linux-kernel - Re: [Patch v3 2/3] iommu: optimize iova_magazine_free

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Date:   Tue, 21 Jan 2020 09:52:11 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        iommu@...ts.linux-foundation.org
Cc:     linux-kernel@...r.kernel.org, joro@...tes.org,
        John Garry <john.garry@...wei.com>
Subject: Re: [Patch v3 2/3] iommu: optimize iova_magazine_free_pfns()

On 18/12/2019 4:39 am, Cong Wang wrote:
> If the magazine is empty, iova_magazine_free_pfns() should
> 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, not much at all.

Have you measured that in any way? AFAICS the only time this can get 
called with a non-full magazine is in the CPU hotplug callback, where 
the impact of taking the rbtree lock and immediately releasing it seems 
unlikely to be significant on top of everything else involved in that 
operation.

Robin.

> Cc: Joerg Roedel <joro@...tes.org>
> Cc: John Garry <john.garry@...wei.com>
> 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))
>   		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)
>   {
>