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From: Cong Wang <xiyou.wangcong@gmail.com>
To: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: David Miller <davem@davemloft.net>,
	Linux Kernel Network Developers <netdev@vger.kernel.org>,
	Jiri Pirko <jiri@resnulli.us>,
	Ariel Levkovich <lariel@mellanox.com>
Subject: Re: [PATCH net-next 1/1] net/sched: Introduce skb hash classifier
Date: Mon, 17 Aug 2020 12:47:49 -0700	[thread overview]
Message-ID: <CAM_iQpX71-jFUddZoSQrXWpd0KRpi0ueoK=h3ugBh5ufYvqLEQ@mail.gmail.com> (raw)
In-Reply-To: <c8722128-71b7-ad83-b142-8d53868dafc6@mojatatu.com>

On Mon, Aug 17, 2020 at 4:19 AM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
>
> On 2020-08-16 2:59 p.m., Cong Wang wrote:
> > On Thu, Aug 13, 2020 at 5:52 AM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
>
>
> [..]
> >> How do you know whether to use hash or mark or both
> >> for that specific key?
> >
> > Hmm, you can just unconditionally pass skb->hash and skb->mark,
> > no? Something like:
> >
> > if (filter_parameter_has_hash) {
> >      match skb->hash with cls->param_hash
> > }
> >
> > if (filter_parameter_has_mark) {
> >      match skb->mark with cls->param_mark
> > }
> >
>  >
> > fw_classify() uses skb->mark unconditionally anyway, without checking
> > whether it is set or not first.
> >
>
> There is no ambiguity of intent in the fw case, there is only one field.
> In the case of having multiple fields it is ambigious if you
> unconditionally look.
>
> Example: policy says to match skb mark of 5 and hash of 3.
> If packet arrives with skb->mark is 5 and skb->hash is 3
> very clearly matched the intent of the policy.
> If packet arrives withj skb->mark 7 and hash 3 it clearly
> did not match the intent. etc.

This example clearly shows no ambiguous, right? ;)


>
> > But if filters were put in a global hashtable, the above would be
> > much harder to implement.
> >
>
> Ok, yes. My assumption has been you will have some global shared
> structure where all filters will be installed on.

Sure, if not hashtable, we could simply put them in a list:

list_for_each_filter {
  if (filter_parameter_has_hash) {
    match skb->hash with cls->param_hash
  }
  if (filter_parameter_has_mark) {
    match skb->mark with cls->param_mark
  }
}


>
> I think i may have misunderstood all along what you were saying
> which is:
>
> a) add the rules so they are each _independent with different
>     priorities_ in a chain.

Yes, because this gives users freedom to pick a different prio
from its value (hash or mark).


>
> b)  when i do lookup for packet arrival, i will only see a filter
>   that matches "match mark 5 and hash 3" (meaning there is no
>   ambiguity on intent). If packet data doesnt match policy then
>   i will iterate to another filter on the chain list with lower
>   priority.

Right. Multiple values mean AND, not OR, so if you specify
mark 5 and hash 3, it will match skb->mark==5 && skb->hash==3.
If not matched, it will continue the iteration until the end.

>
> Am i correct in my understanding?
>
> If i am - then we still have a problem with lookup scale in presence
> of a large number of filters since essentially this approach
> is linear lookup (similar problem iptables has). I am afraid
> a hash table or something with similar principle goals is needed.

Yeah, this is why I asked you whether we have to put them in a
hashtable in previous emails, as hashtable organizes them with
a key, it is hard to combine multiple fields in one key and allow
to extend easily in the future. But other people smarter than me
may have better ideas here.

Thanks.

  reply	other threads:[~2020-08-17 19:48 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-07 22:28 Jamal Hadi Salim
2020-08-09 18:15 ` Cong Wang
2020-08-09 23:41   ` Jamal Hadi Salim
2020-08-11 23:25     ` Cong Wang
2020-08-12 21:07       ` Marcelo Ricardo Leitner
2020-08-13 12:52       ` Jamal Hadi Salim
2020-08-16 18:59         ` Cong Wang
2020-08-17 11:19           ` Jamal Hadi Salim
2020-08-17 19:47             ` Cong Wang [this message]
2020-08-19  9:48               ` Jamal Hadi Salim

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