[PATCH net-next v5] tcp: trace retransmit failures in tcp_retransmit_skb

[PATCH net-next v5] tcp: trace retransmit failures in tcp_retransmit_skb

[fan.yu9]

From: Fan Yu <fan.yu9@zte.com.cn>

Background
==========
When TCP retransmits a packet due to missing ACKs, the
retransmission may fail for various reasons (e.g., packets
stuck in driver queues, receiver zero windows, or routing issues).

The original tcp_retransmit_skb tracepoint:
&apos;commit e086101b150a ("tcp: add a tracepoint for tcp retransmission")&apos;
lacks visibility into these failure causes, making production
diagnostics difficult.

Solution
========
Adds the retval("err") to the tcp_retransmit_skb tracepoint.
Enables users to know why some tcp retransmission failed and
users can filter retransmission failures by retval.

Compatibility description
=========================
This patch extends the tcp_retransmit_skb tracepoint
by adding a new "err" field at the end of its
existing structure (within TP_STRUCT__entry). The
compatibility implications are detailed as follows:

1) Structural compatibility for legacy user-space tools

Legacy tools/BPF programs accessing existing fields
(by offset or name) can still work without modification
or recompilation.The new field is appended to the end,
preserving original memory layout.

2) Note: semantic changes

The original tracepoint primarily only focused on
successfully retransmitted packets. With this patch,
the tracepoint now can figure out packets that may
terminate early due to specific reasons. For accurate
statistics, users should filter using "err" to
distinguish outcomes.

Before patched:
# cat /sys/kernel/debug/tracing/events/tcp/tcp_retransmit_skb/format
field:const void * skbaddr; offset:8; size:8; signed:0;
field:const void * skaddr; offset:16; size:8; signed:0;
field:int state; offset:24; size:4; signed:1;
field:__u16 sport; offset:28; size:2; signed:0;
field:__u16 dport; offset:30; size:2; signed:0;
field:__u16 family; offset:32; size:2; signed:0;
field:__u8 saddr[4]; offset:34; size:4; signed:0;
field:__u8 daddr[4]; offset:38; size:4; signed:0;
field:__u8 saddr_v6[16]; offset:42; size:16; signed:0;
field:__u8 daddr_v6[16]; offset:58; size:16; signed:0;

print fmt: "skbaddr=%p skaddr=%p family=%s sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c state=%s"

After patched:
# cat /sys/kernel/debug/tracing/events/tcp/tcp_retransmit_skb/format
field:const void * skbaddr; offset:8; size:8; signed:0;
field:const void * skaddr; offset:16; size:8; signed:0;
field:int state; offset:24; size:4; signed:1;
field:__u16 sport; offset:28; size:2; signed:0;
field:__u16 dport; offset:30; size:2; signed:0;
field:__u16 family; offset:32; size:2; signed:0;
field:__u8 saddr[4]; offset:34; size:4; signed:0;
field:__u8 daddr[4]; offset:38; size:4; signed:0;
field:__u8 saddr_v6[16]; offset:42; size:16; signed:0;
field:__u8 daddr_v6[16]; offset:58; size:16; signed:0;
field:int err;  offset:76;      size:4; signed:1;

print fmt: "skbaddr=%p skaddr=%p family=%s sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c state=%s err=%d"

Change Log
=========
v4->v5:
Some fixes according to
https://lore.kernel.org/all/20250715072058.12f343bb@kernel.org/
1. Instead of introducing new TCP_RETRANS_* enums, directly
passing the retval to the tracepoint.

v3->v4:
Some fixes according to
https://lore.kernel.org/all/CANn89i+JGSt=_CtWfhDXypWW-34a6SoP3RAzWQ9B9VL4+PHjDw@mail.gmail.com/
1. Consolidate ENOMEMs into a unified TCP_RETRANS_NOMEM.

v2->v3:
Some fixes according to
https://lore.kernel.org/all/CANn89iJvyYjiweCESQL8E-Si7M=gosYvh1BAVWwAWycXW8GSdg@mail.gmail.com/
1. Rename "quit_reason" to "result". Also, keep "key=val" format concise(no space in vals).

