netdev - Re: [PATCH net-next 1/1] net/sched: Introduce skb hash classifier

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Message-ID: <64844778-a3d5-7552-df45-bf663d6498b6@mojatatu.com>
Date:   Thu, 13 Aug 2020 08:52:10 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...nulli.us>,
        Ariel Levkovich <lariel@...lanox.com>
Subject: Re: [PATCH net-next 1/1] net/sched: Introduce skb hash classifier

On 2020-08-11 7:25 p.m., Cong Wang wrote:
> On Sun, Aug 9, 2020 at 4:41 PM Jamal Hadi Salim <jhs@...atatu.com> wrote:

[..]
> 
> Not sure if I get you correctly, but with a combined implementation
> you can do above too, right? Something like:
> 
> (AND case)
> $TC filter add dev $DEV1 parent ffff: protocol ip prio 3 handle 1
> skb hash Y mark X flowid 1:12 action ok
> 
> (OR case)
> $TC filter add dev $DEV1 parent ffff: protocol ip prio 3 handle 1
> skb hash Y flowid 1:12 action ok
> $TC filter add dev $DEV1 parent ffff: protocol ip prio 4 handle 2
> skb mark X flowid 1:12 action ok
> 

It will work but what i was tring to say is it is tricky to implement.
More below

> Side note: you don't have to use handle as the value of hash/mark,
> which gives people freedom to choose different handles.
>

Same comment here as above. More below.

> 
>>
>> Then the question is how to implement? is it one hash table for
>> both or two(one for mark and one for hash), etc.
>>
> 
> Good question. I am not sure, maybe no hash table at all?
> Unless there are a lot of filters, we do not have to organize
> them in a hash table, do we?
>

The _main_ requirement is to scale to a large number of filters
(a million is a good handwave number). Scale means
1) fast datapath lookup time + 2) fast insertion/deletion/get/dump
from control/user space.
fwmark is good at all these goals today for #2. It is good for #1 for
maybe 1K rules (limitation is the 256 buckets, constrained by rcu
trickery). Then you start having collisions in a bucket and your
lookup requires long linked list walks.

Generally something like a hash table with sufficient number of buckets
will work out ok.
There maybe other approaches (IDR in the kernel looks interesting,
but i didnt look closely).

So to the implementation issue:
Major issue is removing ambiguity while at the same time trying
to get good performance.

Lets say we decided to classify skbmark and skbhash at this point.
For a hash table, one simple approach is to set
lookupkey = hash<<32|mark

the key is used as input to the hash algo to find the bucket.

There are two outstanding challenges in my mind:

1)  To use the policy like you describe above
as an example:

$TC filter add dev $DEV1 parent ffff: protocol ip prio 3 handle 1
skb hash Y flowid 1:12 action ok

and say you receive a packet with both skb->hash and skb->mark set
Then there is ambiguity

How do you know whether to use hash or mark or both
for that specific key?
You can probably do some trick but I cant think of a cheap way to 
achieve this goal. Of course this issue doesnt exist if you have
separate classifiers.

2) If you decide tomorrow to add tcindex/prio etc, you will have to
rework this as well.

#2 is not as a big deal as #1.

cheers,
jamal