netdev - Re: [PATCH net] xen-netback: Fix slot estimation

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Message-ID: <538E2EFF.7000103@citrix.com>
Date:	Tue, 3 Jun 2014 21:24:31 +0100
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	David Laight <David.Laight@...LAB.COM>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"ian.campbell@...rix.com" <ian.campbell@...rix.com>,
	"wei.liu2@...rix.com" <wei.liu2@...rix.com>,
	"paul.durrant@...rix.com" <paul.durrant@...rix.com>,
	"linux@...elenboom.it" <linux@...elenboom.it>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"david.vrabel@...rix.com" <david.vrabel@...rix.com>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH net] xen-netback: Fix slot estimation

On 03/06/14 14:52, David Laight wrote:
> From: netdev-owner@...r.kernel.org
>> @@ -615,9 +608,27 @@ static void xenvif_rx_action(struct xenvif *vif)
>>
>>   		/* If the skb may not fit then bail out now */
>>   		if (!xenvif_rx_ring_slots_available(vif, max_slots_needed)) {
>> +			/* If the skb needs more than MAX_SKB_FRAGS slots, it
>> +			 * can happen that the frontend never gives us enough.
>> +			 * To avoid spining on that packet, first we put it back
>> +			 * to the top of the queue, but if the next try fail,
>> +			 * we drop it.
>> +			 */
>> +			if (max_slots_needed > MAX_SKB_FRAGS &&
>> +			    vif->rx_last_skb_slots == MAX_SKB_FRAGS) {
>> +				kfree_skb(skb);
>> +				vif->rx_last_skb_slots = 0;
>> +				continue;
>> +			}
>
> A silent discard here doesn't seem right at all.
> While it stops the kernel crashing, or the entire interface locking
> up; it is likely to leave one connection 'stuck' - a TCP retransmission
> is likely to include the same fragments.
>  From a user point of view this as almost as bad.

Yes, we are aware of this problem for a while. However I have an idea to 
solve that in a way that we don't lose performance, and these packets 
can pass through as well. See my patch called "Fix handling of skbs 
requiring too many slots"

Zoli

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