The patch titled
Subject: mm/page_alloc: remove prefetchw() on freeing page to buddy system
has been added to the -mm mm-unstable branch. Its filename is
mm-page_alloc-remove-prefetchw-on-freeing-page-to-buddy-system.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_alloc-remove-prefetchw-on-freeing-page-to-buddy-system.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Wei Yang <richard.weiyang@xxxxxxxxx>
Subject: mm/page_alloc: remove prefetchw() on freeing page to buddy system
Date: Tue, 2 Jul 2024 02:09:31 +0000
The prefetchw() is introduced from an ancient patch[1].
The change log says:
The basic idea is to free higher order pages instead of going
through every single one. Also, some unnecessary atomic operations
are done away with and replaced with non-atomic equivalents, and
prefetching is done where it helps the most. For a more in-depth
discusion of this patch, please see the linux-ia64 archives (topic
is "free bootmem feedback patch").
So there are several changes improve the bootmem freeing, in which the
most basic idea is freeing higher order pages. And as Matthew says,
"Itanium CPUs of this era had no prefetchers."
I did 10 round bootup tests before and after this change, the data
doesn't prove prefetchw() help speeding up bootmem freeing. The sum of
the 10 round bootmem freeing time after prefetchw() removal even 5.2%
faster than before.
[1]: https://lore.kernel.org/linux-ia64/40F46962.4090604@xxxxxxx/
Link: https://lkml.kernel.org/r/20240702020931.7061-1-richard.weiyang@xxxxxxxxx
Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 13 ++-----------
1 file changed, 2 insertions(+), 11 deletions(-)
--- a/mm/page_alloc.c~mm-page_alloc-remove-prefetchw-on-freeing-page-to-buddy-system
+++ a/mm/page_alloc.c
@@ -1236,16 +1236,11 @@ void __free_pages_core(struct page *page
*/
if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG) &&
unlikely(context == MEMINIT_HOTPLUG)) {
- prefetchw(p);
- for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
- prefetchw(p + 1);
+ for (loop = 0; loop < nr_pages; loop++, p++) {
VM_WARN_ON_ONCE(PageReserved(p));
__ClearPageOffline(p);
set_page_count(p, 0);
}
- VM_WARN_ON_ONCE(PageReserved(p));
- __ClearPageOffline(p);
- set_page_count(p, 0);
/*
* Freeing the page with debug_pagealloc enabled will try to
@@ -1255,14 +1250,10 @@ void __free_pages_core(struct page *page
debug_pagealloc_map_pages(page, nr_pages);
adjust_managed_page_count(page, nr_pages);
} else {
- prefetchw(p);
- for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
- prefetchw(p + 1);
+ for (loop = 0; loop < nr_pages; loop++, p++) {
__ClearPageReserved(p);
set_page_count(p, 0);
}
- __ClearPageReserved(p);
- set_page_count(p, 0);
/* memblock adjusts totalram_pages() manually. */
atomic_long_add(nr_pages, &page_zone(page)->managed_pages);
_
Patches currently in -mm which might be from richard.weiyang@xxxxxxxxx are
kernel-forkc-get-totalram_pages-from-memblock-to-calculate-max_threads.patch
kernel-forkc-put-set_max_threads-task_struct_whitelist-in-__init-section.patch
mm-page_alloc-remove-prefetchw-on-freeing-page-to-buddy-system.patch
