Re: xennet: skb rides the rocket: 20 slots

[Sander Eikelenboom <linux@eikelenboom.it>]

Tuesday, January 8, 2013, 3:12:33 AM, you wrote:



> On 2013-1-7 18:55, Ian Campbell wrote:
>> On Fri, 2013-01-04 at 16:28 +0000, Sander Eikelenboom wrote:
>>> Hi Ian,
>> I'm not actually the netfront maintainer. Adding Konrad. Annie too since
>> IIRC she was at one point looking at various buffer coalescing schemes.
>>
>>> Today i fired up an old VM with a bittorrent client, trying to
>>> download some torrents.
>> How old? Which kernel?
>>
>>> I seem to be hitting the unlikely case of "xennet: skb rides the
>>> rocket: xx slots" and this results in some dropped packets in domU, I
>>> don't see any warnings in dom0.

> If debug is enabled, following netback code should print out messages in 
> netbk_count_requests.

>                  if (unlikely(frags >= MAX_SKB_FRAGS)) {
>                          netdev_dbg(vif->dev, "Too many frags\n");
>                          return -frags;
>                  }

I have added some rate limited warns in this function. However none seems to be triggered while the pv-guest reports the "skb rides the rocket" ..

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index f2d6b78..2f02da9 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -902,11 +902,13 @@ static int netbk_count_requests(struct xenvif *vif,
        do {
                if (frags >= work_to_do) {
                        netdev_dbg(vif->dev, "Need more frags\n");
+                       net_warn_ratelimited("%s: Need more frags:%d, work_to_do:%d \n",vif->dev->name, frags, work_to_do);
                        return -frags;
                }

                if (unlikely(frags >= MAX_SKB_FRAGS)) {
                        netdev_dbg(vif->dev, "Too many frags\n");
+                        net_warn_ratelimited("%s: Too many frags:%d, MAX_SKB_FRAGS:%d \n",vif->dev->name, frags, MAX_SKB_FRAGS);
                        return -frags;
                }

@@ -914,6 +916,7 @@ static int netbk_count_requests(struct xenvif *vif,
                       sizeof(*txp));
                if (txp->size > first->size) {
                        netdev_dbg(vif->dev, "Frags galore\n");
+                        net_warn_ratelimited("%s: Frags galore:%d, txp->size:%d  first->size:%d\n",vif->dev->name, frags, txp->size , first->size);
                        return -frags;
                }

@@ -923,6 +926,7 @@ static int netbk_count_requests(struct xenvif *vif,
                if (unlikely((txp->offset + txp->size) > PAGE_SIZE)) {
                        netdev_dbg(vif->dev, "txp->offset: %x, size: %u\n",
                                 txp->offset, txp->size);
+                        net_warn_ratelimited("%s: Hmm:%d, (txp->offset + txp->size):%d   PAGE_SIZE:%d\n",vif->dev->name, frags,  (txp->offset + txp->size)  ,PAGE_SIZE);
                        return -frags;
                }
        } while ((txp++)->flags & XEN_NETTXF_more_data);




>>>
>>> I have added some extra info, but i don't have enough knowledge if
>>> this could/should be prevented from happening ?
>> MAX_SKB_FRAGS has never, AFAIK, been as big as 19 or 20 so I'm not sure
>> this can have ever worked.
>>
>> These SKBs seem to be pretty big (not quite 64KB), seemingly most of the
>> data is contained in a smallish number of frags, which suggests compound
>> pages and therefore a reasonably modern kernel?
>>
>> This probably relates somewhat to the issues described in the
>> "netchannel vs MAX_SKB_FRAGS" thread last year, or at least the solution
>> to that would necessarily involve fixing this issue.

> If netback complains about "Too many frags", then it should be 
> MAX_SKB_FRAGS limitation in netback results in dropping packets in 
> netfront. It is possible that other netfronts(windows?) also hit this.