v1->v2:
Some fixes according to
https://lore.kernel.org/all/CANn89iK-6kT-ZUpNRMjPY9_TkQj-dLuKrDQtvO1140q4EUsjFg@mail.gmail.com/
1.Rename TCP_RETRANS_QUIT_UNDEFINED to TCP_RETRANS_ERR_DEFAULT.
2.Added detailed compatibility consequences section.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Suggested-by: Eric Dumazet <edumazet@google.com>
Co-developed-by: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Fan Yu <fan.yu9@zte.com.cn>
---
include/trace/events/tcp.h | 27 ++++++++--------------
net/ipv4/tcp_output.c      | 46 ++++++++++++++++++++++++--------------
2 files changed, 38 insertions(+), 35 deletions(-)

diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
index 54e60c6009e3..9d2c36c6a0ed 100644
--- a/include/trace/events/tcp.h
+++ b/include/trace/events/tcp.h
@@ -13,17 +13,11 @@
#include <linux/sock_diag.h>
#include <net/rstreason.h>

-/*
- * tcp event with arguments sk and skb
- *
- * Note: this class requires a valid sk pointer; while skb pointer could
- *       be NULL.
- */
-DECLARE_EVENT_CLASS(tcp_event_sk_skb,
+TRACE_EVENT(tcp_retransmit_skb,

-	TP_PROTO(const struct sock *sk, const struct sk_buff *skb),
+	TP_PROTO(const struct sock *sk, const struct sk_buff *skb, int err),

-	TP_ARGS(sk, skb),
+	TP_ARGS(sk, skb, err),

 	TP_STRUCT__entry(
 		__field(const void *, skbaddr)
@@ -36,6 +30,7 @@ DECLARE_EVENT_CLASS(tcp_event_sk_skb,
 		__array(__u8, daddr, 4)
 		__array(__u8, saddr_v6, 16)
 		__array(__u8, daddr_v6, 16)
+		__field(int, err)
 	),

 	TP_fast_assign(
@@ -58,21 +53,17 @@ DECLARE_EVENT_CLASS(tcp_event_sk_skb,

 		TP_STORE_ADDRS(__entry, inet->inet_saddr, inet->inet_daddr,
 			sk->sk_v6_rcv_saddr, sk->sk_v6_daddr);
+
+		__entry->err = err;
 	),

-	TP_printk("skbaddr=%p skaddr=%p family=%s sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c state=%s",
+	TP_printk("skbaddr=%p skaddr=%p family=%s sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c state=%s err=%d",
 		__entry->skbaddr, __entry->skaddr,
 		show_family_name(__entry->family),
 		__entry->sport, __entry->dport, __entry->saddr, __entry->daddr,
 		__entry->saddr_v6, __entry->daddr_v6,
-		  show_tcp_state_name(__entry->state))
-);
-
-DEFINE_EVENT(tcp_event_sk_skb, tcp_retransmit_skb,
-
-	TP_PROTO(const struct sock *sk, const struct sk_buff *skb),
-
-	TP_ARGS(sk, skb)
+		  show_tcp_state_name(__entry->state),
+		  __entry->err)
);

#undef FN
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index b616776e3354..c31e164693d5 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3330,8 +3330,10 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	if (icsk->icsk_mtup.probe_size)
 		icsk->icsk_mtup.probe_size = 0;

-	if (skb_still_in_host_queue(sk, skb))
-		return -EBUSY;
+	if (skb_still_in_host_queue(sk, skb)) {
+		err = -EBUSY;
+		goto out;
+	}

start:
 	if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
@@ -3342,14 +3344,19 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 		}
 		if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
 			WARN_ON_ONCE(1);
-			return -EINVAL;
+			err = -EINVAL;
+			goto out;
+		}
+		if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq)) {
+			err = -ENOMEM;
+			goto out;
 		}
-		if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
-			return -ENOMEM;
 	}

-	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
-		return -EHOSTUNREACH; /* Routing failure or similar. */
+	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk)) {
+		err = -EHOSTUNREACH; /* Routing failure or similar. */
+		goto out;
+	}

 	cur_mss = tcp_current_mss(sk);
 	avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
@@ -3360,8 +3367,10 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	* our retransmit of one segment serves as a zero window probe.
 	*/
 	if (avail_wnd <= 0) {
-		if (TCP_SKB_CB(skb)->seq != tp->snd_una)
-			return -EAGAIN;
+		if (TCP_SKB_CB(skb)->seq != tp->snd_una) {
+			err = -EAGAIN;
+			goto out;
+		}
 		avail_wnd = cur_mss;
 	}

