Subject: [PATCH 4/9] mm/page_alloc: optimize code layout for __alloc_pages_bulk

From: Mel Gorman <[email-masked]>
Date: Thu, 25 Mar 2021 11:42:23 +0000

From: Jesper Dangaard Brouer <[email-masked]>

Looking at perf-report and ASM-code for __alloc_pages_bulk() it is clear
that the code activated is suboptimal. The compiler guesses wrong and
places unlikely code at the beginning. Due to the use of WARN_ON_ONCE()
macro the UD2 asm instruction is added to the code, which confuse the
I-cache prefetcher in the CPU.

[mgorman: Minor changes and rebasing]
Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
Signed-off-by: Mel Gorman <[email-masked]>
---
 mm/page_alloc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index be1e33a4df39..1ec18121268b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5001,7 +5001,7 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
 	unsigned int alloc_flags;
 	int nr_populated = 0;
 
-	if (WARN_ON_ONCE(nr_pages <= 0))
+	if (unlikely(nr_pages <= 0))
 		return 0;
 
 	/*
@@ -5048,7 +5048,7 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
 	 * If there are no allowed local zones that meets the watermarks then
 	 * try to allocate a single page and reclaim if necessary.
 	 */
-	if (!zone)
+	if (unlikely(!zone))
 		goto failed;
 
 	/* Attempt the batch allocation */
@@ -5066,7 +5066,7 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
 
 		page = __rmqueue_pcplist(zone, ac.migratetype, alloc_flags,
 								pcp, pcp_list);
-		if (!page) {
+		if (unlikely(!page)) {
 			/* Try and get at least one page */
 			if (!nr_populated)
 				goto failed_irq;
-- 
2.26.2