[PATCH RFC net-next v2 2/7] net: page_pool: place frag_* fields in one cacheline

[Alexander Lobakin]

On x86_64, frag_* fields of struct page_pool are scattered across two
cachelines despite the summary size of 24 bytes. The last field,
::frag_users, is pushed out to the next one, sharing it with
::alloc_stats.
All three fields are used in pretty much the same places. There are some
holes and cold members to move around. Move frag_* one block up, placing
them right after &page_pool_params perfectly at the beginning of CL2.
This doesn't do any meaningful to the second block, as those are some
destroy-path cold structures, and doesn't do anything to ::alloc_stats,
which still starts at 200-byte offset, 8 bytes after CL3 (still fitting
into 1 cacheline).
On my setup, this yields 1-2% of Mpps when using PP frags actively.
When it comes to 32-bit architectures with 32-byte CL: &page_pool_params
plus ::pad is 44 bytes, the block taken care of is 16 bytes within one
CL, so there should be at least no regressions from the actual change.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
---
 include/net/page_pool.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index 829dc1f8ba6b..212d72b5cfec 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h
@@ -130,16 +130,16 @@ static inline u64 *page_pool_ethtool_stats_get(u64 *data, void *stats)
 struct page_pool {
 	struct page_pool_params p;
 
+	long frag_users;
+	struct page *frag_page;
+	unsigned int frag_offset;
+	u32 pages_state_hold_cnt;
+
 	struct delayed_work release_dw;
 	void (*disconnect)(void *);
 	unsigned long defer_start;
 	unsigned long defer_warn;
 
-	u32 pages_state_hold_cnt;
-	unsigned int frag_offset;
-	struct page *frag_page;
-	long frag_users;
-
 #ifdef CONFIG_PAGE_POOL_STATS
 	/* these stats are incremented while in softirq context */
 	struct page_pool_alloc_stats alloc_stats;
-- 
2.41.0