From: Tuong Lien tuong.t.lien@dektech.com.au
[ Upstream commit e95584a889e1902fdf1ded9712e2c3c3083baf96 ]
We have identified a problem with the "oversubscription" policy in the link transmission code.
When small messages are transmitted, and the sending link has reached the transmit window limit, those messages will be bundled and put into the link backlog queue. However, bundles of data messages are counted at the 'CRITICAL' level, so that the counter for that level, instead of the counter for the real, bundled message's level is the one being increased. Subsequent, to-be-bundled data messages at non-CRITICAL levels continue to be tested against the unchanged counter for their own level, while contributing to an unrestrained increase at the CRITICAL backlog level.
This leaves a gap in congestion control algorithm for small messages that can result in starvation for other users or a "real" CRITICAL user. Even that eventually can lead to buffer exhaustion & link reset.
We fix this by keeping a 'target_bskb' buffer pointer at each levels, then when bundling, we only bundle messages at the same importance level only. This way, we know exactly how many slots a certain level have occupied in the queue, so can manage level congestion accurately.
By bundling messages at the same level, we even have more benefits. Let consider this: - One socket sends 64-byte messages at the 'CRITICAL' level; - Another sends 4096-byte messages at the 'LOW' level;
When a 64-byte message comes and is bundled the first time, we put the overhead of message bundle to it (+ 40-byte header, data copy, etc.) for later use, but the next message can be a 4096-byte one that cannot be bundled to the previous one. This means the last bundle carries only one payload message which is totally inefficient, as for the receiver also! Later on, another 64-byte message comes, now we make a new bundle and the same story repeats...
With the new bundling algorithm, this will not happen, the 64-byte messages will be bundled together even when the 4096-byte message(s) comes in between. However, if the 4096-byte messages are sent at the same level i.e. 'CRITICAL', the bundling algorithm will again cause the same overhead.
Also, the same will happen even with only one socket sending small messages at a rate close to the link transmit's one, so that, when one message is bundled, it's transmitted shortly. Then, another message comes, a new bundle is created and so on...
We will solve this issue radically by another patch.
Fixes: 365ad353c256 ("tipc: reduce risk of user starvation during link congestion") Reported-by: Hoang Le hoang.h.le@dektech.com.au Acked-by: Jon Maloy jon.maloy@ericsson.com Signed-off-by: Tuong Lien tuong.t.lien@dektech.com.au Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/tipc/link.c | 29 ++++++++++++++++++----------- net/tipc/msg.c | 5 +---- 2 files changed, 19 insertions(+), 15 deletions(-)
--- a/net/tipc/link.c +++ b/net/tipc/link.c @@ -163,6 +163,7 @@ struct tipc_link { struct { u16 len; u16 limit; + struct sk_buff *target_bskb; } backlog[5]; u16 snd_nxt; u16 prev_from; @@ -872,6 +873,7 @@ static void link_prepare_wakeup(struct t void tipc_link_reset(struct tipc_link *l) { struct sk_buff_head list; + u32 imp;
__skb_queue_head_init(&list);
@@ -893,11 +895,10 @@ void tipc_link_reset(struct tipc_link *l __skb_queue_purge(&l->deferdq); __skb_queue_purge(&l->backlogq); __skb_queue_purge(&l->failover_deferdq); - l->backlog[TIPC_LOW_IMPORTANCE].len = 0; - l->backlog[TIPC_MEDIUM_IMPORTANCE].len = 0; - l->backlog[TIPC_HIGH_IMPORTANCE].len = 0; - l->backlog[TIPC_CRITICAL_IMPORTANCE].len = 0; - l->backlog[TIPC_SYSTEM_IMPORTANCE].len = 0; + for (imp = 0; imp <= TIPC_SYSTEM_IMPORTANCE; imp++) { + l->backlog[imp].len = 0; + l->backlog[imp].target_bskb = NULL; + } kfree_skb(l->reasm_buf); kfree_skb(l->failover_reasm_skb); l->reasm_buf = NULL; @@ -938,7 +939,7 @@ int tipc_link_xmit(struct tipc_link *l, u16 bc_ack = l->bc_rcvlink->rcv_nxt - 1; struct sk_buff_head *transmq = &l->transmq; struct sk_buff_head *backlogq = &l->backlogq; - struct sk_buff *skb, *_skb, *bskb; + struct sk_buff *skb, *_skb, **tskb; int pkt_cnt = skb_queue_len(list); int rc = 0;
@@ -988,19 +989,21 @@ int tipc_link_xmit(struct tipc_link *l, seqno++; continue; } - if (tipc_msg_bundle(skb_peek_tail(backlogq), hdr, mtu)) { + tskb = &l->backlog[imp].target_bskb; + if (tipc_msg_bundle(*tskb, hdr, mtu)) { kfree_skb(__skb_dequeue(list)); l->stats.sent_bundled++; continue; } - if (tipc_msg_make_bundle(&bskb, hdr, mtu, l->addr)) { + if (tipc_msg_make_bundle(tskb, hdr, mtu, l->addr)) { kfree_skb(__skb_dequeue(list)); - __skb_queue_tail(backlogq, bskb); - l->backlog[msg_importance(buf_msg(bskb))].len++; + __skb_queue_tail(backlogq, *tskb); + l->backlog[imp].len++; l->stats.sent_bundled++; l->stats.sent_bundles++; continue; } + l->backlog[imp].target_bskb = NULL; l->backlog[imp].len += skb_queue_len(list); skb_queue_splice_tail_init(list, backlogq); } @@ -1016,6 +1019,7 @@ static void tipc_link_advance_backlog(st u16 seqno = l->snd_nxt; u16 ack = l->rcv_nxt - 1; u16 bc_ack = l->bc_rcvlink->rcv_nxt - 1; + u32 imp;
while (skb_queue_len(&l->transmq) < l->window) { skb = skb_peek(&l->backlogq); @@ -1026,7 +1030,10 @@ static void tipc_link_advance_backlog(st break; __skb_dequeue(&l->backlogq); hdr = buf_msg(skb); - l->backlog[msg_importance(hdr)].len--; + imp = msg_importance(hdr); + l->backlog[imp].len--; + if (unlikely(skb == l->backlog[imp].target_bskb)) + l->backlog[imp].target_bskb = NULL; __skb_queue_tail(&l->transmq, skb); /* next retransmit attempt */ if (link_is_bc_sndlink(l)) --- a/net/tipc/msg.c +++ b/net/tipc/msg.c @@ -484,10 +484,7 @@ bool tipc_msg_make_bundle(struct sk_buff bmsg = buf_msg(_skb); tipc_msg_init(msg_prevnode(msg), bmsg, MSG_BUNDLER, 0, INT_H_SIZE, dnode); - if (msg_isdata(msg)) - msg_set_importance(bmsg, TIPC_CRITICAL_IMPORTANCE); - else - msg_set_importance(bmsg, TIPC_SYSTEM_IMPORTANCE); + msg_set_importance(bmsg, msg_importance(msg)); msg_set_seqno(bmsg, msg_seqno(msg)); msg_set_ack(bmsg, msg_ack(msg)); msg_set_bcast_ack(bmsg, msg_bcast_ack(msg));