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tcp: change tcp_skb_pcount() location

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Our goal is to access no more than one cache line access per skb in
a write or receive queue when doing the various walks.

After recent TCP_SKB_CB() reorganizations, it is almost done.

Last part is tcp_skb_pcount() which currently uses
skb_shinfo(skb)->gso_segs, which is a terrible choice, because it needs
3 cache lines in current kernel (skb->head, skb->end, and
shinfo->gso_segs are all in 3 different cache lines, far from skb->cb)

This very simple patch reuses space currently taken by tcp_tw_isn
only in input path, as tcp_skb_pcount is only needed for skb stored in
write queue.

This considerably speeds up tcp_ack(), granted we avoid shinfo->tx_flags
to get SKBTX_ACK_TSTAMP, which seems possible.

This also speeds up all sack processing in general.

This speeds up tcp_sendmsg() because it no longer has to access/dirty
shinfo.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2014-09-24 04:11:22 -07:00 committed by David S. Miller
parent dc83d4d8f6
commit cd7d8498c9
4 changed files with 33 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
include/net/tcp.h
View File

@ -698,7 +698,16 @@ static inline u32 tcp_skb_timestamp(const struct sk_buff *skb)
struct tcp_skb_cb {
__u32 seq; /* Starting sequence number */
__u32 end_seq; /* SEQ + FIN + SYN + datalen */
__u32 tcp_tw_isn; /* isn chosen by tcp_timewait_state_process() */
union {
/* Note : tcp_tw_isn is used in input path only
* (isn chosen by tcp_timewait_state_process())
*
* tcp_gso_segs is used in write queue only,
* cf tcp_skb_pcount()
*/
__u32 tcp_tw_isn;
__u32 tcp_gso_segs;
};
__u8 tcp_flags; /* TCP header flags. (tcp[13]) */
__u8 sacked; /* State flags for SACK/FACK. */
@ -746,7 +755,17 @@ TCP_ECN_create_request(struct request_sock *req, const struct sk_buff *skb,
*/
static inline int tcp_skb_pcount(const struct sk_buff *skb)
{
return skb_shinfo(skb)->gso_segs;
return TCP_SKB_CB(skb)->tcp_gso_segs;
}
static inline void tcp_skb_pcount_set(struct sk_buff *skb, int segs)
{
TCP_SKB_CB(skb)->tcp_gso_segs = segs;
}
static inline void tcp_skb_pcount_add(struct sk_buff *skb, int segs)
{
TCP_SKB_CB(skb)->tcp_gso_segs += segs;
}
/* This is valid iff tcp_skb_pcount() > 1. */

4
net/ipv4/tcp.c
View File

@ -963,7 +963,7 @@ new_segment:
skb->ip_summed = CHECKSUM_PARTIAL;
tp->write_seq += copy;
TCP_SKB_CB(skb)->end_seq += copy;
skb_shinfo(skb)->gso_segs = 0;
tcp_skb_pcount_set(skb, 0);
if (!copied)
TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_PSH;
@ -1261,7 +1261,7 @@ new_segment:
tp->write_seq += copy;
TCP_SKB_CB(skb)->end_seq += copy;
skb_shinfo(skb)->gso_segs = 0;
tcp_skb_pcount_set(skb, 0);
from += copy;
copied += copy;

8
net/ipv4/tcp_input.c
View File

@ -1295,9 +1295,9 @@ static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
TCP_SKB_CB(prev)->end_seq += shifted;
TCP_SKB_CB(skb)->seq += shifted;
skb_shinfo(prev)->gso_segs += pcount;
BUG_ON(skb_shinfo(skb)->gso_segs < pcount);
skb_shinfo(skb)->gso_segs -= pcount;
tcp_skb_pcount_add(prev, pcount);
BUG_ON(tcp_skb_pcount(skb) < pcount);
tcp_skb_pcount_add(skb, -pcount);
/* When we're adding to gso_segs == 1, gso_size will be zero,
* in theory this shouldn't be necessary but as long as DSACK
@ -1310,7 +1310,7 @@ static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
}
/* CHECKME: To clear or not to clear? Mimics normal skb currently */
if (skb_shinfo(skb)->gso_segs <= 1) {
if (tcp_skb_pcount(skb) <= 1) {
skb_shinfo(skb)->gso_size = 0;
skb_shinfo(skb)->gso_type = 0;
}

9
net/ipv4/tcp_output.c
View File

@ -384,7 +384,7 @@ static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
TCP_SKB_CB(skb)->tcp_flags = flags;
TCP_SKB_CB(skb)->sacked = 0;
shinfo->gso_segs = 1;
tcp_skb_pcount_set(skb, 1);
shinfo->gso_size = 0;
shinfo->gso_type = 0;
@ -972,6 +972,9 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
tcp_skb_pcount(skb));
/* OK, its time to fill skb_shinfo(skb)->gso_segs */
skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
/* Our usage of tstamp should remain private */
skb->tstamp.tv64 = 0;
@ -1019,11 +1022,11 @@ static void tcp_set_skb_tso_segs(const struct sock *sk, struct sk_buff *skb,
/* Avoid the costly divide in the normal
* non-TSO case.
*/
shinfo->gso_segs = 1;
tcp_skb_pcount_set(skb, 1);
shinfo->gso_size = 0;
shinfo->gso_type = 0;
} else {
shinfo->gso_segs = DIV_ROUND_UP(skb->len, mss_now);
tcp_skb_pcount_set(skb, DIV_ROUND_UP(skb->len, mss_now));
shinfo->gso_size = mss_now;
shinfo->gso_type = sk->sk_gso_type;
}