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 clearing several
fields at once using an u32 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

v1  -> v2:
 - use struct_group (Alexander and Alex)

RFC -> v1:
 - use __struct_group to delimit the zeroed area (Alexander)

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/gro.h b/include/net/gro.h
index 8f75802..a765fed 100644 (file)
--- a/include/net/gro.h
+++ b/include/net/gro.h
@@ -29,46 +29,50 @@ 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 */
+       struct_group(zeroed,
+
+               /* 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;
+               /* This is non-zero if the packet may be of the same flow. */
+               u8      same_flow:1;
 
-       /* Used in tunnel GRO receive */
-       u8      encap_mark:1;
+               /* Used in tunnel GRO receive */
+               u8      encap_mark:1;
 
-       /* GRO checksum is valid */
-       u8      csum_valid:1;
+               /* GRO checksum is valid */
+               u8      csum_valid:1;
 
-       /* Number of checksums via CHECKSUM_UNNECESSARY */
-       u8      csum_cnt:3;
+               /* Number of checksums via CHECKSUM_UNNECESSARY */
+               u8      csum_cnt:3;
 
-       /* Free the skb? */
-       u8      free:2;
+               /* Free the skb? */
+               u8      free:2;
 #define NAPI_GRO_FREE            1
 #define NAPI_GRO_FREE_STOLEN_HEAD 2
 
-       /* Used in foo-over-udp, set in udp[46]_gro_receive */
-       u8      is_ipv6:1;
+               /* Used in foo-over-udp, set in udp[46]_gro_receive */
+               u8      is_ipv6:1;
 
-       /* Used in GRE, set in fou/gue_gro_receive */
-       u8      is_fou:1;
+               /* Used in GRE, set in fou/gue_gro_receive */
+               u8      is_fou:1;
 
-       /* Used to determine if flush_id can be ignored */
-       u8      is_atomic:1;
+               /* Used to determine if flush_id can be ignored */
+               u8      is_atomic:1;
 
-       /* Number of gro_receive callbacks this packet already went through */
-       u8 recursion_counter:4;
+               /* Number of gro_receive callbacks this packet already went through */
+               u8 recursion_counter:4;
 
-       /* GRO is done by frag_list pointer chaining. */
-       u8      is_flist:1;
+               /* GRO is done by frag_list pointer chaining. */
+               u8      is_flist:1;
+       );
 
        /* used to support CHECKSUM_COMPLETE for tunneling protocols */
        __wsum  csum;

diff --git a/net/core/gro.c b/net/core/gro.c
index d43d422..ee5e7e8 100644 (file)
--- a/net/core/gro.c
+++ b/net/core/gro.c
@@ -459,29 +459,22 @@ 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(sizeof_field(struct napi_gro_cb, zeroed) != sizeof(u32));
+               BUILD_BUG_ON(!IS_ALIGNED(offsetof(struct napi_gro_cb, zeroed),
+                                        sizeof(u32))); /* Avoid slow unaligned acc */
+               *(u32 *)&NAPI_GRO_CB(skb)->zeroed = 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,