Subject: Re: [PATCH] sctp: don't break the loop while meeting the active_path so as to find the matched transport

From: Vlad Yasevich <[email-masked]>
Date: Tue, 12 Mar 2013 12:00:32 -0400

On 03/12/2013 11:44 AM, Neil Horman wrote:
> On Tue, Mar 12, 2013 at 08:11:34AM -0400, Vlad Yasevich wrote:
>> On 03/11/2013 09:31 AM, Neil Horman wrote:
>>> On Mon, Mar 11, 2013 at 10:14:50AM +0800, Xufeng Zhang wrote:
>>>> On 3/8/13, Neil Horman <[email-masked]> wrote:
>>>>> On Fri, Mar 08, 2013 at 03:39:37PM +0800, Xufeng Zhang wrote:
>>>>>> sctp_assoc_lookup_tsn() function searchs which transport a certain TSN
>>>>>> was sent on, if not found in the active_path transport, then go search
>>>>>> all the other transports in the peer's transport_addr_list, however, we
>>>>>> should continue to the next entry rather than break the loop when meet
>>>>>> the active_path transport.
>>>>>>
>>>>>> Signed-off-by: Xufeng Zhang <[email-masked]>
>>>>>> ---
>>>>>>   net/sctp/associola.c |    2 +-
>>>>>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>>>>>
>>>>>> diff --git a/net/sctp/associola.c b/net/sctp/associola.c
>>>>>> index 43cd0dd..d2709e2 100644
>>>>>> --- a/net/sctp/associola.c
>>>>>> +++ b/net/sctp/associola.c
>>>>>> @@ -1079,7 +1079,7 @@ struct sctp_transport *sctp_assoc_lookup_tsn(struct
>>>>>> sctp_association *asoc,
>>>>>>   			transports) {
>>>>>>
>>>>>>   		if (transport == active)
>>>>>> -			break;
>>>>>> +			continue;
>>>>>>   		list_for_each_entry(chunk, &transport->transmitted,
>>>>>>   				transmitted_list) {
>>>>>>   			if (key == chunk->subh.data_hdr->tsn) {
>>>>>> --
>>>>>> 1.7.0.2
>>>>>>
>>>>>>
>>>>>
>>>>> This works, but what might be better would be if we did a move to front
>>>>> heuristic in sctp_assoc_set_primary.  E.g. when we set the active_path, move
>>>>> the
>>>>> requisite transport to the front of the transport_addr_list.  If we did
>>>>> that,
>>>>> then we could just do one for loop in sctp_assoc_lookup_tsn and wind up
>>>>> implicitly check the active path first without having to check it seprately
>>>>> and
>>>>> skip it in the second for loop.
>>>>
>>>> Thanks for your review, Neil!
>>>>
>>>> I know what you mean, yes, it's most probably that the searched TSN was
>>>> transmitted in the currently active_path, but what should we do if not.
>>>>
>>>> Check the comment in sctp_assoc_lookup_tsn() function:
>>>> /* Let's be hopeful and check the active_path first. */
>>>> /* If not found, go search all the other transports. */
>>>>
>>>> It has checked the active_path first and then traverse all the other
>>>> transports in
>>>> the transport_addr_list except the active_path.
>>>>
>>>> So what I want to fix here is the inconsistency between the function
>>>> should do and
>>>> the code actually does.
>>>>
>>> I understand what you're doing, and I agree that the fix is functional (Hence
>>> my "This works" statement in my last note).  What I'm suggesting is that, since
>>> you're messing about in that code anyway that you clean it up while your at it,
>>> so that we don't need to have the if (transport == active) check at all.  We
>>> trade in some extra work in a non-critical path (sctp_assoc_set_primary), for
>>> the removal of an extra for loop operation and a conditional check in a much
>>> hotter path.  Something like this (completely untested), is what I was thinking
>>>
>>>
>>> diff --git a/net/sctp/associola.c b/net/sctp/associola.c
>>> index 43cd0dd..8ae873c 100644
>>> --- a/net/sctp/associola.c
>>> +++ b/net/sctp/associola.c
>>> @@ -505,6 +505,9 @@ void sctp_assoc_set_primary(struct sctp_association *asoc,
>>>
>>>   	asoc->peer.primary_path = transport;
>>>
>>> +	list_del_rcu(&transport->transports);
>>> +	list_add_rcu(&transport->transports, &asoc->peer.transport_addr_list);
>>> +
>>>   	/* Set a default msg_name for events. */
>>>   	memcpy(&asoc->peer.primary_addr, &transport->ipaddr,
>>>   	       sizeof(union sctp_addr));
>>> @@ -1040,7 +1043,6 @@ struct sctp_chunk *sctp_get_ecne_prepend(struct sctp_association *asoc)
>>>   struct sctp_transport *sctp_assoc_lookup_tsn(struct sctp_association *asoc,
>>>   					     __u32 tsn)
>>>   {
>>> -	struct sctp_transport *active;
>>>   	struct sctp_transport *match;
>>>   	struct sctp_transport *transport;
>>>   	struct sctp_chunk *chunk;
>>> @@ -1057,29 +1059,16 @@ struct sctp_transport *sctp_assoc_lookup_tsn(struct sctp_association *asoc,
>>>   	 * The general strategy is to search each transport's transmitted
>>>   	 * list.   Return which transport this TSN lives on.
>>>   	 *
>>> -	 * Let's be hopeful and check the active_path first.
>>> -	 * Another optimization would be to know if there is only one
>>> -	 * outbound path and not have to look for the TSN at all.
>>> +	 * Note, that sctp_assoc_set_primary does a move to front operation
>>> +	 * on the active_path transport, so this code implicitly checks
>>> +	 * the active_path first, as we most commonly expect to find our TSN
>>> +	 * there.
>>>   	 *
>>>   	 */
>>
>> Neil, active_patch != primary_path all the time.  In fact, when you
>> have path primary path failure, active path will change while
>> primary
>> may only change when the user says so.
> Thats a good point, thank you Vlad.  We would need to only update the
> transport_addr_list in set_primary if its state is ACTIVE or UNKNOWN.  We would
> also need to update it if the active path changes in
> sctp_assoc_control_transport, if the new active path is different from the old.
> Both of those paths however are not intended to be run frequently, so I think
> this is still a viable optimization.  I'm working on it now.
> Neil

You also need to take special care if an active transport is removed. 
The active path will get reassigned then.  I am not sure if it is worth 
doing all these changes to address a corner case optimization (CWR is a 
corner case).

-vlad

>
>>
>> So, you may still get into a situation where primary and active
>> paths are different.
>>
>> The optimization here may not work at all under those circumstances.
>>
>> -vlad
>>
>>>
>>> -	active = asoc->peer.active_path;
>>> -
>>> -	list_for_each_entry(chunk, &active->transmitted,
>>> -			transmitted_list) {
>>> -
>>> -		if (key == chunk->subh.data_hdr->tsn) {
>>> -			match = active;
>>> -			goto out;
>>> -		}
>>> -	}
>>> -
>>> -	/* If not found, go search all the other transports. */
>>>   	list_for_each_entry(transport, &asoc->peer.transport_addr_list,
>>>   			transports) {
>>>
>>> -		if (transport == active)
>>> -			break;
>>>   		list_for_each_entry(chunk, &transport->transmitted,
>>>   				transmitted_list) {
>>>   			if (key == chunk->subh.data_hdr->tsn) {
>>>
>>
>>