netdev - [PATCH net-next v3 4/6] net: skbuff: avoid accessing page_pool if !napi

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Date: Thu,  3 Aug 2023 20:20:36 +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 4/6] net: skbuff: avoid accessing page_pool if !napi_safe when returning page

Currently, pp->p.napi is always read, but the actual variable it gets
assigned to is read-only when @napi_safe is true. For the !napi_safe
cases, which yet is still a pack, it's an unneeded operation.
Moreover, it can lead to premature or even redundant page_pool
cacheline access. For example, when page_pool_is_last_frag() returns
false (with the recent frag improvements).
Thus, read it only when @napi_safe is true. This also allows moving
@napi inside the condition block itself. Constify it while we are
here, because why not.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
---
 net/core/skbuff.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index acc5844a0de1..85f82a6a08dc 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -882,9 +882,8 @@ static void skb_clone_fraglist(struct sk_buff *skb)
 #if IS_ENABLED(CONFIG_PAGE_POOL)
 bool napi_pp_put_page(struct page *page, bool napi_safe)
 {
-	struct napi_struct *napi;
+	bool allow_direct = false;
 	struct page_pool *pp;
-	bool allow_direct;
 
 	page = compound_head(page);
 
@@ -904,9 +903,12 @@ bool napi_pp_put_page(struct page *page, bool napi_safe)
 	 * in the same context as the consumer would run, so there's
 	 * no possible race.
 	 */
-	napi = READ_ONCE(pp->p.napi);
-	allow_direct = napi_safe && napi &&
-		READ_ONCE(napi->list_owner) == smp_processor_id();
+	if (napi_safe) {
+		const struct napi_struct *napi = READ_ONCE(pp->p.napi);
+
+		allow_direct = napi &&
+			READ_ONCE(napi->list_owner) == smp_processor_id();
+	}
 
 	/* Driver set this to memory recycling info. Reset it on recycle.
 	 * This will *not* work for NIC using a split-page memory model.
-- 
2.41.0