Re: [PATCH net-next v2 6/7] net/sched: act_ct: offload UDP NEW connections

[Vlad Buslov <vladbu@nvidia.com>]

On Tue 17 Jan 2023 at 14:56, Marcelo Ricardo Leitner <marcelo.leitner@gmail.com> wrote:
> On Tue, Jan 17, 2023 at 07:36:42PM +0200, Vlad Buslov wrote:
>> 
>> On Tue 17 Jan 2023 at 12:41, Marcelo Ricardo Leitner <marcelo.leitner@gmail.com> wrote:
>> > On Fri, Jan 13, 2023 at 05:55:47PM +0100, Vlad Buslov wrote:
>> >> When processing connections allow offloading of UDP connections that don't
>> >> have IPS_ASSURED_BIT set as unidirectional. When performing table lookup
>> >
>> > Hmm. Considering that this is now offloading one direction only
>> > already, what about skipping this grace period:
>> >
>> > In nf_conntrack_udp_packet(), it does:
>> >
>> >         /* If we've seen traffic both ways, this is some kind of UDP
>> >          * stream. Set Assured.
>> >          */
>> >         if (test_bit(IPS_SEEN_REPLY_BIT, &ct->status)) {
>> > 		...
>> >                 /* Still active after two seconds? Extend timeout. */
>> >                 if (time_after(jiffies, ct->proto.udp.stream_ts)) {
>> >                         extra = timeouts[UDP_CT_REPLIED];
>> >                         stream = true;
>> >                 }
>> > 		...
>> >                 /* Also, more likely to be important, and not a probe */
>> >                 if (stream && !test_and_set_bit(IPS_ASSURED_BIT, &ct->status))
>> >                         nf_conntrack_event_cache(IPCT_ASSURED, ct);
>> >
>> > Maybe the patch should be relying on IPS_SEEN_REPLY_BIT instead of
>> > ASSURED for UDP? Just a thought here, but I'm not seeing why not.
>> 
>> The issue with this is that if we offload both directions early, then
>> conntrack state machine will not receive any more packets and,
>> consecutively, will never change the flow state to assured. I guess that
>
> I'm missing how it would offload each direction independently.
> Wouldn't CT state machine see the 1st reply, because it is not
> offloaded yet, and match it to the original direction?

What I meant is that if both directions are offloaded as soon as
IPS_SEEN_REPLY_BIT is set, then nf_conntrack_udp_packet() will not be
called for that connection anymore and would never be able to transition
the connection to assured state. But main thing is, as I said in the
previous reply, that we don't need to offload such connection ATM. It
could be done in a follow-up if there is a use-case for it, maybe even
made somehow configurable (with BPF! :)), so it could be dynamically
controlled.