> Thanks
> Annie
>>
>> In the meantime you could try disabling sg and/or various offloads for
>> that domain's vif.
>>
>>> [16798.629141] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:106 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:4 page_size:4096 prot:0800
>>> [16800.575182] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:2154 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:4 page_size:4096 prot:0800
>>> [16801.589166] xennet: skb rides the rocket: 20 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:19 offset:2154 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:5 page_size:4096 prot:0800
>>> [16803.279039] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:106 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:3 page_size:4096 prot:0800
>>> [16809.973268] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:2154 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:4 page_size:4096 prot:0800
>>> [16811.420048] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:2154 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:6 page_size:4096 prot:0800
>>> [16814.872686] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:106 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:5 page_size:4096 prot:0800
>>> [16815.359099] xennet: skb rides the rocket: 21 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:20 offset:2154 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:5 page_size:4096 prot:0800
>>> [16825.851906] xennet: skb rides the rocket: 20 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:19 offset:106 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:4 page_size:4096 prot:0800
>>> [16828.295083] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:2154 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:5 page_size:4096 prot:0800
>>> [16837.386684] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:106 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:4 page_size:4096 prot:0800
>>> [16838.609683] xennet: skb rides the rocket: 20 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:19 offset:106 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:4 page_size:4096 prot:0800
>>> [16841.783729] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:106 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:6 page_size:4096 prot:0800
>>> [16843.841678] xennet: skb rides the rocket: 20 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:19 offset:2154 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:4 page_size:4096 prot:0800
>>> [16849.847614] xennet: skb rides the rocket: 19 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:18 offset:3690 skb_headlen:54 skb->len:64294, skb->data_len:64240 skb->truesize:65008 nr_frags:4 page_size:4096 prot:0800
>>> [16853.787128] xennet: skb rides the rocket: 20 slots MAX_SKB_FRAGS: 17 div_roundup:1 xennet_count_skb_frag_slots:19 offset:2154 skb_headlen:1622 skb->len:64294, skb->data_len:62672 skb->truesize:64976 nr_frags:4 page_size:4096 prot:0800
>>>
>>> # ifconfig
>>> eth0      Link encap:Ethernet  HWaddr 00:16:3e:c4:20:46
>>>            inet addr:192.168.1.12  Bcast:192.168.1.255  Mask:255.255.255.0
>>>            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>>>            RX packets:9090214 errors:0 dropped:0 overruns:0 frame:0
>>>            TX packets:5902090 errors:0 dropped:304 overruns:0 carrier:0
>>>            collisions:0 txqueuelen:1000
>>>            RX bytes:19757934770 (18.4 GiB)  TX bytes:16855238200 (15.6 GiB)
>>>            Interrupt:25
>>>
>>>
>>>
>>>
>>> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
>>> index c26e28b..50ac403 100644
>>> --- a/drivers/net/xen-netfront.c
>>> +++ b/drivers/net/xen-netfront.c
>>> @@ -552,7 +552,11 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>>>                  xennet_count_skb_frag_slots(skb);
>>>          if (unlikely(slots>  MAX_SKB_FRAGS + 1)) {
>>>                  net_alert_ratelimited(
>>> -                       "xennet: skb rides the rocket: %d slots\n", slots);
>>> +                       "xennet: skb rides the rocket: %d slots MAX_SKB_FRAGS: %d div_roundup:%d xennet_count_skb_frag_slots:%d offset:%d skb_headlen:%d skb->len:%d, skb->data_len:%d skb->truesize:%d nr_frags:%d page_size:%d prot:%04x \n
>>> +                             slots, MAX_SKB_FRAGS,DIV_ROUND_UP(offset + len, PAGE_SIZE),
>>> +                             xennet_count_skb_frag_slots(skb), offset, len, skb->len,
>>> +                             skb->data_len, skb->truesize, skb_shinfo(skb)->nr_frags,
>>> +                             PAGE_SIZE, ntohs(skb->protocol));
>>>                  goto drop;
>>>          }
>>>
>>