1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index cb473ddce4cf..184d4c0e20b5 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -797,13 +797,23 @@ static void iova_magazine_free(struct iova_magazine *mag)
>   	kfree(mag);
>   }
>   
> +static bool iova_magazine_full(struct iova_magazine *mag)
> +{
> +	return (mag && mag->size == IOVA_MAG_SIZE);
> +}
> +
> +static bool iova_magazine_empty(struct iova_magazine *mag)
> +{
> +	return (!mag || mag->size == 0);
> +}
> +
>   static void
>   iova_magazine_free_pfns(struct iova_magazine *mag, struct iova_domain *iovad)
>   {
>   	unsigned long flags;
>   	int i;
>   
> -	if (!mag)
> +	if (iova_magazine_empty(mag))

The only hot path we this call is 
__iova_rcache_insert()->iova_magazine_free_pfns(mag_to_free) and 
mag_to_free is full in this case, so I am sure how the additional check 
helps, right?

Thanks,
John

>   		return;
>   
>   	spin_lock_irqsave(&iovad->iova_rbtree_lock, flags);
> @@ -820,16 +830,6 @@ iova_magazine_free_pfns(struct iova_magazine *mag, struct iova_domain *iovad)
>   	mag->size = 0;
>   }
>   
> -static bool iova_magazine_full(struct iova_magazine *mag)
> -{
> -	return (mag && mag->size == IOVA_MAG_SIZE);
> -}
> -
> -static bool iova_magazine_empty(struct iova_magazine *mag)
> -{
> -	return (!mag || mag->size == 0);
> -}
> -
>   static unsigned long iova_magazine_pop(struct iova_magazine *mag,
>   				       unsigned long limit_pfn)
>   {
>