Subject: Re: [PATCH net-next 2/2] xdp: Add devmap_idx map type for looking up devices by ifindex

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 22 Feb 2019 00:02:23 +0100

Jakub Kicinski <[email-masked]> writes:

> On Thu, 21 Feb 2019 12:56:54 +0100, Toke Høiland-Jørgensen wrote:
>> A common pattern when using xdp_redirect_map() is to create a device map
>> where the lookup key is simply ifindex. Because device maps are arrays,
>> this leaves holes in the map, and the map has to be sized to fit the
>> largest ifindex, regardless of how many devices actually are actually
>> needed in the map.
>> 
>> This patch adds a second type of device map where the key is interpreted as
>> an ifindex and looked up using a hashmap, instead of being used as an array
>> index. This leads to maps being densely packed, so they can be smaller.
>> 
>> The default maps used by xdp_redirect() are changed to use the new map
>> type, which means that xdp_redirect() is no longer limited to ifindex < 64,
>> but instead to 64 total simultaneous interfaces per network namespace. This
>> also provides an easy way to compare the performance of devmap and
>> devmap_idx:
>> 
>> xdp_redirect_map (devmap): 8394560 pkt/s
>> xdp_redirect (devmap_idx): 8179480 pkt/s
>> 
>> Difference: 215080 pkt/s or 3.1 nanoseconds per packet.
>
> Could you share what the ifindex mix was here, to arrive at these
> numbers? How does it compare to using an array but not keying with
> ifindex?

Just the standard set on my test machine; ifindex 1 through 9, except 8
in this case. So certainly no more than 1 ifindex in each hash bucket
for those numbers.

>> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>
>> +static int dev_map_idx_update_elem(struct bpf_map *map, void *key, void *value,
>> +				   u64 map_flags)
>> +{
>> +	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
>> +	struct bpf_dtab_netdev *dev, *old_dev;
>> +	u32 idx = *(u32 *)key;
>> +	u32 val = *(u32 *)value;
>> +	u32 bit;
>> +
>> +	if (unlikely(map_flags > BPF_EXIST))
>> +		return -EINVAL;
>> +	if (unlikely(map_flags == BPF_NOEXIST))
>> +		return -EEXIST;
>> +
>> +	old_dev = __dev_map_idx_lookup_elem(map, idx);
>> +	if (!val) {
>> +		if (!old_dev)
>> +			return 0;
>
> IMHO this is a fairly strange mix of array and hashmap semantics. I
> think you should stick to hashmap behaviour AFA flags and
> update/delete goes.

Yeah, the double book-keeping is a bit strange, but it allows the actual
forwarding and flush code to be reused between both types of maps. I
think this is worth the slight semantic confusion :)

-Toke