[PATCH] iommu/iova: Don't reset cached_node in dac deallocation

[Zaid Alali <zaidal@os.amperecomputing.com>]

The iova allocator has two rbtrees for allocations that are not satisfied
by rcache. The two rbtrees track iovas for the ranges of 32bit address
space and larger address space >32bit. On deallocation, the cached_node
is updated to point to the deallocated iova.

Because the cached_node is moved to point to the recently deallocated
iova with higher address, the first-fit allocator needs to walk the
rbtree backwards skipping holes that do not fit while holding
iova_rbtree_lock, which impacts performance and can cause soft-lockups.
On deallocation, do not reset the cached_node to the freed iova for the
rbtree tracking the dac addresses and keep moving forward with new
allocations. This only affects addresses > 32bit.

This patch was tested with ‘iommu.forcedac=1’ and 20 dd read instances
of 8GB from nvme as well as kernel compilation running in parallel.
The test results obtained from /proc/lock_stat shows the following
improvements for iovad->iova_rbtree_lock:

	Wait time average: reduced by 31%
	Hold time average: reduced by 60%

Signed-off-by: D Scott Phillips <scott@os.amperecomputing.com>
Signed-off-by: Zaid Alali <zaidal@os.amperecomputing.com>
---
 drivers/iommu/iova.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index fe452ce46..d2a6cb573 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -106,7 +106,7 @@ __cached_rbnode_delete_update(struct iova_domain *iovad, struct iova *free)
		iovad->max32_alloc_size = iovad->dma_32bit_pfn;
 
	cached_iova = to_iova(iovad->cached_node);
-	if (free->pfn_lo >= cached_iova->pfn_lo)
+	if (free == cached_iova)
		iovad->cached_node = rb_next(&free->node);
 }
 
-- 
2.34.1