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net: gro: minor optimization for dev_gro_receive()
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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

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

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
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@intel-lab-lkp
Paolo Abeni authored and intel-lab-lkp committed Feb 3, 2022
1 parent 3038325 commit 5684b57
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Showing 2 changed files with 35 additions and 35 deletions.
52 changes: 28 additions & 24 deletions include/net/gro.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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(/* no tag */, zeroed, /* no attrs */,

/* 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;
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18 changes: 7 additions & 11 deletions net/core/gro.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
Expand All @@ -459,29 +462,22 @@ 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,
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