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 08:11:34 -0400

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.

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) {
>