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

Mel Gorman
2021-03-25T11:44:06Z

From: Jesper Dangaard Brouer <brouer@redhat.com>

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 <brouer@redhat.com>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
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