netdev - Re: pktgen and spin_lock

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Message-ID: <4ADE7C0D.5070208@gmail.com>
Date:	Wed, 21 Oct 2009 05:12:13 +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

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)

Thanks, I'll take care of official patches submission

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