From d4589926d7a9a4b88e650bb9154bab71e8d2a7dd Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Fri, 13 Dec 2013 13:51:23 -0800 Subject: tcp: refine TSO splits While investigating performance problems on small RPC workloads, I noticed linux TCP stack was always splitting the last TSO skb into two parts (skbs). One being a multiple of MSS, and a small one with the Push flag. This split is done even if TCP_NODELAY is set, or if no small packet is in flight. Example with request/response of 4K/4K IP A > B: . ack 68432 win 2783 IP A > B: . 65537:68433(2896) ack 69632 win 2783 IP A > B: P 68433:69633(1200) ack 69632 win 2783 IP B > A: . ack 68433 win 2768 IP B > A: . 69632:72528(2896) ack 69633 win 2768 IP B > A: P 72528:73728(1200) ack 69633 win 2768 IP A > B: . ack 72528 win 2783 IP A > B: . 69633:72529(2896) ack 73728 win 2783 IP A > B: P 72529:73729(1200) ack 73728 win 2783 We can avoid this split by including the Nagle tests at the right place. Note : If some NIC had trouble sending TSO packets with a partial last segment, we would have hit the problem in GRO/forwarding workload already. tcp_minshall_update() is moved to tcp_output.c and is updated as we might feed a TSO packet with a partial last segment. This patch tremendously improves performance, as the traffic now looks like : IP A > B: . ack 98304 win 2783 IP A > B: P 94209:98305(4096) ack 98304 win 2783 IP B > A: . ack 98305 win 2768 IP B > A: P 98304:102400(4096) ack 98305 win 2768 IP A > B: . ack 102400 win 2783 IP A > B: P 98305:102401(4096) ack 102400 win 2783 IP B > A: . ack 102401 win 2768 IP B > A: P 102400:106496(4096) ack 102401 win 2768 IP A > B: . ack 106496 win 2783 IP A > B: P 102401:106497(4096) ack 106496 win 2783 IP B > A: . ack 106497 win 2768 IP B > A: P 106496:110592(4096) ack 106497 win 2768 Before : lpq83:~# nstat >/dev/null;perf stat ./super_netperf 200 -t TCP_RR -H lpq84 -l 20 -- -r 4K,4K 280774 Performance counter stats for './super_netperf 200 -t TCP_RR -H lpq84 -l 20 -- -r 4K,4K': 205719.049006 task-clock # 9.278 CPUs utilized 8,449,968 context-switches # 0.041 M/sec 1,935,997 CPU-migrations # 0.009 M/sec 160,541 page-faults # 0.780 K/sec 548,478,722,290 cycles # 2.666 GHz [83.20%] 455,240,670,857 stalled-cycles-frontend # 83.00% frontend cycles idle [83.48%] 272,881,454,275 stalled-cycles-backend # 49.75% backend cycles idle [66.73%] 166,091,460,030 instructions # 0.30 insns per cycle # 2.74 stalled cycles per insn [83.39%] 29,150,229,399 branches # 141.699 M/sec [83.30%] 1,943,814,026 branch-misses # 6.67% of all branches [83.32%] 22.173517844 seconds time elapsed lpq83:~# nstat | egrep "IpOutRequests|IpExtOutOctets" IpOutRequests 16851063 0.0 IpExtOutOctets 23878580777 0.0 After patch : lpq83:~# nstat >/dev/null;perf stat ./super_netperf 200 -t TCP_RR -H lpq84 -l 20 -- -r 4K,4K 280877 Performance counter stats for './super_netperf 200 -t TCP_RR -H lpq84 -l 20 -- -r 4K,4K': 107496.071918 task-clock # 4.847 CPUs utilized 5,635,458 context-switches # 0.052 M/sec 1,374,707 CPU-migrations # 0.013 M/sec 160,920 page-faults # 0.001 M/sec 281,500,010,924 cycles # 2.619 GHz [83.28%] 228,865,069,307 stalled-cycles-frontend # 81.30% frontend cycles idle [83.38%] 142,462,742,658 stalled-cycles-backend # 50.61% backend cycles idle [66.81%] 95,227,712,566 instructions # 0.34 insns per cycle # 2.40 stalled cycles per insn [83.43%] 16,209,868,171 branches # 150.795 M/sec [83.20%] 874,252,952 branch-misses # 5.39% of all branches [83.37%] 22.175821286 seconds time elapsed lpq83:~# nstat | egrep "IpOutRequests|IpExtOutOctets" IpOutRequests 11239428 0.0 IpExtOutOctets 23595191035 0.0 Indeed, the occupancy of tx skbs (IpExtOutOctets/IpOutRequests) is higher : 2099 instead of 1417, thus helping GRO to be more efficient when using FQ packet scheduler. Many thanks to Neal for review and ideas. Signed-off-by: Eric Dumazet Cc: Yuchung Cheng Cc: Neal Cardwell Cc: Nandita Dukkipati Cc: Van Jacobson Acked-by: Neal Cardwell Tested-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_output.c | 93 ++++++++++++++++++++++++++++++--------------------- 1 file changed, 54 insertions(+), 39 deletions(-) (limited to 'net') diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 2a69f42e51ca..9e7aec7ee67e 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1384,23 +1384,51 @@ static void tcp_cwnd_validate(struct sock *sk) } } -/* Returns the portion of skb which can be sent right away without - * introducing MSS oddities to segment boundaries. In rare cases where - * mss_now != mss_cache, we will request caller to create a small skb - * per input