netdev - [PATCH net-next v4 4/4] skbuff: Optimization of SKB coalescing for page pool

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Message-Id: <20231129031201.32014-5-liangchen.linux@gmail.com>
Date: Wed, 29 Nov 2023 11:12:01 +0800
From: Liang Chen <liangchen.linux@...il.com>
To: davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	hawk@...nel.org,
	ilias.apalodimas@...aro.org,
	linyunsheng@...wei.com
Cc: netdev@...r.kernel.org,
	linux-mm@...ck.org,
	liangchen.linux@...il.com
Subject: [PATCH net-next v4 4/4] skbuff: Optimization of SKB coalescing for page pool

In order to address the issues encountered with commit 1effe8ca4e34
("skbuff: fix coalescing for page_pool fragment recycling"), the
combination of the following condition was excluded from skb coalescing:

from->pp_recycle = 1
from->cloned = 1
to->pp_recycle = 1

However, with page pool environments, the aforementioned combination can
be quite common(ex. NetworkMananger may lead to the additional
packet_type being registered, thus the cloning). In scenarios with a
higher number of small packets, it can significantly affect the success
rate of coalescing. For example, considering packets of 256 bytes size,
our comparison of coalescing success rate is as follows:

Without page pool: 70%
With page pool: 13%

Consequently, this has an impact on performance:

Without page pool: 2.57 Gbits/sec
With page pool: 2.26 Gbits/sec

Therefore, it seems worthwhile to optimize this scenario and enable
coalescing of this particular combination. To achieve this, we need to
ensure the correct increment of the "from" SKB page's page pool
reference count (pp_ref_count).

Following this optimization, the success rate of coalescing measured in
our environment has improved as follows:

With page pool: 60%

This success rate is approaching the rate achieved without using page
pool, and the performance has also been improved:

With page pool: 2.52 Gbits/sec

Below is the performance comparison for small packets before and after
this optimization. We observe no impact to packets larger than 4K.

packet size     before      after       improved
(bytes)         (Gbits/sec) (Gbits/sec)
128             1.19        1.27        7.13%
256             2.26        2.52        11.75%
512             4.13        4.81        16.50%
1024            6.17        6.73        9.05%
2048            14.54       15.47       6.45%
4096            25.44       27.87       9.52%

Signed-off-by: Liang Chen <liangchen.linux@...il.com>
---
 net/core/skbuff.c | 41 +++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 310207389f51..893d03264afc 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -947,6 +947,24 @@ static bool skb_pp_recycle(struct sk_buff *skb, void *data, bool napi_safe)
 	return napi_pp_put_page(virt_to_page(data), napi_safe);
 }
 
+/**
+ * skb_pp_get_frag_ref() - Increase fragment reference count of a page
+ * @page:	page of the fragment on which to increase a reference
+ *
+ * Increase fragment reference count (pp_ref_count) on a page, but if it is
+ * not a page pool page, fallback to increase a reference(_refcount) on a
+ * normal page.
+ */
+static inline void skb_pp_get_frag_ref(struct page *page)
+{
+	struct page *head_page = compound_head(page);
+
+	if (likely(skb_frag_is_pp_page(head_page)))
+		atomic_long_inc(&head_page->pp_ref_count);
+	else
+		get_page(head_page);
+}
+
 static void skb_kfree_head(void *head, unsigned int end_offset)
 {
 	if (end_offset == SKB_SMALL_HEAD_HEADROOM)
@@ -5769,17 +5787,12 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
 		return false;
 
 	/* In general, avoid mixing page_pool and non-page_pool allocated
-	 * pages within the same SKB. Additionally avoid dealing with clones
-	 * with page_pool pages, in case the SKB is using page_pool fragment
-	 * references (page_pool_alloc_frag()). Since we only take full page
-	 * references for cloned SKBs at the moment that would result in
-	 * inconsistent reference counts.
-	 * In theory we could take full references if @from is cloned and
-	 * !@to->pp_recycle but its tricky (due to potential race with
-	 * the clone disappearing) and rare, so not worth dealing with.
+	 * pages within the same SKB. In theory we could take full
+	 * references if @from is cloned and !@to->pp_recycle but its
+	 * tricky (due to potential race with the clone disappearing) and
+	 * rare, so not worth dealing with.
 	 */
-	if (to->pp_recycle != from->pp_recycle ||
-	    (from->pp_recycle && skb_cloned(from)))
+	if (to->pp_recycle != from->pp_recycle)
 		return false;
 
 	if (len <= skb_tailroom(to)) {
@@ -5836,8 +5849,12 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
 	/* if the skb is not cloned this does nothing
 	 * since we set nr_frags to 0.
 	 */
-	for (i = 0; i < from_shinfo->nr_frags; i++)
-		__skb_frag_ref(&from_shinfo->frags[i]);
+	if (from->pp_recycle)
+		for (i = 0; i < from_shinfo->nr_frags; i++)
+			skb_pp_get_frag_ref(skb_frag_page(&from_shinfo->frags[i]));
+	else
+		for (i = 0; i < from_shinfo->nr_frags; i++)
+			__skb_frag_ref(&from_shinfo->frags[i]);
 
 	to->truesize += delta;
 	to->len += len;
-- 
2.31.1