Subject: Re: [PATCH net-next] net: allow large number of tx queues

From: Jason Wang <[email-masked]>
Date: Fri, 21 Jun 2013 14:41:48 +0800

On 06/20/2013 04:35 PM, Michael S. Tsirkin wrote:
> On Thu, Jun 20, 2013 at 01:15:51AM -0700, Eric Dumazet wrote:
>> From: Eric Dumazet <[email-masked]>
>>
>> On Wed, 2013-06-19 at 21:07 +0300, Michael S. Tsirkin wrote:
>>
>>> As we have explicit code to recover, maybe set __GFP_NOWARN?
>>> If we are out of memory, vzalloc will warn too, we don't
>>> need two warnings.
>> Yes, that's absolutely the right thing to do.
>>
>> Its yet not clear this patch is really needed, but here it is.
>>
>> Thanks
>>
>> [PATCH net-next] net: allow large number of tx queues
>>
>> netif_alloc_netdev_queues() uses kcalloc() to allocate memory
>> for the "struct netdev_queue *_tx" array.
>>
>> For large number of tx queues, kcalloc() might fail, so this
>> patch does a fallback to vzalloc().
>>
>> As vmalloc() adds overhead on a critical network path, add __GFP_REPEAT
>> to kzalloc() flags to do this fallback only when really needed.
>>
>> Signed-off-by: Eric Dumazet <[email-masked]>
> FWIW
>
> Acked-by: Michael S. Tsirkin <[email-masked]>
>
> This makes it possible to go up to 1K queues which should
> be enough, so we can revert the first chunk in
> edfb6a148ce62e5e19354a1dcd9a34e00815c2a1.

1K queues (about 80 pages) looks a little bit aggressive which means we
may always fall back to vmalloc()?
>
>
>> ---
>>  net/core/dev.c |   26 +++++++++++++++++++-------
>>  1 file changed, 19 insertions(+), 7 deletions(-)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index fa007db..722f633 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -130,6 +130,7 @@
>>  #include <linux/cpu_rmap.h>
>>  #include <linux/static_key.h>
>>  #include <linux/hashtable.h>
>> +#include <linux/vmalloc.h>
>>  
>>  #include "net-sysfs.h"
>>  
>> @@ -5253,17 +5254,28 @@ static void netdev_init_one_queue(struct net_device *dev,
>>  #endif
>>  }
>>  
>> +static void netif_free_tx_queues(struct net_device *dev)
>> +{
>> +	if (is_vmalloc_addr(dev->_tx))
>> +		vfree(dev->_tx);
>> +	else
>> +		kfree(dev->_tx);
>> +}
>> +
>>  static int netif_alloc_netdev_queues(struct net_device *dev)
>>  {
>>  	unsigned int count = dev->num_tx_queues;
>>  	struct netdev_queue *tx;
>> +	size_t sz = count * sizeof(*tx);
>>  
>> -	BUG_ON(count < 1);
>> -
>> -	tx = kcalloc(count, sizeof(struct netdev_queue), GFP_KERNEL);
>> -	if (!tx)
>> -		return -ENOMEM;
>> +	BUG_ON(count < 1 || count > 0xffff);
>>  
>> +	tx = kzalloc(sz, GFP_KERNEL | __GFP_NOWARN | __GFP_REPEAT);
>> +	if (!tx) {
>> +		tx = vzalloc(sz);
>> +		if (!tx)
>> +			return -ENOMEM;
>> +	}
>>  	dev->_tx = tx;
>>  
>>  	netdev_for_each_tx_queue(dev, netdev_init_one_queue, NULL);
>> @@ -5811,7 +5823,7 @@ free_all:
>>  
>>  free_pcpu:
>>  	free_percpu(dev->pcpu_refcnt);
>> -	kfree(dev->_tx);
>> +	netif_free_tx_queues(dev);
>>  #ifdef CONFIG_RPS
>>  	kfree(dev->_rx);
>>  #endif
>> @@ -5836,7 +5848,7 @@ void free_netdev(struct net_device *dev)
>>  
>>  	release_net(dev_net(dev));
>>  
>> -	kfree(dev->_tx);
>> +	netif_free_tx_queues(dev);
>>  #ifdef CONFIG_RPS
>>  	kfree(dev->_rx);
>>  #endif
>>