netdev - [RFC PATCH 2/3] net: gro: minor optimization for dev_gro

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Message-Id: <35d722e246b7c4afb6afb03760df6f664db4ef05.1642519257.git.pabeni@redhat.com>
Date:   Tue, 18 Jan 2022 16:24:19 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     Eric Dumazet <edumazet@...gle.com>
Subject: [RFC PATCH 2/3] net: gro: minor optimization for dev_gro_receive()

While inspecting some perf report, I noticed that the compiler
emits suboptimal code for the napi CB initialization, fetching
and storing multiple times the memory for flags bitfield.
This is with gcc 10.3.1, but I observed the same with older compiler
versions.

We can help the compiler to do a nicer work e.g. initially setting
all the bitfield to 0 using an u16 alias. The generated code is quite
smaller, with the same number of conditional

Before:
objdump -t net/core/gro.o | grep " F .text"
0000000000000bb0 l     F .text	0000000000000357 dev_gro_receive

After:
0000000000000bb0 l     F .text	000000000000033c dev_gro_receive

Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 include/net/gro.h | 13 +++++++++----
 net/core/gro.c    | 16 +++++-----------
 2 files changed, 14 insertions(+), 15 deletions(-)

diff --git a/include/net/gro.h b/include/net/gro.h
index 8f75802d50fd..a068b27d341f 100644
--- a/include/net/gro.h
+++ b/include/net/gro.h
@@ -29,14 +29,17 @@ struct napi_gro_cb {
 	/* Number of segments aggregated. */
 	u16	count;
 
-	/* Start offset for remote checksum offload */
-	u16	gro_remcsum_start;
+	/* Used in ipv6_gro_receive() and foo-over-udp */
+	u16	proto;
 
 	/* jiffies when first packet was created/queued */
 	unsigned long age;
 
-	/* Used in ipv6_gro_receive() and foo-over-udp */
-	u16	proto;
+	/* portion of the cb set to zero at every gro iteration */
+	u32	zeroed_start[0];
+
+	/* Start offset for remote checksum offload */
+	u16	gro_remcsum_start;
 
 	/* This is non-zero if the packet may be of the same flow. */
 	u8	same_flow:1;
@@ -70,6 +73,8 @@ struct napi_gro_cb {
 	/* GRO is done by frag_list pointer chaining. */
 	u8	is_flist:1;
 
+	u32	zeroed_end[0];
+
 	/* used to support CHECKSUM_COMPLETE for tunneling protocols */
 	__wsum	csum;
 
diff --git a/net/core/gro.c b/net/core/gro.c
index d43d42215bdb..b9ebe9298731 100644
--- a/net/core/gro.c
+++ b/net/core/gro.c
@@ -435,6 +435,9 @@ static void gro_flush_oldest(struct napi_struct *napi, struct list_head *head)
 	napi_gro_complete(napi, oldest);
 }
 
+#define zeroed_len	(offsetof(struct napi_gro_cb, zeroed_end) - \
+			 offsetof(struct napi_gro_cb, zeroed_start))
+
 static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
 {
 	u32 bucket = skb_get_hash_raw(skb) & (GRO_HASH_BUCKETS - 1);
@@ -459,29 +462,20 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
 
 		skb_set_network_header(skb, skb_gro_offset(skb));
 		skb_reset_mac_len(skb);
-		NAPI_GRO_CB(skb)->same_flow = 0;
+		BUILD_BUG_ON(zeroed_len != sizeof(NAPI_GRO_CB(skb)->zeroed_start[0]));
+		NAPI_GRO_CB(skb)->zeroed_start[0] = 0;
 		NAPI_GRO_CB(skb)->flush = skb_is_gso(skb) || skb_has_frag_list(skb);
-		NAPI_GRO_CB(skb)->free = 0;
-		NAPI_GRO_CB(skb)->encap_mark = 0;
-		NAPI_GRO_CB(skb)->recursion_counter = 0;
-		NAPI_GRO_CB(skb)->is_fou = 0;
 		NAPI_GRO_CB(skb)->is_atomic = 1;
-		NAPI_GRO_CB(skb)->gro_remcsum_start = 0;
 
 		/* Setup for GRO checksum validation */
 		switch (skb->ip_summed) {
 		case CHECKSUM_COMPLETE:
 			NAPI_GRO_CB(skb)->csum = skb->csum;
 			NAPI_GRO_CB(skb)->csum_valid = 1;
-			NAPI_GRO_CB(skb)->csum_cnt = 0;
 			break;
 		case CHECKSUM_UNNECESSARY:
 			NAPI_GRO_CB(skb)->csum_cnt = skb->csum_level + 1;
-			NAPI_GRO_CB(skb)->csum_valid = 0;
 			break;
-		default:
-			NAPI_GRO_CB(skb)->csum_cnt = 0;
-			NAPI_GRO_CB(skb)->csum_valid = 0;
 		}
 
 		pp = INDIRECT_CALL_INET(ptype->callbacks.gro_receive,
-- 
2.34.1