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 <richard.weiyang@gmail.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3f3d83def9be9f3341d92c7814df307d753fbaf0..480e4416131f967c6529be47682527011c9a460b 100644 (file)
--- 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);