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Date:   Fri, 22 Feb 2019 13:30:29 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH net-next 2/2] xdp: Add devmap_idx map type for looking
 up devices by ifindex

On Fri, 22 Feb 2019 10:47:10 +0100, Toke Høiland-Jørgensen wrote:
> Jakub Kicinski <jakub.kicinski@...ronome.com> writes:
> 
> > On Fri, 22 Feb 2019 00:02:23 +0100, Toke Høiland-Jørgensen wrote:  
> >> Jakub Kicinski <jakub.kicinski@...ronome.com> 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.  
> >
> > Oh, I clearly misread your numbers, it's still slower than array, you
> > just don't need the size limit.  
> 
> Yeah, this is not about speeding up devmap, it's about lifting the size
> restriction.
> 
> >> >> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>    
> >> >    
> >> >> +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 :)  
> >
> > I'm not sure I was clear, let me try again :) Your get_next_key only
> > reports existing indexes if I read the code right, so that's not an
> > array - in an array indexes always exist. What follows inserting 0
> > should not be equivalent to delete and BPF_NOEXIST should be handled
> > appropriately.  
> 
> Ah, I see what you mean. Yeah, sure, I guess I can restrict deletion to
> only working through explicit delete.
> 
> I could also add a fail on NOEXIST, but since each index is tied to a
> particular value, you can't actually change the contents of each index,
> only insert and remove. So why would you ever set that flag?

The reason user would have for setting the flag is not clear :)  But 
if you want to reject it because it's unsupported/makes no sense, you
should do EINVAL, not EEXIST ;)

> > Different maps behave differently, I think it's worth trying to limit
> > the divergence in how things behave to the basic array and a hashmap
> > models when possible.  
> 
> So I don't actually think of this as a hashmap in the general sense;
> after all, you can only store ifindexes in it, and key and value are
> tied to one another. So it's an ifindex'ed devmap (which is also why I
> named it devmap_idx and not devmap_hash); the fact that it's implemented
> as a hashmap is just incidental.
> 
> So I guess it's a choice between being consistent with the other devmap
> type, or with a general hashmap. I'm not actually sure that the latter
> is less surprising? :)

The distinction is that if entry is not in the map get_next won't
return its key.  As you say the construct is not really a hash map
(probably a set is the closest) but it most definitely is not an
array, so no hard EEXIST on NOEXIST flag :)

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