skb which could be mostly avoided here (if desired). - * - * We explicitly want to create a request for splitting write queue tail - * to a small skb for Nagle purposes while avoiding unnecessary modulos, - * thus all the complexity (cwnd_len is always MSS multiple which we - * return whenever allowed by the other factors). Basically we need the - * modulo only when the receiver window alone is the limiting factor or - * when we would be allowed to send the split-due-to-Nagle skb fully. +/* Minshall's variant of the Nagle send check. */ +static bool tcp_minshall_check(const struct tcp_sock *tp) +{ + return after(tp->snd_sml, tp->snd_una) && + !after(tp->snd_sml, tp->snd_nxt); +} + +/* Update snd_sml if this skb is under mss + * Note that a TSO packet might end with a sub-mss segment + * The test is really : + * if ((skb->len % mss) != 0) + * tp->snd_sml = TCP_SKB_CB(skb)->end_seq; + * But we can avoid doing the divide again given we already have + * skb_pcount = skb->len / mss_now */ -static unsigned int tcp_mss_split_point(const struct sock *sk, const struct sk_buff *skb, - unsigned int mss_now, unsigned int max_segs) +static void tcp_minshall_update(struct tcp_sock *tp, unsigned int mss_now, + const struct sk_buff *skb) +{ + if (skb->len < tcp_skb_pcount(skb) * mss_now) + tp->snd_sml = TCP_SKB_CB(skb)->end_seq; +} + +/* Return false, if packet can be sent now without violation Nagle's rules: + * 1. It is full sized. (provided by caller in %partial bool) + * 2. Or it contains FIN. (already checked by caller) + * 3. Or TCP_CORK is not set, and TCP_NODELAY is set. + * 4. Or TCP_CORK is not set, and all sent packets are ACKed. + * With Minshall's modification: all sent small packets are ACKed. + */ +static bool tcp_nagle_check(bool partial, const struct tcp_sock *tp, + unsigned int mss_now, int nonagle) +{ + return partial && + ((nonagle & TCP_NAGLE_CORK) || + (!nonagle && tp->packets_out && tcp_minshall_check(tp))); +} +/* Returns the portion of skb which can be sent right away */ +static unsigned int tcp_mss_split_point(const struct sock *sk, + const struct sk_buff *skb, + unsigned int mss_now, + unsigned int max_segs, + int nonagle) { const struct tcp_sock *tp = tcp_sk(sk); - u32 needed, window, max_len; + u32 partial, needed, window, max_len; window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq; max_len = mss_now * max_segs; @@ -1413,7 +1441,15 @@ static unsigned int tcp_mss_split_point(const struct sock *sk, const struct sk_b if (max_len <= needed) return max_len; - return needed - needed % mss_now; + partial = needed % mss_now; + /* If last segment is not a full MSS, check if Nagle rules allow us + * to include this last segment in this skb. + * Otherwise, we'll split the skb at last MSS boundary + */ + if (tcp_nagle_check(partial != 0, tp, mss_now, nonagle)) + return needed - partial; + + return needed; } /* Can at least one segment of SKB be sent right now, according to the @@ -1453,28 +1489,6 @@ static int tcp_init_tso_segs(const struct sock *sk, struct sk_buff *skb, return tso_segs; } -/* Minshall's variant of the Nagle send check. */ -static inline bool tcp_minshall_check(const struct tcp_sock *tp) -{ - return after(tp->snd_sml, tp->snd_una) && - !after(tp->snd_sml, tp->snd_nxt); -} - -/* Return false, if packet can be sent now without violation Nagle's rules: - * 1. It is full sized. - * 2. Or it contains FIN. (already checked by caller) - * 3. Or TCP_CORK is not set, and TCP_NODELAY is set. - * 4. Or TCP_CORK is not set, and all sent packets are ACKed. - * With Minshall's modification: all sent small packets are ACKed. - */ -static inline bool tcp_nagle_check(const struct tcp_sock *tp, - const struct sk_buff *skb, - unsigned int mss_now, int nonagle) -{ - return skb->len < mss_now && - ((nonagle & TCP_NAGLE_CORK) || - (!nonagle && tp->packets_out && tcp_minshall_check(tp))); -} /* Return true if the Nagle test allows this packet to be * sent now. @@ -1495,7 +1509,7 @@ static inline bool tcp_nagle_test(const struct tcp_sock *tp, const struct sk_buf if (tcp_urg_mode(tp) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)) return true; - if (!tcp_nagle_check(tp, skb, cur_mss, nonagle)) + if (!tcp_nagle_check(skb->len < cur_mss, tp, cur_mss, nonagle)) return true; return false; @@ -1898,7 +1912,8 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, limit = tcp_mss_split_point(sk, skb, mss_now, min_t(unsigned int, cwnd_quota, - sk->sk_gso_max_segs)); + sk->sk_gso_max_segs), + nonagle); if (skb->len > limit && unlikely(tso_fragment(sk, skb, limit, mss_now, gfp))) -- cgit v1.2.3-59-g8ed1b