[PATCH net-next 3/3] net: gro: move L3 flush checks to tcp_gro_receive

Richard Gobert
2024-02-22T21:12:36Z

{inet,ipv6}_gro_receive functions perform flush checks (ttl, flags,
iph->id, ...) against all packets in a loop. These flush checks are
relevant only to tcp flows, and as such they're used to determine whether
the packets can be merged later in tcp_gro_receive.

These checks are not relevant to UDP packets. Furthermore, they need to be
done only once in tcp_gro_receive and only against the found p skb, since
they only affect flush and not same_flow.

Leveraging the initial commit in the series, correct network header offsets
are saved for both outer and inner network headers - allowing these checks
to be done only once, in tcp_gro_receive. As a result,
NAPI_GRO_CB(p)->flush is not used at all. In addition - flush_id checks are
more declarative and contained in inet_gro_flush_id, thus removing the need
for flush_id in napi_gro_cb.

This results in less parsing code for UDP flows and non-loop flush tests
for TCP flows.

For example, running 64 IP/UDP netperf connections:
./super_netperf.sh 64 -H 1.1.1.2 -t UDP_STREAM -l 120

Running perf top for 90s we can see that relatively less time is spent on
inet_gro_receive:

net-next:

Samples: 330K of event 'cycles:P', 4000 Hz, Event count (approx.): 45945660156 lost: 0/0 drop: 0/0
1.43% [kernel] [k] inet_gro_receive

patch applied:

Samples: 295K of event 'cycles:P', 4000 Hz, Event count (approx.): 44726408099 lost: 0/0 drop: 0/0
1.22% [kernel] [k] inet_gro_receive

Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
---
include/net/gro.h | 9 ++----
net/core/gro.c | 3 --
net/ipv4/af_inet.c | 36 ---------------------
net/ipv4/tcp_offload.c | 71 ++++++++++++++++++++++++++++++++++--------
net/ipv6/ip6_offload.c | 11 -------
5 files changed, 61 insertions(+), 69 deletions(-)

diff --git a/include/net/gro.h b/include/net/gro.h
index 89502a7e35ed..e6db591cdc32 100644
--- a/include/net/gro.h
+++ b/include/net/gro.h
@@ -35,15 +35,15 @@ struct napi_gro_cb {
/* This is non-zero if the packet cannot be merged with the new skb. */
u16 flush;

- /* Save the IP ID here and check when we get to the transport layer */
- u16 flush_id;
-
/* Number of segments aggregated. */
u16 count;

/* Used in ipv6_gro_receive() and foo-over-udp and esp-in-udp */
u16 proto;

+ /* used to support CHECKSUM_COMPLETE for tunneling protocols */
+ __wsum csum;
+
/* Used in napi_gro_cb::free */
#define NAPI_GRO_FREE 1
#define NAPI_GRO_FREE_STOLEN_HEAD 2
@@ -83,9 +83,6 @@ struct napi_gro_cb {
/* GRO is done by frag_list pointer chaining. */
u8 is_flist:1;
);
-
- /* used to support CHECKSUM_COMPLETE for tunneling protocols */
- __wsum csum;
};

#define NAPI_GRO_CB(skb) ((struct napi_gro_cb *)(skb)->cb)
diff --git a/net/core/gro.c b/net/core/gro.c
index 0759277dc14e..b7dced5bcef7 100644
--- a/net/core/gro.c
+++ b/net/core/gro.c
@@ -332,8 +332,6 @@ static void gro_list_prepare(const struct list_head *head,
list_for_each_entry(p, head, list) {
unsigned long diffs;

