Re: [PATCH] sctp: do trace_sctp_probe after SACK validation and check

[Kevin Kou <qdkevin.kou@gmail.com>]

On 2019/12/23 21:26, Marcelo Ricardo Leitner wrote:
> On Sun, Dec 22, 2019 at 12:22:24PM +0800, Kevin Kou wrote:
>> On 2019/12/21 0:17, Marcelo Ricardo Leitner wrote:
>>> On Fri, Dec 20, 2019 at 04:47:03AM +0000, Kevin Kou wrote:
>>>> The function sctp_sf_eat_sack_6_2 now performs
>>>> the Verification Tag validation, Chunk length validation, Bogu check,
>>>> and also the detection of out-of-order SACK based on the RFC2960
>>>> Section 6.2 at the beginning, and finally performs the further
>>>> processing of SACK. The trace_sctp_probe now triggered before
>>>> the above necessary validation and check.
>>>>
>>>> This patch is to do the trace_sctp_probe after the necessary check
>>>> and validation to SACK.
>>>>
>>>> Signed-off-by: Kevin Kou <qdkevin.kou@gmail.com>
>>>> ---
>>>>    net/sctp/sm_statefuns.c | 3 ++-
>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/net/sctp/sm_statefuns.c b/net/sctp/sm_statefuns.c
>>>> index 42558fa..b4a54df 100644
>>>> --- a/net/sctp/sm_statefuns.c
>>>> +++ b/net/sctp/sm_statefuns.c
>>>> @@ -3281,7 +3281,6 @@ enum sctp_disposition sctp_sf_eat_sack_6_2(struct net *net,
>>>>    	struct sctp_sackhdr *sackh;
>>>>    	__u32 ctsn;
>>>> -	trace_sctp_probe(ep, asoc, chunk);
>>>>    	if (!sctp_vtag_verify(chunk, asoc))
>>>>    		return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
>>>> @@ -3319,6 +3318,8 @@ enum sctp_disposition sctp_sf_eat_sack_6_2(struct net *net,
>>>>    	if (!TSN_lt(ctsn, asoc->next_tsn))
>>>>    		return sctp_sf_violation_ctsn(net, ep, asoc, type, arg, commands);
>>>> +	trace_sctp_probe(ep, asoc, chunk);
>>>> +
>>>
>>> Moving it here will be after the check against ctsn_ack_point, which
>>> could cause duplicated SACKs to be missed from the log.
>>
>>
>> As this SCTP trace used to trace the changes of SCTP association state in
>> response to incoming packets(SACK). It is used for debugging SCTP congestion
>> control algorithms, so according to the code in include/trace/events/sctp.h,
>> the trace event mainly focus on congestion related information, and there is
>> no SACK Chunk related information printed. So it is hard to point out
>> whether the SACK is duplicate one or not based on this trace event.
> 
> I see. Yet, it's quite odd to do debugging of congestion control
> algorithms without knowing how many TSNs/bytes are being acked by this
> ack, but let's keep that aside for now.
> 
> I still can't agree with filtering out based the out-of-order SACK check
> (the TSN_lt(ctsn, asoc->ctsn_ack_point) check. That is valuable to
> congestion control debugging, because it will likely mean that the
> sender is working with fewer acks than it would like/expect.
> 
> If you need to filter out them and have a "clean" list of what got in,
> then the fix it needs lies in adding support for logging the ctsn in
> the trace point itself (similarly to the pr_debug in there) and filter
> it on post-processing of the logs.
> 
> I don't know how much of UAPI cover probe points. Hopefully we can add
> that information without having to create new probe points.
> 
Thanks for your comments, In order to avoid affecting the UAPI, let's 
keep the existing print fields at present.

> PS: You can invert the check in
>          if (!TSN_lt(ctsn, asoc->next_tsn))
> to
>          if (TSN_lte(asoc->next_tsn, ctsn))
> and move it above, so it is done before the out-of-order check, and
> the trace point in between them.
> 

I will make this change and commit another patch.

>>
>> include/trace/events/sctp.h
>> 1. TRACE_EVENT(sctp_probe,
>>
>> TP_printk("asoc=%#llx mark=%#x bind_port=%d peer_port=%d pathmtu=%d "
>> 		  "rwnd=%u unack_data=%d",
>> 		  __entry->asoc, __entry->mark, __entry->bind_port,
>> 		  __entry->peer_port, __entry->pathmtu, __entry->rwnd,
>> 		  __entry->unack_data)
>>
>> 2. TRACE_EVENT(sctp_probe_path,
>>
>> TP_printk("asoc=%#llx%s ipaddr=%pISpc state=%u cwnd=%u ssthresh=%u "
>> 		  "flight_size=%u partial_bytes_acked=%u pathmtu=%u",
>> 		  __entry->asoc, __entry->primary ? "(*)" : "",
>> 		  __entry->ipaddr, __entry->state, __entry->cwnd,
>> 		  __entry->ssthresh, __entry->flight_size,
>> 		  __entry->partial_bytes_acked, __entry->pathmtu)
>>
>>>
>>> Yes, from the sender-side CC we don't care about it (yet), but it
>>> helps to spot probably avoidable retransmissions.
>>>
>>> I think this is cleaning up the noise too much. I can agree with
>>> moving it to after the chunk sanity tests, though.
>>>
>>>>    	/* Return this SACK for further processing.  */
>>>>    	sctp_add_cmd_sf(commands, SCTP_CMD_PROCESS_SACK, SCTP_CHUNK(chunk));
>>>> -- 
>>>> 1.8.3.1
>>>>
>>