[Cake] fq_codel leveraging the skb->hash now in net-next

Dave Taht dave.taht at gmail.com
Fri Jan 20 16:47:00 EST 2017

On Fri, Jan 20, 2017 at 1:36 PM, Eric Dumazet <edumazet at google.com> wrote:
> The 0 case is checked.
> If skb->hash == 0 or a non L4 hash was stored in skb->hash, we call
> the same flow dissector code than before ;)
> And each host has normally :
> 1) Boot time generated RSS keys on NIC providing skb->hash
> 2) A boot time random number
> static u32 hashrnd __read_mostly;
> static __always_inline void __flow_hash_secret_init(void)
> {
>         net_get_random_once(&hashrnd, sizeof(hashrnd));
> }
> u32 flow_hash_from_keys(struct flow_keys *keys)
> {
>         __flow_hash_secret_init();
>         return __flow_hash_from_keys(keys, hashrnd);
> }
> EXPORT_SYMBOL(flow_hash_from_keys);
> static inline u32 ___skb_get_hash(const struct sk_buff *skb,
>                                   struct flow_keys *keys, u32 keyval)
> {
>         skb_flow_dissect_flow_keys(skb, keys,
>                                    FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL);
>         return __flow_hash_from_keys(keys, keyval);
> }
> So an attacker has no way to guess in which slot of the hash table a
> particular flow will end up.

Groovy. safe to backport to 4.4? (lede/openwrt)?

> For the record, I will add (optional) pacing to fq_codel.

BBR is one less step away from world domination then!

My dream has always been to have all timestamping being on ingress,
thus eliminating that within fq_codel also, thus measuring cpu
overload on queuing within linux itself. This would also tend toward
favoring local tcp flows (I think), slightly, in the fq_codel case.

But perhaps I'm still dreaming too hard.

> On Fri, Jan 20, 2017 at 1:29 PM, Dave Taht <dave.taht at gmail.com> wrote:
>> It's not clear to me if all the encapsulation types (6rd for
>> example?), or drivers? are generating an skb->hash (or as of what
>> release of linux they did), and there's no error checking for 0, and
>> whether or not they are being permuted in skb->hash,  (otherwise all
>> linux implementations in the world will end up hashing the same way on
>> the same combination of ips and ports),
>> but I tend to trust eric to get it right, and hashing here was always
>> the 2nd or 3rd biggest hotspot in fq_codel.
>> https://www.mail-archive.com/netdev@vger.kernel.org/msg148598.html
>> --
>> Dave Täht
>> Let's go make home routers and wifi faster! With better software!
>> http://blog.cerowrt.org

Dave Täht
Let's go make home routers and wifi faster! With better software!

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