- NAPI_GRO_CB(p)->flush = 0;
-
if (hash != skb_get_hash_raw(p)) {
NAPI_GRO_CB(p)->same_flow = 0;
continue;
@@ -466,7 +464,6 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
sizeof(u32))); /* Avoid slow unaligned acc */
*(u32 *)&NAPI_GRO_CB(skb)->zeroed = 0;
NAPI_GRO_CB(skb)->flush = skb_has_frag_list(skb);
- NAPI_GRO_CB(skb)->is_atomic = 1;
NAPI_GRO_CB(skb)->count = 1;
if (unlikely(skb_is_gso(skb))) {
NAPI_GRO_CB(skb)->count = skb_shinfo(skb)->gso_segs;
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index c0f3c162bf73..fd7378ecc4e3 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1509,7 +1509,6 @@ struct sk_buff *inet_gro_receive(struct list_head *head, struct sk_buff *skb)

list_for_each_entry(p, head, list) {
struct iphdr *iph2;
- u16 flush_id;

if (!NAPI_GRO_CB(p)->same_flow)
continue;
@@ -1526,43 +1525,8 @@ struct sk_buff *inet_gro_receive(struct list_head *head, struct sk_buff *skb)
NAPI_GRO_CB(p)->same_flow = 0;
continue;
}
-
- /* All fields must match except length and checksum. */
- NAPI_GRO_CB(p)->flush |=
- (iph->ttl ^ iph2->ttl) |
- (iph->tos ^ iph2->tos) |
- ((iph->frag_off ^ iph2->frag_off) & htons(IP_DF));
-
- NAPI_GRO_CB(p)->flush |= flush;
-
- /* We need to store of the IP ID check to be included later
- * when we can verify that this packet does in fact belong
- * to a given flow.
- */
- flush_id = (u16)(id - ntohs(iph2->id));
-
- /* This bit of code makes it much easier for us to identify
- * the cases where we are doing atomic vs non-atomic IP ID
- * checks. Specifically an atomic check can return IP ID
- * values 0 - 0xFFFF, while a non-atomic check can only
- * return 0 or 0xFFFF.
- */
- if (!NAPI_GRO_CB(p)->is_atomic ||
- !(iph->frag_off & htons(IP_DF))) {
- flush_id ^= NAPI_GRO_CB(p)->count;
- flush_id = flush_id ? 0xFFFF : 0;
- }
-
- /* If the previous IP ID value was based on an atomic
- * datagram we can overwrite the value and ignore it.
- */
- if (NAPI_GRO_CB(skb)->is_atomic)
- NAPI_GRO_CB(p)->flush_id = flush_id;
- else
- NAPI_GRO_CB(p)->flush_id |= flush_id;
}

- NAPI_GRO_CB(skb)->is_atomic = !!(iph->frag_off & htons(IP_DF));
NAPI_GRO_CB(skb)->flush |= flush;
if (NAPI_GRO_CB(skb)->encap_mark)
skb_set_inner_network_header(skb, off);
diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
index 8bbcd3f502ac..962c1b900e11 100644
--- a/net/ipv4/tcp_offload.c
+++ b/net/ipv4/tcp_offload.c
@@ -178,6 +178,60 @@ struct sk_buff *tcp_gso_segment(struct sk_buff *skb,
return segs;
}

