Subject: Re: [PATCH 1/2] NET: Multiple queue network device support

From: Jarek Poplawski <[email-masked]>
Date: Mon, 12 Mar 2007 09:58:02 +0100

On 09-03-2007 14:40, Thomas Graf wrote:
> * Kok, Auke <[email-masked]> 2007-02-08 16:09
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 455d589..42b635c 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -1477,6 +1477,49 @@ gso:
>>  	skb->tc_verd = SET_TC_AT(skb->tc_verd,AT_EGRESS);
>>  #endif
>>  	if (q->enqueue) {
>> +#ifdef CONFIG_NET_MULTI_QUEUE_DEVICE
>> +		int queue_index;
>> +		/* If we're a multi-queue device, get a queue index to lock */
>> +		if (netif_is_multiqueue(dev))
>> +		{
>> +			/* Get the queue index and lock it. */
>> +			if (likely(q->ops->map_queue)) {
>> +				queue_index = q->ops->map_queue(skb, q);
>> +				spin_lock(&dev->egress_subqueue[queue_index].queue_lock);
>> +				rc = q->enqueue(skb, q);

I'm not sure Dave Miller thought about this place, when he proposed
to save the mapping, but I think this could be not enough. This place
is racy: ->map_queue() is called 2 times and with some filters (and
policies/actions) results could differ. And of course the subqueue
lock doesn't prevent any filter from a config change in the meantime.

After second reading of this patch I have doubts it's the proper way
to solve the problem: there are many subqueues but we need a top queue
(prio here) to mange them, anyway. So, why not to build this
functionality directly into the queue? There is no difference for a
device if skbs are going from the subqueue or a class, it is only
interested in the mapping result and a possibility to stop and start
a subqueue and to query its status. All this could be done by adding
the callbacks directly to any classful scheduler or, if not enough,
to write some specialized qdisc based on prio. The possibility to lock
only a subqueue instead of a queue could be analized independently -
current proposal doesn't solve this anyway.

Regards,
Jarek P.