Commit cd7d8498 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

tcp: change tcp_skb_pcount() location 



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: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent dc83d4d8

include/net/tcp.h

+21 −2
Original line number Diff line number Diff line
@@ -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. */

net/ipv4/tcp.c

+2 −2
Original line number Diff line number Diff line
@@ -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;

net/ipv4/tcp_input.c

+4 −4
Original line number Diff line number Diff line
@@ -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;

	}

net/ipv4/tcp_output.c

+6 −3
Original line number Diff line number Diff line
@@ -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;

	}

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