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

[Vladimir Oltean <olteanv@gmail.com>]

On Tue, Aug 10, 2021 at 02:35:32PM +0300, Vladimir Oltean wrote:
> On Tue, Aug 10, 2021 at 02:41:07AM -0700, Florian Fainelli wrote:
> > On 8/9/2021 8:33 PM, DENG Qingfang wrote:
> > > On Mon, Aug 09, 2021 at 10:03:18PM +0300, Vladimir Oltean wrote:
> > > > Ever since Vivien's conversion of the ds->ports array into a dst->ports
> > > > list, and the introduction of dsa_to_port, iterations through the ports
> > > > of a switch became quadratic whenever dsa_to_port was needed.
> > > 
> > > So, what is the benefit of a linked list here? Do we allow users to
> > > insert/delete a dsa_port at runtime? If not, how about using a
> > > dynamically allocated array instead?
> > 
> > The goal was to flatten the space while doing cross switch operations, which
> > would have otherwise required iterating over dsa_switch instances within a
> > dsa_switch_tree, and then over dsa_port within each dsa_switch.
> 
> To expand on that: technically dsa_port_touch() _does_ happen at
> runtime, since multiple switches in a cross-chip tree probe
> asynchronously. To use a dynamically allocated array would mean to
> preallocate the sum of all DSA switch ports' worth of memory, and to
> preallocate an index for each DSA switch within that single array.
> Overall a list is simpler.

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 ]