netdev - Re: [PATCH net-next] net: better IFF_XMIT_DST

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Date:	Thu, 2 Oct 2014 22:34:34 +0300 (EEST)
From:	Julian Anastasov <ja@....bg>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: better IFF_XMIT_DST_RELEASE support


	Hello,

On Thu, 2 Oct 2014, Eric Dumazet wrote:

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Testing xmit_more support with netperf and connected UDP sockets,
> I found strange dst refcount false sharing.
> 
> Current handling of IFF_XMIT_DST_RELEASE is not optimal.
> 
> dropping dst in validate_xmit_skb() is certainly too late in case
> packet was queued by cpu X but dequeued by cpu Y
> 
> The logical point to take care of drop/force is in __dev_queue_xmit()
> before even taking qdisc lock.

	Does it hurt skb_dst usage in net/sched/, for example,
dst->tclassid in net/sched/cls_route.c ?

> @@ -2895,6 +2887,14 @@ static int __dev_queue_xmit(struct sk_buff *skb, void *accel_priv)
>  
>  	skb_update_prio(skb);
>  
> +	/* If device doesn't need skb->dst, release it right now while
> +	 * its hot in this cpu cache
> +	 */
> +	if (dev->priv_flags & IFF_XMIT_DST_RELEASE)
> +		skb_dst_drop(skb);
> +	else
> +		skb_dst_force(skb);
> +
>  	txq = netdev_pick_tx(dev, skb, accel_priv);
>  	q = rcu_dereference_bh(txq->qdisc);

Regards

--
Julian Anastasov <ja@....bg>
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