netdev - [PATCH net-next v3 3/6] page_pool: place frag

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Date: Thu,  3 Aug 2023 20:20:35 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>
Cc: Alexander Lobakin <aleksander.lobakin@...el.com>,
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
	Larysa Zaremba <larysa.zaremba@...el.com>,
	Yunsheng Lin <linyunsheng@...wei.com>,
	Alexander Duyck <alexanderduyck@...com>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Ilias Apalodimas <ilias.apalodimas@...aro.org>,
	Simon Horman <simon.horman@...igine.com>,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH net-next v3 3/6] page_pool: place frag_* fields in one cacheline

On x86_64, frag_* fields of struct page_pool are scattered across two
cachelines despite the summary size of 24 bytes. All three fields are
used in pretty much the same places, but the last field, ::frag_users,
is pushed out to the next CL, provoking unwanted false-sharing on
hotpath (frags allocation code).
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.
::pages_state_hold_cnt is not related directly to that triple, but is
paired currently with ::frags_offset and decoupling them would mean
either two 4-byte holes or more invasive layout changes.

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

diff --git a/include/net/page_pool/types.h b/include/net/page_pool/types.h
index fcb846523398..887e7946a597 100644
--- a/include/net/page_pool/types.h
+++ b/include/net/page_pool/types.h
@@ -123,16 +123,16 @@ struct page_pool_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 *pool);
 	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