Subject: Re: [PATCH] sctp: optimize searching the active path for tsns

From: Vlad Yasevich <[email-masked]>
Date: Tue, 12 Mar 2013 21:43:20 -0400

On 03/12/2013 09:20 PM, Neil Horman wrote:
> On Tue, Mar 12, 2013 at 05:01:50PM -0400, Vlad Yasevich wrote:
>> Hi Neil
>>
>> On 03/12/2013 01:29 PM, Neil Horman wrote:
>>> SCTP currently attempts to optimize the search for tsns on a transport by first
>>> checking the active_path, then searching alternate transports.  This operation
>>> however is a bit convoluted, as we explicitly search the active path, then serch
>>> all other transports, skipping the active path, when its detected.  Lets
>>> optimize this by preforming a move to front on the transport_addr_list every
>>> time the active_path is changed.  The active_path changes occur in relatively
>>> non-critical paths, and doing so allows us to just search the
>>> transport_addr_list in order, avoiding an extra conditional check in the
>>> relatively hot tsn lookup path.  This also happens to fix a bug where we break
>>> out of the for loop early in the tsn lookup.
>>>
>>> CC: Xufeng Zhang <[email-masked]>
>>> CC: [email-masked]
>>> CC: [email-masked]
>>> CC: [email-masked]
>>> ---
>>>   net/sctp/associola.c | 31 ++++++++++++-------------------
>>>   1 file changed, 12 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/net/sctp/associola.c b/net/sctp/associola.c
>>> index 43cd0dd..7af96b3 100644
>>> --- a/net/sctp/associola.c
>>> +++ b/net/sctp/associola.c
>>> @@ -513,8 +513,11 @@ void sctp_assoc_set_primary(struct sctp_association *asoc,
>>>   	 * user wants to use this new path.
>>>   	 */
>>>   	if ((transport->state == SCTP_ACTIVE) ||
>>> -	    (transport->state == SCTP_UNKNOWN))
>>> +	    (transport->state == SCTP_UNKNOWN)) {
>>> +		list_del_rcu(&transport->transports);
>>> +		list_add_rcu(&transport->transports, &asoc->peer.transport_addr_list);
>>>   		asoc->peer.active_path = transport;
>>> +	}
>>
>> What would happen if at the same time someone is walking the list
>> through the proc interfaces?
>>
>> Since you are effectively changing the .next pointer, I think it is
>> possible to get a duplicate transport output essentially corrupting
>> the output.
>>
> It would be the case, but you're using the rcu variants of the list_add macros
> at all the points where we modify the list (some of which we do at call sites
> right before we call set_primary, see sctp_assoc_add_peer, where
> list_add_tail_rcu also modifies a next pointer).  So if this is a problem, its a
> problem without this patch.  In fact looking at it, our list access to
> transport_addr_list is broken, as we use rcu apis to modify the list but non-rcu
> apis to traverse the list.  I'll need to fix that first.

As long as we are under lock, we don't need rcu variants for traversal. 
  The traversals done by the sctp_seq_ functions already use correct rcu 
variants.

I don't see this as a problem right now since we either delete the
transport, or add it.  We don't move it to a new location in the list.
With the move, what could happen is that while sctp_seq_ is printing
a transport, you might move it to another spot, and the when you grab
the .next pointer on the next iteration, it points to a completely 
different spot.

-vlad

>
>> Personally, I don't think that this particular case is worth the
>> optimization since we are trying to optimize a TSN search that only
>> happens when ECNE chunk is received.  You say that it is a hot path.
>> Is ECNE really such a common occurrence?
>>
> Not particularly, but I'm guessing its more used than the path that switches the
> active path.  Plus its just more elegant, modifying the list so we don't have to
> have two identical for loops in the lookup_tsn function, plus a check for the
> transport that we already checked.
>
>> Additionally, I don't think this is really an optimization as the
>> current and new code do exactly the same thing:
>>   1) look at active path
>>   2) look at the rest of the paths
>>
> Thats not the optimization, its trading where we do work in the interests of
> making the more used path faster (we drop an if condition, and remove some
> duplicated code).
>
>> This just makes cleaner code at the expense of list shuffling.
>>
> Yes, that sounds like a good trade to me. But as noted above, we probably need
> to fix the transport_addr_list access first.
> Neil
>
>> -vlad
>>>
>>>   	/*
>>>   	 * SFR-CACC algorithm:
>>> @@ -964,6 +967,10 @@ void sctp_assoc_control_transport(struct sctp_association *asoc,
>>>   	}
>>>
>>>   	/* Set the active and retran transports.  */
>>> +	if (asoc->peer.active_path != first) {
>>> +		list_del_rcu(first);
>>> +		list_add_rcu(first, &asoc->peer.transport_addr_list);
>>> +	}
>>>   	asoc->peer.active_path = first;
>>>   	asoc->peer.retran_path = second;
>>>   }
>>> @@ -1040,7 +1047,6 @@ 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 +1063,16 @@ 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.
>>>   	 *
>>>   	 */
>>>
>>> -	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) {
>>>
>>
>>