@@ -3373,11 +3382,15 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	}
 	if (skb->len > len) {
 		if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
-				 cur_mss, GFP_ATOMIC))
-			return -ENOMEM; /* We&apos;ll try again later. */
+				 cur_mss, GFP_ATOMIC)) {
+			err = -ENOMEM;  /* We&apos;ll try again later. */
+			goto out;
+		}
 	} else {
-		if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
-			return -ENOMEM;
+		if (skb_unclone_keeptruesize(skb, GFP_ATOMIC)) {
+			err = -ENOMEM;
+			goto out;
+		}

 		diff = tcp_skb_pcount(skb);
 		tcp_set_skb_tso_segs(skb, cur_mss);
@@ -3431,17 +3444,16 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
 				TCP_SKB_CB(skb)->seq, segs, err);

-	if (likely(!err)) {
-		trace_tcp_retransmit_skb(sk, skb);
-	} else if (err != -EBUSY) {
+	if (err != -EBUSY)
 		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);
-	}

 	/* To avoid taking spuriously low RTT samples based on a timestamp
 	* for a transmit that never happened, always mark EVER_RETRANS
 	*/
 	TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;

+out:
+	trace_tcp_retransmit_skb(sk, skb, err);
 	return err;
}

--
2.25.1

Re: [PATCH net-next v5] tcp: trace retransmit failures in tcp_retransmit_skb

[Kuniyuki Iwashima]

[ updated my email address, please use this one for v6+ ]

From: <fan.yu9@zte.com.cn>
Date: Tue, 15 Jul 2025 23:52:49 +0800 (CST)
> From: Fan Yu <fan.yu9@zte.com.cn>
> 
> Background
> ==========
> When TCP retransmits a packet due to missing ACKs, the
> retransmission may fail for various reasons (e.g., packets
> stuck in driver queues, receiver zero windows, or routing issues).
> 
> The original tcp_retransmit_skb tracepoint:
> &apos;commit e086101b150a ("tcp: add a tracepoint for tcp retransmission")&apos;

Looks like your email client converts "'" into "&apos;" and
corrupts the diff, see below.


