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

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Date: Sun, 18 Feb 2024 12:22:59 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: baolu.lu@...ux.intel.com, 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 2024/2/16 1:47, Jason Gunthorpe wrote:
> 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?

Yes. This is not expected, and it is worth a WARN_ON().

Best regards,
baolu