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Commit 0e280af0 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

tcp: introduce TCPSpuriousRtxHostQueues SNMP counter 



Host queues (Qdisc + NIC) can hold packets so long that TCP can
eventually retransmit a packet before the first transmit even left
the host.

Its not clear right now if we could avoid this in the first place :

- We could arm RTO timer not at the time we enqueue packets, but
  at the time we TX complete them (tcp_wfree())

- Cancel the sending of the new copy of the packet if prior one
  is still in queue.

This patch adds instrumentation so that we can at least see how
often this problem happens.

TCPSpuriousRtxHostQueues SNMP counter is incremented every time
we detect the fast clone is not yet freed in tcp_transmit_skb()

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 5a5967d8

include/uapi/linux/snmp.h

+1 −0
Original line number Diff line number Diff line
@@ -247,6 +247,7 @@ enum 
	LINUX_MIB_TCPFASTOPENPASSIVEFAIL,	/* TCPFastOpenPassiveFail */

	LINUX_MIB_TCPFASTOPENLISTENOVERFLOW,	/* TCPFastOpenListenOverflow */

	LINUX_MIB_TCPFASTOPENCOOKIEREQD,	/* TCPFastOpenCookieReqd */

	LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES, /* TCPSpuriousRtxHostQueues */

	__LINUX_MIB_MAX

};

net/ipv4/proc.c

+1 −0
Original line number Diff line number Diff line
@@ -269,6 +269,7 @@ static const struct snmp_mib snmp4_net_list[] = { 
	SNMP_MIB_ITEM("TCPFastOpenPassiveFail", LINUX_MIB_TCPFASTOPENPASSIVEFAIL),

	SNMP_MIB_ITEM("TCPFastOpenListenOverflow", LINUX_MIB_TCPFASTOPENLISTENOVERFLOW),

	SNMP_MIB_ITEM("TCPFastOpenCookieReqd", LINUX_MIB_TCPFASTOPENCOOKIEREQD),

	SNMP_MIB_ITEM("TCPSpuriousRtxHostQueues", LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES),

	SNMP_MIB_SENTINEL

};

net/ipv4/tcp_output.c

+7 −0
Original line number Diff line number Diff line
@@ -846,6 +846,13 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it, 
		__net_timestamp(skb);



	if (likely(clone_it)) {

		const struct sk_buff *fclone = skb + 1;



		if (unlikely(skb->fclone == SKB_FCLONE_ORIG &&

			     fclone->fclone == SKB_FCLONE_CLONE))

			NET_INC_STATS_BH(sock_net(sk),

					 LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);



		if (unlikely(skb_cloned(skb)))

			skb = pskb_copy(skb, gfp_mask);

		else