From 689d92cc81ac57ca6b674be8728b9c5ea5c725fd Mon Sep 17 00:00:00 2001 From: Wei Yang Date: Tue, 2 Jul 2024 02:09:31 +0000 Subject: [PATCH] mm/page_alloc: remove prefetchw() on freeing page to buddy system 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@sgi.com/ Link: https://lkml.kernel.org/r/20240702020931.7061-1-richard.weiyang@gmail.com Signed-off-by: Wei Yang Suggested-by: Matthew Wilcox Reviewed-by: Matthew Wilcox Reviewed-by: David Hildenbrand Signed-off-by: Andrew Morton --- mm/page_alloc.c | 13 ++----------- 1 file changed, 2 insertions(+), 11 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 3f3d83def9be..480e4416131f 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1236,16 +1236,11 @@ void __free_pages_core(struct page *page, unsigned int order, */ 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, unsigned int order, 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);