On Tue, 11 May 2021 22:47:02 +0100
"Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> wrote:
> We also waste a lot of instructions ensuring that we're not looking at
> a tail page. Almost every call to PageFoo() contains one or more
> hidden calls to compound_head(). This also happens for get_page(),
> put_page() and many more functions. There does not appear to be a
> way to tell gcc that it can cache the result of compound_head(), nor
> is there a way to tell it that compound_head() is idempotent.
>
Maybe it's not effective in all situations but the following hint to
the compiler seems to have an effect, at least according to bloat-o-meter:
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -179,7 +179,7 @@ enum pageflags {
struct page; /* forward declaration */
-static inline struct page *compound_head(struct page *page)
+static inline __attribute_const__ struct page *compound_head(struct page *page)
{
unsigned long head = READ_ONCE(page->compound_head);
$ scripts/bloat-o-meter vmlinux.o.orig vmlinux.o
add/remove: 3/13 grow/shrink: 65/689 up/down: 21080/-198089 (-177009)
Function old new delta
ntfs_mft_record_alloc 14414 16627 +2213
migrate_pages 8891 10819 +1928
ext2_get_page.isra 1029 2343 +1314
kfence_init 180 1331 +1151
page_remove_rmap 754 1893 +1139
f2fs_fsync_node_pages 4378 5406 +1028
deferred_split_huge_page 1279 2286 +1007
relock_page_lruvec_irqsave - 975 +975
f2fs_file_write_iter 3508 4408 +900
__pagevec_lru_add 704 1311 +607
[...]
pagevec_move_tail_fn 5333 3215 -2118
__activate_page 6183 4021 -2162
__unmap_and_move 2190 - -2190
__page_cache_release 4738 2547 -2191
migrate_page_states 7088 4842 -2246
lru_deactivate_fn 5925 3652 -2273
move_pages_to_lru 7259 4980 -2279
check_move_unevictable_pages 7131 4594 -2537
release_pages 6940 4386 -2554
lru_lazyfree_fn 6798 4198 -2600
ntfs_mft_record_format 2940 - -2940
lru_deactivate_file_fn 9220 5631 -3589
shrink_page_list 20653 15749 -4904
page_memcg 5149 193 -4956
Total: Before=388863526, After=388686517, chg -0.05%
I don't know if it breaks something though, nor if it gives some real
improvement.
--
per aspera ad upstream