> lacks visibility into these failure causes, making production
> diagnostics difficult.
> 
> Solution
> ========
> Adds the retval("err") to the tcp_retransmit_skb tracepoint.
> Enables users to know why some tcp retransmission failed and
> users can filter retransmission failures by retval.
> 
> Compatibility description
> =========================
> This patch extends the tcp_retransmit_skb tracepoint
> by adding a new "err" field at the end of its
> existing structure (within TP_STRUCT__entry). The
> compatibility implications are detailed as follows:
> 
> 1) Structural compatibility for legacy user-space tools
> 
> Legacy tools/BPF programs accessing existing fields
> (by offset or name) can still work without modification
> or recompilation.The new field is appended to the end,
> preserving original memory layout.
> 
> 2) Note: semantic changes
> 
> The original tracepoint primarily only focused on
> successfully retransmitted packets. With this patch,
> the tracepoint now can figure out packets that may
> terminate early due to specific reasons. For accurate
> statistics, users should filter using "err" to
> distinguish outcomes.
> 
> Before patched:
> # cat /sys/kernel/debug/tracing/events/tcp/tcp_retransmit_skb/format
> field:const void * skbaddr; offset:8; size:8; signed:0;
> field:const void * skaddr; offset:16; size:8; signed:0;
> field:int state; offset:24; size:4; signed:1;
> field:__u16 sport; offset:28; size:2; signed:0;
> field:__u16 dport; offset:30; size:2; signed:0;
> field:__u16 family; offset:32; size:2; signed:0;
> field:__u8 saddr[4]; offset:34; size:4; signed:0;
> field:__u8 daddr[4]; offset:38; size:4; signed:0;
> field:__u8 saddr_v6[16]; offset:42; size:16; signed:0;
> field:__u8 daddr_v6[16]; offset:58; size:16; signed:0;
> 
> print fmt: "skbaddr=%p skaddr=%p family=%s sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c state=%s"
> 
> After patched:
> # cat /sys/kernel/debug/tracing/events/tcp/tcp_retransmit_skb/format
> field:const void * skbaddr; offset:8; size:8; signed:0;
> field:const void * skaddr; offset:16; size:8; signed:0;
> field:int state; offset:24; size:4; signed:1;
> field:__u16 sport; offset:28; size:2; signed:0;
> field:__u16 dport; offset:30; size:2; signed:0;
> field:__u16 family; offset:32; size:2; signed:0;
> field:__u8 saddr[4]; offset:34; size:4; signed:0;
> field:__u8 daddr[4]; offset:38; size:4; signed:0;
> field:__u8 saddr_v6[16]; offset:42; size:16; signed:0;
> field:__u8 daddr_v6[16]; offset:58; size:16; signed:0;
> field:int err;  offset:76;      size:4; signed:1;
> 
> print fmt: "skbaddr=%p skaddr=%p family=%s sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c state=%s err=%d"
> 
> Change Log
> =========
> v4->v5:
> Some fixes according to
> https://lore.kernel.org/all/20250715072058.12f343bb@kernel.org/
> 1. Instead of introducing new TCP_RETRANS_* enums, directly
> passing the retval to the tracepoint.
> 
> v3->v4:
> Some fixes according to
> https://lore.kernel.org/all/CANn89i+JGSt=_CtWfhDXypWW-34a6SoP3RAzWQ9B9VL4+PHjDw@mail.gmail.com/
> 1. Consolidate ENOMEMs into a unified TCP_RETRANS_NOMEM.
> 
> v2->v3:
> Some fixes according to
> https://lore.kernel.org/all/CANn89iJvyYjiweCESQL8E-Si7M=gosYvh1BAVWwAWycXW8GSdg@mail.gmail.com/
> 1. Rename "quit_reason" to "result". Also, keep "key=val" format concise(no space in vals).
> 
> v1->v2:
> Some fixes according to
> https://lore.kernel.org/all/CANn89iK-6kT-ZUpNRMjPY9_TkQj-dLuKrDQtvO1140q4EUsjFg@mail.gmail.com/
> 1.Rename TCP_RETRANS_QUIT_UNDEFINED to TCP_RETRANS_ERR_DEFAULT.
> 2.Added detailed compatibility consequences section.
> 
> Suggested-by: Jakub Kicinski <kuba@kernel.org>
> Suggested-by: Eric Dumazet <edumazet@google.com>
> Co-developed-by: xu xin <xu.xin16@zte.com.cn>
> Signed-off-by: xu xin <xu.xin16@zte.com.cn>
> Signed-off-by: Fan Yu <fan.yu9@zte.com.cn>
> ---

Please add changelog here so it will disappear when the patch is merged.


