netdev - Re: Throughput regression with `tcp: refine TSO autosizing`

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Message-ID: <1422926330.21689.138.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 02 Feb 2015 17:18:50 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Kazior <michal.kazior@...to.com>
Cc:	linux-wireless <linux-wireless@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	eyalpe@....mellanox.co.il
Subject: Re: Throughput regression with `tcp: refine TSO autosizing`

On Mon, 2015-02-02 at 10:52 -0800, Eric Dumazet wrote:

> It seems to break ACK clocking badly (linux stack has a somewhat buggy
> tcp_tso_should_defer(), which relies on ACK being received smoothly, as
> no timer is setup to split the TSO packet.)

Following patch might help the TSO split defer logic.

It would avoid setting the TSO defer 'pseudo timer' twice, if/when TCP
Small Queue logic prevented the xmit at the expiration of first 'timer'.

This patch clears the tso_deferred variable only if we could really
send something.

Please try it, thanks !


diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 65caf8b95e17..e735f38557db 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1821,7 +1821,6 @@ static bool tcp_tso_should_defer(struct sock *sk,
struct sk_buff *skb,
 	return true;
 
 send_now:
-	tp->tso_deferred = 0;
 	return false;
 }
 
@@ -2070,6 +2069,7 @@ static bool tcp_write_xmit(struct sock *sk,
unsigned int mss_now, int nonagle,
 		if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
 			break;
 
+		tp->tso_deferred = 0;
 repair:
 		/* Advance the send_head.  This one is sent out.
 		 * This call will increment packets_out.


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