netdev - Re: [PATCH net-next] pkt_sched: avoid requeues if possible

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Date:	Wed, 12 Dec 2012 00:24:17 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	erdnetdev@...il.com
Cc:	netdev@...r.kernel.org, jhs@...atatu.com,
	john.r.fastabend@...el.com
Subject: Re: [PATCH net-next] pkt_sched: avoid requeues if possible

From: Eric Dumazet <erdnetdev@...il.com>
Date: Tue, 11 Dec 2012 17:54:33 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> With BQL being deployed, we can more likely have following behavior :
> 
> We dequeue a packet from qdisc in dequeue_skb(), then we realize target
> tx queue is in XOFF state in sch_direct_xmit(), and we have to hold the
> skb into gso_skb for later.
> 
> This shows in stats (tc -s qdisc dev eth0) as requeues.
> 
> Problem of these requeues is that high priority packets can not be
> dequeued as long as this (possibly low prio and big TSO packet) is not
> removed from gso_skb.
> 
> At 1Gbps speed, a full size TSO packet is 500 us of extra latency.
> 
> In some cases, we know that all packets dequeued from a qdisc are
> for a particular and known txq :
> 
> - If device is non multi queue
> - For all MQ/MQPRIO slave qdiscs
> 
> This patch introduces a new qdisc flag, TCQ_F_ONETXQUEUE to mark
> this capability, so that dequeue_skb() is allowed to dequeue a packet
> only if the associated txq is not stopped.
> 
> This indeed reduce latencies for high prio packets (or improve fairness
> with sfq/fq_codel), and almost remove qdisc 'requeues'.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied.
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