> include/trace/events/tcp.h | 27 ++++++++--------------
> net/ipv4/tcp_output.c      | 46 ++++++++++++++++++++++++--------------
> 2 files changed, 38 insertions(+), 35 deletions(-)
> 
> diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
> index 54e60c6009e3..9d2c36c6a0ed 100644
> --- a/include/trace/events/tcp.h
> +++ b/include/trace/events/tcp.h
> @@ -13,17 +13,11 @@
> #include <linux/sock_diag.h>
> #include <net/rstreason.h>
> 
> -/*
> - * tcp event with arguments sk and skb
> - *
> - * Note: this class requires a valid sk pointer; while skb pointer could
> - *       be NULL.
> - */
> -DECLARE_EVENT_CLASS(tcp_event_sk_skb,
> +TRACE_EVENT(tcp_retransmit_skb,
> 
> -	TP_PROTO(const struct sock *sk, const struct sk_buff *skb),
> +	TP_PROTO(const struct sock *sk, const struct sk_buff *skb, int err),
> 
> -	TP_ARGS(sk, skb),
> +	TP_ARGS(sk, skb, err),
> 
>  	TP_STRUCT__entry(
>  		__field(const void *, skbaddr)
> @@ -36,6 +30,7 @@ DECLARE_EVENT_CLASS(tcp_event_sk_skb,
>  		__array(__u8, daddr, 4)
>  		__array(__u8, saddr_v6, 16)
>  		__array(__u8, daddr_v6, 16)
> +		__field(int, err)
>  	),
> 
>  	TP_fast_assign(
> @@ -58,21 +53,17 @@ DECLARE_EVENT_CLASS(tcp_event_sk_skb,
> 
>  		TP_STORE_ADDRS(__entry, inet->inet_saddr, inet->inet_daddr,
>  			sk->sk_v6_rcv_saddr, sk->sk_v6_daddr);
> +
> +		__entry->err = err;
>  	),
> 
> -	TP_printk("skbaddr=%p skaddr=%p family=%s sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c state=%s",
> +	TP_printk("skbaddr=%p skaddr=%p family=%s sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c state=%s err=%d",
>  		__entry->skbaddr, __entry->skaddr,
>  		show_family_name(__entry->family),
>  		__entry->sport, __entry->dport, __entry->saddr, __entry->daddr,
>  		__entry->saddr_v6, __entry->daddr_v6,
> -		  show_tcp_state_name(__entry->state))
> -);
> -
> -DEFINE_EVENT(tcp_event_sk_skb, tcp_retransmit_skb,
> -
> -	TP_PROTO(const struct sock *sk, const struct sk_buff *skb),
> -
> -	TP_ARGS(sk, skb)
> +		  show_tcp_state_name(__entry->state),
> +		  __entry->err)
> );
> 
> #undef FN
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index b616776e3354..c31e164693d5 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -3330,8 +3330,10 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
>  	if (icsk->icsk_mtup.probe_size)
>  		icsk->icsk_mtup.probe_size = 0;
> 
> -	if (skb_still_in_host_queue(sk, skb))
> -		return -EBUSY;
> +	if (skb_still_in_host_queue(sk, skb)) {
> +		err = -EBUSY;
> +		goto out;
> +	}
> 
> start:
>  	if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
> @@ -3342,14 +3344,19 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
>  		}
>  		if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
>  			WARN_ON_ONCE(1);
> -			return -EINVAL;
> +			err = -EINVAL;
> +			goto out;
> +		}
> +		if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq)) {
> +			err = -ENOMEM;
> +			goto out;
>  		}
> -		if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
> -			return -ENOMEM;
>  	}
> 
> -	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
> -		return -EHOSTUNREACH; /* Routing failure or similar. */
> +	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk)) {
> +		err = -EHOSTUNREACH; /* Routing failure or similar. */
> +		goto out;
> +	}
> 
>  	cur_mss = tcp_current_mss(sk);
>  	avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
> @@ -3360,8 +3367,10 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
>  	* our retransmit of one segment serves as a zero window probe.
>  	*/
>  	if (avail_wnd <= 0) {
> -		if (TCP_SKB_CB(skb)->seq != tp->snd_una)
> -			return -EAGAIN;
> +		if (TCP_SKB_CB(skb)->seq != tp->snd_una) {
> +			err = -EAGAIN;
> +			goto out;
> +		}
>  		avail_wnd = cur_mss;
>  	}
> 
> @@ -3373,11 +3382,15 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
>  	}
>  	if (skb->len > len) {
>  		if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
> -				 cur_mss, GFP_ATOMIC))
> -			return -ENOMEM; /* We&apos;ll try again later. */
> +				 cur_mss, GFP_ATOMIC)) {
> +			err = -ENOMEM;  /* We&apos;ll try again later. */

This chunk is corrupted by "'" conversion.
Please check your email client config.



> +			goto out;
> +		}
>  	} else {
> -		if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
> -			return -ENOMEM;
> +		if (skb_unclone_keeptruesize(skb, GFP_ATOMIC)) {
> +			err = -ENOMEM;
> +			goto out;
> +		}
> 
>  		diff = tcp_skb_pcount(skb);
>  		tcp_set_skb_tso_segs(skb, cur_mss);
> @@ -3431,17 +3444,16 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
>  		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
>  				TCP_SKB_CB(skb)->seq, segs, err);
> 
> -	if (likely(!err)) {
> -		trace_tcp_retransmit_skb(sk, skb);
> -	} else if (err != -EBUSY) {
> +	if (err != -EBUSY)
>  		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);

Now this is counted when !err.


> -	}
> 
>  	/* To avoid taking spuriously low RTT samples based on a timestamp
>  	* for a transmit that never happened, always mark EVER_RETRANS
>  	*/
>  	TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
> 
> +out:
> +	trace_tcp_retransmit_skb(sk, skb, err);
>  	return err;
> }
> 
> --
> 2.25.1
>