netdev - Re: [PATCH bpf-next 0/3] xdp: Add devmap

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Date:   Mon, 08 Jul 2019 09:50:07 -0700
From:   "Jonathan Lemon" <jlemon@...gsvamp.com>
To:     "Toke Høiland-Jørgensen" <toke@...hat.com>
Cc:     "Daniel Borkmann" <daniel@...earbox.net>,
        "Alexei Starovoitov" <ast@...nel.org>, netdev@...r.kernel.org,
        "David Miller" <davem@...emloft.net>,
        "Jesper Dangaard Brouer" <brouer@...hat.com>,
        "Jakub Kicinski" <jakub.kicinski@...ronome.com>,
        "Björn Töpel" <bjorn.topel@...il.com>
Subject: Re: [PATCH bpf-next 0/3] xdp: Add devmap_hash map type



On 8 Jul 2019, at 8:40, Toke Høiland-Jørgensen wrote:

> "Jonathan Lemon" <jlemon@...gsvamp.com> writes:
>
>> On 5 Jul 2019, at 10:56, Toke Høiland-Jørgensen wrote:
>>
>>> This series adds a new map type, devmap_hash, that works like the
>>> existing
>>> devmap type, but using a hash-based indexing scheme. This is useful
>>> for the use
>>> case where a devmap is indexed by ifindex (for instance for use with
>>> the routing
>>> table lookup helper). For this use case, the regular devmap needs to
>>> be sized
>>> after the maximum ifindex number, not the number of devices in it. A
>>> hash-based
>>> indexing scheme makes it possible to size the map after the number of
>>> devices it
>>> should contain instead.
>>
>> This device hash map is sized at NETDEV_HASHENTRIES == 2^8 == 256. Is
>> this actually smaller than an array? What ifindex values are you
>> seeing?
>
> Well, not in all cases, certainly. But machines with lots of virtual
> interfaces (e.g., container hosts) can easily exceed that. Also, for a
> devmap we charge the full size of max_entries * struct bpf_dtab_netdev
> towards the locked memory cost on map creation. And since sizeof(struct
> bpf_dtab_netdev) is 64, the size of the hashmap only corresponds to 32
> entries...
>
> But more importantly, it's a UI issue: Say you want to create a simple
> program that uses the fib_lookup helper (something like the xdp_fwd
> example under samples/bpf/). You know that you only want to route
> between a couple of interfaces, so you naturally create a devmap that
> can hold, say, 8 entries (just to be sure). This works fine on your
> initial test, where the machine only has a couple of physical interfaces
> brought up at boot. But then you try to run the same program on your
> production server, where the interfaces you need to use just happen to
> have ifindexes higher than 8, and now it breaks for no discernible
> reason. Or even worse, if you remove and re-add an interface, you may no
> longer be able to insert it into your map because the ifindex changed...

Thanks for the explanation, that makes sense.
-- 
Jonathan