linux-kernel - Re: [PATCH 1/2] iommu/vt-d: Use rbtree to track iommu probed devices

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Date: Thu, 15 Feb 2024 13:47:08 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Lu Baolu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Kevin Tian <kevin.tian@...el.com>,
	Huang Jiaqing <jiaqing.huang@...el.com>,
	Ethan Zhao <haifeng.zhao@...ux.intel.com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] iommu/vt-d: Use rbtree to track iommu probed devices

On Thu, Feb 15, 2024 at 03:22:48PM +0800, Lu Baolu wrote:
> Use a red-black tree(rbtree) to track devices probed by the driver's
> probe_device callback. These devices need to be looked up quickly by
> a source ID when the hardware reports a fault, either recoverable or
> unrecoverable.
> 
> Fault reporting paths are critical. Searching a list in this scenario
> is inefficient, with an algorithm complexity of O(n). An rbtree is a
> self-balancing binary search tree, offering an average search time
> complexity of O(log(n)). This significant performance improvement
> makes rbtrees a better choice.
> 
> Furthermore, rbtrees are implemented on a per-iommu basis, eliminating
> the need for global searches and further enhancing efficiency in
> critical fault paths. The rbtree is protected by a spin lock with
> interrupts disabled to ensure thread-safe access even within interrupt
> contexts.
> 
> Co-developed-by: Huang Jiaqing <jiaqing.huang@...el.com>
> Signed-off-by: Huang Jiaqing <jiaqing.huang@...el.com>
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  drivers/iommu/intel/iommu.h |  7 +++++
>  drivers/iommu/intel/dmar.c  |  3 +-
>  drivers/iommu/intel/iommu.c | 62 +++++++++++++++++++++++++++++++++++--
>  3 files changed, 69 insertions(+), 3 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

> +static int device_rbtree_insert(struct intel_iommu *iommu,
> +				struct device_domain_info *info)
> +{
> +	struct rb_node *curr;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&iommu->device_rbtree_lock, flags);
> +	curr = rb_find_add(&info->node, &iommu->device_rbtree, device_rid_cmp);
> +	spin_unlock_irqrestore(&iommu->device_rbtree_lock, flags);
> +	if (curr)
> +		dev_warn(info->dev, "device already in rbtree\n");

I would suggest

WARN_ON(curr);

Something has gone really wonky at this point, right?

Jason