Linux Virtualization - Jan 2014 - [PATCH net-next v3 4/5] net-sysfs: add support for device-specific rx queue sysfs attributes

On Thu, 2014-01-16 at 01:38 -0800, Michael Dalton wrote:
[...]
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
[...]
> @@ -2401,6 +2416,23 @@ static inline int netif_copy_real_num_queues(struct
net_device *to_dev,
>  #endif
>  }
>  
> +#ifdef CONFIG_SYSFS
> +static inline unsigned int get_netdev_rx_queue_index(
> +		struct netdev_rx_queue *queue)
> +{
> +	struct net_device *dev = queue->dev;
> +	int i;
> +
> +	for (i = 0; i < dev->num_rx_queues; i++)
> +		if (queue == &dev->_rx[i])
> +			break;
Why write a loop when you can do:
	i = queue - dev->_rx;

Ben.
> +	BUG_ON(i >= dev->num_rx_queues);
> +
> +	return i;
> +}
> +#endif
-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

On Jan 16, 2014 at 10:57 AM, Ben Hutchings <bhutchings at solarflare.com>
wrote:
> Why write a loop when you can do:
>         i = queue - dev->_rx;
Good point, the loop approach was done in get_netdev_queue_index --
I agree your fix is faster and simpler. I'll fix in next patchset.
Thanks!

Best,

Mike

On Thu, 2014-01-16 at 11:07 -0800, Michael Dalton wrote:
> On Jan 16, 2014 at 10:57 AM, Ben Hutchings <bhutchings at
solarflare.com> wrote:
> > Why write a loop when you can do:
> >         i = queue - dev->_rx;
> Good point, the loop approach was done in get_netdev_queue_index --
> I agree your fix is faster and simpler. I'll fix in next patchset.
It's simpler but we don't know if it's faster (and I don't
believe that
matters for the current usage).

If one of these functions starts to be used in the data path, at that
point it could be worth optimising, e.g. by doing a test for queue 0 and
only then doing the pointer arithmetic with its implicit division.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.