Re: [RFC PATCH net-next 2/4] net: dsa: remove the "dsa_to_port in a loop" antipattern from the core

[DENG Qingfang <dqfext@gmail.com>]

On Tue, Aug 10, 2021 at 07:35:33PM +0300, Vladimir Oltean wrote:
> If I were to guess where Qingfang was hinting at, is that the receive
> path now needs to iterate over a list, whereas before it simply indexed
> an array:
> 
> static inline struct net_device *dsa_master_find_slave(struct net_device *dev,
> 						       int device, int port)
> {
> 	struct dsa_port *cpu_dp = dev->dsa_ptr;
> 	struct dsa_switch_tree *dst = cpu_dp->dst;
> 	struct dsa_port *dp;
> 
> 	list_for_each_entry(dp, &dst->ports, list)
> 		if (dp->ds->index == device && dp->index == port &&
> 		    dp->type == DSA_PORT_TYPE_USER)
> 			return dp->slave;
> 
> 	return NULL;
> }
> 
> I will try in the following days to make a prototype implementation of
> converting back the linked list into an array and see if there is any
> justifiable performance improvement.
> 
> [ even if this would make the "multiple CPU ports in LAG" implementation
>   harder ]

Yes, you got my point.

There is RTL8390M series SoC, which has 52+ ports but a weak CPU (MIPS
34kc 700MHz). In that case the linear lookup time and the potential cache
miss could make a difference.