resource: avoid unnecessary lookups in find_next_iomem_res()

find_next_iomem_res() shows up to be a source for overhead in dax
benchmarks.

Improve performance by not considering children of the tree if the top
level does not match.  Since the range of the parents should include the
range of the children such check is redundant.

Running sysbench on dax (pmem emulation, with write_cache disabled):

  sysbench fileio --file-total-size=3G --file-test-mode=rndwr \
   --file-io-mode=mmap --threads=4 --file-fsync-mode=fdatasync run

Provides the following results:

		events (avg/stddev)
		-------------------
  5.2-rc3:	1247669.0000/16075.39
  w/patch:	1286320.5000/16402.72	(+3%)

Link: http://lkml.kernel.org/r/20190613045903.4922-3-namit@vmware.com
Signed-off-by: Nadav Amit <namit@vmware.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/resource.c b/kernel/resource.c
index 3ced0cd..7ea4306 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -342,6 +342,7 @@
 			       unsigned long flags, unsigned long desc,
 			       bool first_lvl, struct resource *res)
 {
+	bool siblings_only = true;
 	struct resource *p;
 
 	if (!res)
@@ -352,17 +353,31 @@
 
 	read_lock(&resource_lock);
 
-	for (p = iomem_resource.child; p; p = next_resource(p, first_lvl)) {
-		if ((p->flags & flags) != flags)
-			continue;
-		if ((desc != IORES_DESC_NONE) && (desc != p->desc))
-			continue;
+	for (p = iomem_resource.child; p; p = next_resource(p, siblings_only)) {
+		/* If we passed the resource we are looking for, stop */
 		if (p->start > end) {
 			p = NULL;
 			break;
 		}
-		if ((p->end >= start) && (p->start <= end))
-			break;
+
+		/* Skip until we find a range that matches what we look for */
+		if (p->end < start)
+			continue;
+
+		/*
+		 * Now that we found a range that matches what we look for,
+		 * check the flags and the descriptor. If we were not asked to
+		 * use only the first level, start looking at children as well.
+		 */
+		siblings_only = first_lvl;
+
+		if ((p->flags & flags) != flags)
+			continue;
+		if ((desc != IORES_DESC_NONE) && (desc != p->desc))
+			continue;
+
+		/* Found a match, break */
+		break;
 	}
 
 	if (p) {