netdev - Re: [PATCH net-next-2.6] rps: allocate rx queues in register

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Date:	Fri, 24 Sep 2010 09:54:14 -0700
From:	John Fastabend <john.r.fastabend@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-net-drivers@...arflare.com" <linux-net-drivers@...arflare.com>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH net-next-2.6] rps: allocate rx queues in register_netdevice
 only

On 9/24/2010 2:21 AM, Eric Dumazet wrote:
> Le vendredi 24 septembre 2010 à 01:15 -0700, John Fastabend a écrit :
>> On 9/23/2010 8:26 PM, Eric Dumazet wrote:
>>>
>>>> Also, I dont understand why we need to restrict
>>>> netif_set_real_num_rx_queues() to lower the count.
>>>> This wastes memory.
>>>>
>>>> Why dont we allocate dev->_rx once we know the real count, not in
>>>> alloc_netdev_mq() but in register_netdevice() ?
>>>>
>>
>> Eric,
>>
>> At least in the TX case we may not "know" until later how many
>> tx_queues we want to use. For example it could change based on
>> enabling/disabling features or available interrupts. So we use
>> num_tx_queues as the max we ever expect to use and then
>> netif_set_real_num_tx_queues() sets the number we want to use.
>>
>> I presume for rx queues there are similar cases where features and
>> available interrupts may determine how many rx queues are needed.
>>
>> Moving the allocation later could help drivers make better max number
>> of queue decisions. But, I think we still need the
>> netif_set_num_rx_queues() and netif_set_num_tx_queues(). Although this
>> does end up wasting memory as you pointed out.
>>
> 
> Note I am not against having netif_set_num_rx_queues() and
> netif_set_num_tx_queues(). My patch was a cleanup, not an alternative.
> 
> 
> If I take a look at sysfs stuff, on a machine with a bnx2 adapter,
> single queue, I get :
> 
> /sys/class/net/eth0/queues/rx-0/rps_cpus
> /sys/class/net/eth0/queues/rx-0/rps_flow_cnt
> /sys/class/net/eth0/queues/rx-1/rps_cpus
> /sys/class/net/eth0/queues/rx-1/rps_flow_cnt
> /sys/class/net/eth0/queues/rx-2/rps_cpus
> /sys/class/net/eth0/queues/rx-2/rps_flow_cnt
> /sys/class/net/eth0/queues/rx-3/rps_cpus
> /sys/class/net/eth0/queues/rx-3/rps_flow_cnt
> /sys/class/net/eth0/queues/rx-4/rps_cpus
> /sys/class/net/eth0/queues/rx-4/rps_flow_cnt
> /sys/class/net/eth0/queues/rx-5/rps_cpus
> /sys/class/net/eth0/queues/rx-5/rps_flow_cnt
> /sys/class/net/eth0/queues/rx-6/rps_cpus
> /sys/class/net/eth0/queues/rx-6/rps_flow_cnt
> /sys/class/net/eth0/queues/rx-7/rps_cpus
> /sys/class/net/eth0/queues/rx-7/rps_flow_cnt
> 
> Thats a lot of extra memory and administrator confusion.
> 
> We all agree :)
> 
> 

Thanks for the clarification Eric.
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