Subject: Re: Stable regression with 'tcp: allow splice() to build full TSO packets'

From: Willy Tarreau <[email-masked]>
Date: Fri, 18 May 2012 00:29:27 +0200

David,

After several tests, we finally agreed on this one. I can't make the
transfers fail anymore. And they're damn fast thanks to Eric's recent
work!

Best regards,
Willy

---

>>From 39c3f73176118a274ec9dea9c22c83d97a7fbfe0 Mon Sep 17 00:00:00 2001
From: Willy Tarreau <[email-masked]>
Date: Thu, 17 May 2012 22:43:20 +0200
Subject: [PATCH] tcp: do_tcp_sendpages() must try to push data out on oom conditions

Since recent changes on TCP splicing (starting with commits 2f533844
and 35f9c09f), I started seeing massive stalls when forwarding traffic
between two sockets using splice() when pipe buffers were larger than
socket buffers.

Latest changes (net: netdev_alloc_skb() use build_skb()) made the
problem even more apparent.

The reason seems to be that if do_tcp_sendpages() fails on out of memory
condition without being able to send at least one byte, tcp_push() is not
called and the buffers cannot be flushed.

After applying the attached patch, I cannot reproduce the stalls at all
and the data rate it perfectly stable and steady under any condition
which previously caused the problem to be permanent.

The issue seems to have been there since before the kernel migrated to
git, which makes me think that the stalls I occasionally experienced
with tux during stress-tests years ago were probably related to the
same issue.

This issue was first encountered on 3.0.31 and 3.2.17, so please backport
to -stable.

Signed-off-by: Willy Tarreau <[email-masked]>
Cc: <[email-masked]>
Acked-by: Eric Dumazet <[email-masked]>
---
 net/ipv4/tcp.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 63ddaee..231fe53 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -917,8 +917,7 @@ new_segment:
 wait_for_sndbuf:
 		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 wait_for_memory:
-		if (copied)
-			tcp_push(sk, flags & ~MSG_MORE, mss_now, TCP_NAGLE_PUSH);
+		tcp_push(sk, flags & ~MSG_MORE, mss_now, TCP_NAGLE_PUSH);
 
 		if ((err = sk_stream_wait_memory(sk, &timeo)) != 0)
 			goto do_error;