[PATCH] kernel/resource: optimize find_next_iomem_res

[Chia-I Wu <olvaffe@gmail.com>]

We can skip children resources when the parent resource does not cover
the range.

This should help vmf_insert_* users on x86, such as several DRM drivers.
On my AMD Ryzen 5 7520C, when streaming data from cpu memory into amdgpu
bo, the throughput goes from 5.1GB/s to 6.6GB/s.  perf report says

  34.69%--__do_fault
  34.60%--amdgpu_gem_fault
  34.00%--ttm_bo_vm_fault_reserved
  32.95%--vmf_insert_pfn_prot
  25.89%--track_pfn_insert
  24.35%--lookup_memtype
  21.77%--pat_pagerange_is_ram
  20.80%--walk_system_ram_range
  17.42%--find_next_iomem_res

before this change, and

  26.67%--__do_fault
  26.57%--amdgpu_gem_fault
  25.83%--ttm_bo_vm_fault_reserved
  24.40%--vmf_insert_pfn_prot
  14.30%--track_pfn_insert
  12.20%--lookup_memtype
  9.34%--pat_pagerange_is_ram
  8.22%--walk_system_ram_range
  5.09%--find_next_iomem_res

after.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
---
 kernel/resource.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/resource.c b/kernel/resource.c
index fcbca39dbc450..19b84b4f9a577 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -326,6 +326,7 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
 			       unsigned long flags, unsigned long desc,
 			       struct resource *res)
 {
+	bool skip_children = false;
 	struct resource *p;
 
 	if (!res)
@@ -336,7 +337,7 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
 
 	read_lock(&resource_lock);
 
-	for_each_resource(&iomem_resource, p, false) {
+	for_each_resource(&iomem_resource, p, skip_children) {
 		/* If we passed the resource we are looking for, stop */
 		if (p->start > end) {
 			p = NULL;
@@ -344,8 +345,11 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
 		}
 
 		/* Skip until we find a range that matches what we look for */
-		if (p->end < start)
+		if (p->end < start) {
+			skip_children = true;
 			continue;
+		}
+		skip_children = false;
 
 		if ((p->flags & flags) != flags)
 			continue;
-- 
2.45.1.288.g0e0cd299f1-goog