netdev - Re: [PATCH 2/2] virtio_net: remove send completion interrupts and avoid TX queue overrun through packet drop

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Message-ID: <874o72gwm5.fsf@rustcorp.com.au>
Date:	Thu, 17 Mar 2011 15:40:58 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Shirley Ma <mashirle@...ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] virtio_net: remove send completion interrupts and avoid TX queue overrun through packet drop

On Wed, 16 Mar 2011 17:12:55 -0700, Shirley Ma <mashirle@...ibm.com> wrote:
> Signed-off-by: Shirley Ma <xma@...ibm.com>

This is fascinating... and deeply weird.

OK, what's the difference between calling xmit_skb and ignoring failure,
and this patch which figures out it's going to fail before calling
xmit_skb?

ie. what if you *just* delete this:

> @@ -605,20 +620,6 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	skb_orphan(skb);
>  	nf_reset(skb);
>  
> -	/* Apparently nice girls don't return TX_BUSY; stop the queue
> -	 * before it gets out of hand.  Naturally, this wastes entries. */
> -	if (capacity < 2+MAX_SKB_FRAGS) {
> -		netif_stop_queue(dev);
> -		if (unlikely(!virtqueue_enable_cb(vi->svq))) {
> -			/* More just got used, free them then recheck. */
> -			capacity += free_old_xmit_skbs(vi);
> -			if (capacity >= 2+MAX_SKB_FRAGS) {
> -				netif_start_queue(dev);
> -				virtqueue_disable_cb(vi->svq);
> -			}
> -		}
> -	}
> -
>  	return NETDEV_TX_OK;
>  }

Thanks!
Rusty.
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