+static inline bool inet_gro_flush(struct iphdr *iph, struct iphdr *iph2)
+{
+ /* All fields must match except length and checksum. */
+ return (iph->ttl ^ iph2->ttl) |
+ (iph->tos ^ iph2->tos) |
+ ((iph->frag_off ^ iph2->frag_off) & htons(IP_DF));
+}
+
+static inline bool ipv6_gro_flush(struct ipv6hdr *iph, struct ipv6hdr *iph2)
+{
+ /* <Version:4><Traffic_Class:8><Flow_Label:20> */
+ __be32 first_word = *(__be32 *)iph ^ *(__be32 *)iph2;
+
+ /* Flush if Traffic Class fields are different. */
+ return (first_word & htonl(0x0FF00000)) |
+ (__force __be32)(iph->hop_limit ^ iph2->hop_limit);
+}
+
+static inline bool inet_gro_flush_id(struct sk_buff *p, struct iphdr *iph,
+ struct iphdr *iph2)
+{
+ u16 flush_id = ntohs(iph->id) - ntohs(iph2->id);
+ u16 is_atomic = !!(iph->frag_off & htons(IP_DF));
+ u16 count_masked;
+
+ /* When we receive our second frame we can make a decision on if we
+ * continue this flow as an atomic flow with a fixed ID or if we use
+ * an incrementing ID.
+ */
+ if (NAPI_GRO_CB(p)->count == 1)
+ NAPI_GRO_CB(p)->is_atomic = flush_id == 0;
+
+ /* Ignore outer IP ID value if based on atomic datagram. */
+ if (NAPI_GRO_CB(p)->encap_mark && is_atomic && iph2 == ip_hdr(p))
+ return false;
+
+ is_atomic &= NAPI_GRO_CB(p)->is_atomic;
+ count_masked = NAPI_GRO_CB(p)->count & ~(-is_atomic | is_atomic);
+
+ return flush_id ^ count_masked;
+}
+
+static bool tcp_gro_network_flush(struct sk_buff *p, struct sk_buff *skb,
+ int offset)
+{
+ void *nh = (NAPI_GRO_CB(skb)->frag0 ?: skb->data) + offset;
+ void *nh2 = p->data + offset;
+
+ if (((struct iphdr *)nh)->version == 6)
+ return ipv6_gro_flush(nh, nh2);
+
+ return inet_gro_flush(nh, nh2) | inet_gro_flush_id(p, nh, nh2);
+}
+
struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb)
{
struct sk_buff *pp = NULL;
@@ -232,8 +286,10 @@ struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb)
goto out_check_final;

found:
- /* Include the IP ID check below from the inner most IP hdr */
- flush = NAPI_GRO_CB(p)->flush;
+ flush = tcp_gro_network_flush(p, skb, skb_network_offset(skb));
+ if (NAPI_GRO_CB(skb)->encap_mark)
+ flush |= tcp_gro_network_flush(p, skb, skb_inner_network_offset(skb));
+
flush |= (__force int)(flags & TCP_FLAG_CWR);
flush |= (__force int)((flags ^ tcp_flag_word(th2)) &
~(TCP_FLAG_CWR | TCP_FLAG_FIN | TCP_FLAG_PSH));
@@ -242,17 +298,6 @@ struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb)
flush |= *(u32 *)((u8 *)th + i) ^
*(u32 *)((u8 *)th2 + i);

- /* When we receive our second frame we can made a decision on if we
- * continue this flow as an atomic flow with a fixed ID or if we use
- * an incrementing ID.
- */
- if (NAPI_GRO_CB(p)->flush_id != 1 ||
- NAPI_GRO_CB(p)->count != 1 ||
- !NAPI_GRO_CB(p)->is_atomic)
- flush |= NAPI_GRO_CB(p)->flush_id;
- else
- NAPI_GRO_CB(p)->is_atomic = false;
-
mss = skb_shinfo(p)->gso_size;

/* If skb is a GRO packet, make sure its gso_size matches prior packet mss.
diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
index db7e3db587b9..8bf93a0e9669 100644
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c
@@ -291,19 +291,8 @@ INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
nlen - sizeof(struct ipv6hdr)))
goto not_same_flow;
}
- /* flush if Traffic Class fields are different */
- NAPI_GRO_CB(p)->flush |= !!((first_word & htonl(0x0FF00000)) |
- (__force __be32)(iph->hop_limit ^ iph2->hop_limit));
- NAPI_GRO_CB(p)->flush |= flush;
-
- /* If the previous IP ID value was based on an atomic
- * datagram we can overwrite the value and ignore it.
- */
- if (NAPI_GRO_CB(skb)->is_atomic)
- NAPI_GRO_CB(p)->flush_id = 0;
}

- NAPI_GRO_CB(skb)->is_atomic = true;
NAPI_GRO_CB(skb)->flush |= flush;

skb_gro_postpull_rcsum(skb, iph, nlen);
--
2.36.1