netdev - Re: [PATCH 3/3] net: allocate tx queues in register

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Date:	Mon, 18 Oct 2010 17:12:30 -0700
From:	John Fastabend <john.r.fastabend@...el.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	Tom Herbert <therbert@...gle.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>
Subject: Re: [PATCH 3/3] net: allocate tx queues in register_netdevice

On 10/18/2010 2:41 PM, Ben Hutchings wrote:
> On Mon, 2010-10-18 at 11:02 -0700, Tom Herbert wrote:
>> This patch introduces netif_alloc_netdev_queues which is called from
>> register_device instead of alloc_netdev_mq.  This makes TX queue
>> allocation symmetric with RX allocation similarly allows drivers to
>> change dev->num_tx_queues after allocating netdev and before
>> registering it.
> 
> Changing num_tx_queues is probably *not* desirable, same as for
> num_rx_queues.
> 

Right, this will break ixgbe and other drivers that may call netif_set_real_num_{rx|tx}_queues() to increase the number of
queues. Returning an error code seems like a good idea though.

John.

> [...]
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -1553,18 +1553,24 @@ static void dev_queue_xmit_nit(struct sk_buff *skb, struct net_device *dev)
>>   * Routine to help set real_num_tx_queues. To avoid skbs mapped to queues
>>   * greater then real_num_tx_queues stale skbs on the qdisc must be flushed.
>>   */
>> -void netif_set_real_num_tx_queues(struct net_device *dev, unsigned int txq)
>> +int netif_set_real_num_tx_queues(struct net_device *dev, unsigned int txq)
>>  {
>> -	unsigned int real_num = dev->real_num_tx_queues;
>> +	if (txq < 1)
>> +		return -EINVAL;
>>  
>> -	if (unlikely(txq > dev->num_tx_queues))
>> -		;
>> -	else if (txq > real_num)
>> -		dev->real_num_tx_queues = txq;
>> -	else if (txq < real_num) {
>> -		dev->real_num_tx_queues = txq;
>> -		qdisc_reset_all_tx_gt(dev, txq);
>> -	}
>> +	if (dev->reg_state == NETREG_REGISTERED) {
>> +		ASSERT_RTNL();
>> +
>> +		if (txq > dev->num_tx_queues)
>> +			return -EINVAL;
>> +
>> +		if (txq < dev->real_num_tx_queues)
>> +			qdisc_reset_all_tx_gt(dev, txq);
>> +	} else
>> +		dev->num_tx_queues = txq;
> [...]
> 
> The kernel-doc comment should be updated to reflect the locking
> requirement and the possibility of failure when called after
> registration.
> 
> Ben.
> 

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