Subject: Re: [PATCH net] net: sctp: wake up all assocs if sndbuf policy is per socket

From: Daniel Borkmann <[email-masked]>
Date: Tue, 08 Apr 2014 16:52:35 +0200

On 04/08/2014 04:41 PM, Vlad Yasevich wrote:
> On 04/08/2014 09:33 AM, Daniel Borkmann wrote:
>> SCTP charges chunks for wmem accounting via skb->truesize in
>> sctp_set_owner_w(), and sctp_wfree() respectively as the
>> reverse operation. If a sender runs out of wmem, it needs to
>> wait via sctp_wait_for_sndbuf(), and gets woken up by a call
>> to __sctp_write_space() mostly via sctp_wfree().
>>
>> __sctp_write_space() is being called per association. Although
>> we assign sk->sk_write_space() to sctp_write_space(), which
>> is then being done per socket, it is only used if send space
>> is increased per socket option (SO_SNDBUF), as SOCK_USE_WRITE_QUEUE
>> is set and therefore not invoked in sock_wfree().
>>
>> Commit 4c3a5bdae293 ("sctp: Don't charge for data in sndbuf
>> again when transmitting packet") fixed an issue where in case
>> sctp_packet_transmit() manages to queue up more than sndbuf
>> bytes, sctp_wait_for_sndbuf() will never be woken up again
>> unless it is interrupted by a signal. However, a still
>> remaining issue is that if net.sctp.sndbuf_policy=0, that is
>> accounting per socket, and one-to-many sockets are in use,
>> the reclaimed write space from sctp_wfree() is 'unfairly'
>> handed back on the server to the association that is the lucky
>> one to be woken up again via __sctp_write_space(), while
>> the remaining associations are never be woken up again
>> (unless by a signal).
>>
>> The effect disappears with net.sctp.sndbuf_policy=1, that
>> is wmem accounting per association, as it guarantees a fair
>> share of wmem among associations.
>>
>> Therefore, if we have reclaimed memory in case of per socket
>> accouting, wake all related associations to a socket in a
>> fair manner, that is, traverse the socket association list
>> starting from the current neighbour of the association and
>> issue a __sctp_write_space() to everyone until we end up
>> waking ourselves. This guarantees that no association is
>> preferred over another and even if more associations are
>> taken into the one-to-many session, all receivers will get
>> messages from the server and are not stalled forever on
>> high load. This setting still leaves the advantage of per
>> socket accounting in touch as an association can still use
>> up global limits if unused by others.
>>
>> Fixes: 4eb701dfc618 ("[SCTP] Fix SCTP sendbuffer accouting.")
>> Signed-off-by: Daniel Borkmann <[email-masked]>
>> Cc: Thomas Graf <[email-masked]>
>> Cc: Neil Horman <[email-masked]>
>> ---
>>   [ When net-next opens up again, we need to think how
>>     we can ideally make a new list interface and simplify
>>     both open-coded list traversals. ]
>>
>>   net/sctp/socket.c | 31 ++++++++++++++++++++++++++++++-
>>   1 file changed, 30 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
>> index 981aaf8..a4c8c1f 100644
>> --- a/net/sctp/socket.c
>> +++ b/net/sctp/socket.c
>> @@ -6593,6 +6593,35 @@ static void __sctp_write_space(struct sctp_association *asoc)
>>   	}
>>   }
>>
>> +static void sctp_wake_up_waiters(struct sock *sk,
>> +				 struct sctp_association *asoc)
>> +{
>> +	struct sctp_association *tmp = asoc;
>> +
>> +	/* We do accounting for the sndbuf space per association,
>> +	 * so we only need to wake our own association.
>> +	 */
>> +	if (asoc->ep->sndbuf_policy)
>> +		return __sctp_write_space(asoc);
>> +
>> +	/* Accounting for the sndbuf space is per socket, so we need
>> +	 * to wake up others, try to be fair and in case of other
>> +	 * associations, let them have a go first instead of just
>> +	 * doing a sctp_write_space() call.
>> +	 */
>
> May be a note saying that we are here only when association frees
> queued up chunks and thus we are under lock and the list is guaranteed
> not to change.

Ok, will add that to the comment and respin, thanks Vlad.

>> +	for (tmp = list_next_entry(tmp, asocs); 1;
>
> Why not change the stop condition to tmp == asoc.  It should work
> since it will not be head pointer.

If I see this correctly, wouldn't we then exclude to eventually
call __sctp_write_space(tmp) on ourselves as we also need to make
sure to wake us up?

> -vlad
>
>> +	     tmp = list_next_entry(tmp, asocs)) {
>> +		/* Manually skip the head element. */
>> +		if (&tmp->asocs == &((sctp_sk(sk))->ep->asocs))
>> +			continue;
>> +		/* Wake up association. */
>> +		__sctp_write_space(tmp);
>> +		/* We've reached the end. */
>> +		if (tmp == asoc)
>> +			break;
>> +	}
>> +}
>> +
>>   /* Do accounting for the sndbuf space.
>>    * Decrement the used sndbuf space of the corresponding association by the
>>    * data size which was just transmitted(freed).
>> @@ -6620,7 +6649,7 @@ static void sctp_wfree(struct sk_buff *skb)
>>   	sk_mem_uncharge(sk, skb->truesize);
>>
>>   	sock_wfree(skb);
>> -	__sctp_write_space(asoc);
>> +	sctp_wake_up_waiters(sk, asoc);
>>
>>   	sctp_association_put(asoc);
>>   }
>>
>