netdev - Re: pktgen and spin_lock

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Date:	Wed, 21 Oct 2009 05:59:59 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Greear <greearb@...delatech.com>
CC:	NetDev <netdev@...r.kernel.org>, robert@...julf.net,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: pktgen and spin_lock_bh in xmit path

Eric Dumazet a écrit :
> Ben Greear a écrit :
>> Ben Greear wrote:
>>> On 10/20/2009 02:57 PM, Eric Dumazet wrote:
>>>> Ben Greear a écrit :
>>>>> That's definitely a nasty little issue.  Using skb_set_queue_mapping
>>>>> in pktgen makes it run for me, but may just be getting lucky with the
>>>>> mac-vlan interfaces which will do the dev_queue_xmit (but, I don't
>>>>> so much
>>>>> care exactly what queue is used as long as things don't crash and the
>>>>> link doesn't reset).
>>>>>
>>>>> Don't worry about a quick patch on my account.  I seem to have it
>>>>> working
>>>>> to at least some degree (no funny crashes, no link watchdog timeouts).
>>>>>
>>>> Could you try following patch ?
>>>>
>>>> This makes queue_mapping invariant if set in range [0 ...
>>>> real_num_tx_queues-1]
>>> Yes, that runs w/out causing link resets and without crashes (just
>>> tested it for
>>> a few minutes).
>>>
>>> Interestingly, the pkts sent by pktgen on the mac-vlans end up in
>>> tx-queues that match processor ID, even though I'm on .31 where mac-vlans
>>> have only one tx-queue and pktgen is setting the queue to 0 in the skb
>>> (per your previous patch).
>> Ok, this is because ixgbe implements the ndo_select_queue, which is
>> called from
>> dev_pick_tx.
>>
>> So, as far as I can tell, as long as you are using ixgbe with 82559, it
>> doesn't matter what you set
>> for the queue-map in the skb..it's always over-written.
>>
>> I don't know if this is a bug or a feature, but it explains the behavior
>> with tx and rx queues
>> that I saw when using pktgen on mac-vlans...
>>
> 
> We have many bugs in this area :)
> 
> So we probably need to reset skb_set_queue_mapping(skb, queue_map);
> each time skb is transmitted by pktgen.
> 
> Or else, pktgen will break if using bonding driver -> ixgbe, since bonding
> uses only one txqueue (it is not yet multiqueue aware)
> 

After some thoughts, I believe user is in error :)

pktgen should not use "clone XXX" pkts if macvlan is used (or any other driver
that ultimatly calls dev_queue_xmit() and queue packet), since skb queue anchor
is shared and would be overwritten.

1) Only way to use "clone XXXX" pkts is when using real device.

2) Also, using macvlan in pktgen is sub-optimal, since you can already put any
MAC addresses in pktgen pkts, you dont need to go through macvlan layer.

3) If ixgbe overwrites skb->queue_mapping to current cpu, you should setup pktgen
 queue_map_min and queue_map_max to match you cpu number, or use QUEUE_MAP_CPU pktgen flag
Or else, pktgen wont get  the appropriate txq (and lock) before calling driver start_xmit()

Unfortunatly, the (queue_map_min==queue_map_max) case needs a patch that might be not present in 2.6.31
(commit 896a7cf8d846a9e86fb823be16f4f14ffeb7f074 : pktgen: Fix multiqueue handling)

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