>
>> could be mitigated somehow by periodically checking the hw stats and
>> transitioning the flow to assured based on them, but as I said in
>> previous email we don't want to over-complicate this series even more.
>> Also, offloading to hardware isn't free and costs both memory and CPU,
>> so it is not like offloading as early as possible is strictly beneficial
>> for all cases...
>
> Yup.
>
>> 
>> >
>> >> for reply packets check the current connection status: If UDP
>> >> unidirectional connection became assured also promote the corresponding
>> >> flow table entry to bidirectional and set the 'update' bit, else just set
>> >> the 'update' bit since reply directional traffic will most likely cause
>> >> connection status to become 'established' which requires updating the
>> >> offload state.
>> >> 
>> >> Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
>> >> ---
>> >>  net/sched/act_ct.c | 48 ++++++++++++++++++++++++++++++++++------------
>> >>  1 file changed, 36 insertions(+), 12 deletions(-)
>> >> 
>> >> diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
>> >> index bfddb462d2bc..563cbdd8341c 100644
>> >> --- a/net/sched/act_ct.c
>> >> +++ b/net/sched/act_ct.c
>> >> @@ -369,7 +369,7 @@ static void tcf_ct_flow_tc_ifidx(struct flow_offload *entry,
>> >>  
>> >>  static void tcf_ct_flow_table_add(struct tcf_ct_flow_table *ct_ft,
>> >>  				  struct nf_conn *ct,
>> >> -				  bool tcp)
>> >> +				  bool tcp, bool bidirectional)
>> >>  {
>> >>  	struct nf_conn_act_ct_ext *act_ct_ext;
>> >>  	struct flow_offload *entry;
>> >> @@ -388,6 +388,8 @@ static void tcf_ct_flow_table_add(struct tcf_ct_flow_table *ct_ft,
>> >>  		ct->proto.tcp.seen[0].flags |= IP_CT_TCP_FLAG_BE_LIBERAL;
>> >>  		ct->proto.tcp.seen[1].flags |= IP_CT_TCP_FLAG_BE_LIBERAL;
>> >>  	}
>> >> +	if (bidirectional)
>> >> +		__set_bit(NF_FLOW_HW_BIDIRECTIONAL, &entry->flags);
>> >>  
>> >>  	act_ct_ext = nf_conn_act_ct_ext_find(ct);
>> >>  	if (act_ct_ext) {
>> >> @@ -411,26 +413,34 @@ static void tcf_ct_flow_table_process_conn(struct tcf_ct_flow_table *ct_ft,
>> >>  					   struct nf_conn *ct,
>> >>  					   enum ip_conntrack_info ctinfo)
>> >>  {
>> >> -	bool tcp = false;
>> >> -
>> >> -	if ((ctinfo != IP_CT_ESTABLISHED && ctinfo != IP_CT_ESTABLISHED_REPLY) ||
>> >> -	    !test_bit(IPS_ASSURED_BIT, &ct->status))
>> >> -		return;
>> >> +	bool tcp = false, bidirectional = true;
>> >>  
>> >>  	switch (nf_ct_protonum(ct)) {
>> >>  	case IPPROTO_TCP:
>> >> -		tcp = true;
>> >> -		if (ct->proto.tcp.state != TCP_CONNTRACK_ESTABLISHED)
>> >> +		if ((ctinfo != IP_CT_ESTABLISHED &&
>> >> +		     ctinfo != IP_CT_ESTABLISHED_REPLY) ||
>> >> +		    !test_bit(IPS_ASSURED_BIT, &ct->status) ||
>> >> +		    ct->proto.tcp.state != TCP_CONNTRACK_ESTABLISHED)
>> >>  			return;
>> >> +
>> >> +		tcp = true;
>> >>  		break;
>> >>  	case IPPROTO_UDP:
>> >> +		if (!nf_ct_is_confirmed(ct))
>> >> +			return;
>> >> +		if (!test_bit(IPS_ASSURED_BIT, &ct->status))
>> >> +			bidirectional = false;
>> >>  		break;
>> >>  #ifdef CONFIG_NF_CT_PROTO_GRE
>> >>  	case IPPROTO_GRE: {
>> >>  		struct nf_conntrack_tuple *tuple;
>> >>  
>> >> -		if (ct->status & IPS_NAT_MASK)
>> >> +		if ((ctinfo != IP_CT_ESTABLISHED &&
>> >> +		     ctinfo != IP_CT_ESTABLISHED_REPLY) ||
>> >> +		    !test_bit(IPS_ASSURED_BIT, &ct->status) ||
>> >> +		    ct->status & IPS_NAT_MASK)
>> >>  			return;
>> >> +
>> >>  		tuple = &ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple;
>> >>  		/* No support for GRE v1 */
>> >>  		if (tuple->src.u.gre.key || tuple->dst.u.gre.key)
>> >> @@ -446,7 +456,7 @@ static void tcf_ct_flow_table_process_conn(struct tcf_ct_flow_table *ct_ft,
>> >>  	    ct->status & IPS_SEQ_ADJUST)
>> >>  		return;
>> >>  
>> >> -	tcf_ct_flow_table_add(ct_ft, ct, tcp);
>> >> +	tcf_ct_flow_table_add(ct_ft, ct, tcp, bidirectional);
>> >>  }
>> >>  
>> >>  static bool
>> >> @@ -625,13 +635,27 @@ static bool tcf_ct_flow_table_lookup(struct tcf_ct_params *p,
>> >>  	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
>> >>  	ct = flow->ct;
>> >>  
>> >> +	if (dir == FLOW_OFFLOAD_DIR_REPLY &&
>> >> +	    !test_bit(NF_FLOW_HW_BIDIRECTIONAL, &flow->flags)) {
>> >> +		/* Only offload reply direction after connection became
>> >> +		 * assured.
>> >> +		 */
>> >> +		if (test_bit(IPS_ASSURED_BIT, &ct->status))
>> >> +			set_bit(NF_FLOW_HW_BIDIRECTIONAL, &flow->flags);
>> >> +		set_bit(NF_FLOW_HW_UPDATE, &flow->flags);
>> >> +		return false;
>> >> +	}
>> >> +
>> >>  	if (tcph && (unlikely(tcph->fin || tcph->rst))) {
>> >>  		flow_offload_teardown(flow);
>> >>  		return false;
>> >>  	}
>> >>  
>> >> -	ctinfo = dir == FLOW_OFFLOAD_DIR_ORIGINAL ? IP_CT_ESTABLISHED :
>> >> -						    IP_CT_ESTABLISHED_REPLY;
>> >> +	if (dir == FLOW_OFFLOAD_DIR_ORIGINAL)
>> >> +		ctinfo = test_bit(IPS_SEEN_REPLY_BIT, &ct->status) ?
>> >> +			IP_CT_ESTABLISHED : IP_CT_NEW;
>> >> +	else
>> >> +		ctinfo = IP_CT_ESTABLISHED_REPLY;
>> >>  
>> >>  	flow_offload_refresh(nf_ft, flow);
>> >>  	nf_conntrack_get(&ct->ct_general);
>> >> -- 
>> >> 2.38.1
>> >> 
>>