[net-next 1/1] tipc: reduce risk of wakeup queue starvation

[net-next 1/1] tipc: reduce risk of wakeup queue starvation

[Jon Maloy]

In commit 365ad353c256 ("tipc: reduce risk of user starvation during
link congestion") we allowed senders to add exactly one list of extra
buffers to the link backlog queues during link congestion (aka
"oversubscription"). However, the criteria for when to stop adding
wakeup messages to the input queue when the overload abates is
inaccurate, and may cause starvation problems during very high load.

Currently, we stop adding wakeup messages after 10 total failed attempts
where we find that there is no space left in the backlog queue for a
certain importance level. The counter for this is accumulated across all
levels, which may lead the algorithm to leave the loop prematurely,
although there may still be plenty of space available at some levels.
The result is sometimes that messages near the wakeup queue tail are not
added to the input queue as they should be.

We now introduce a more exact algorithm, where we keep adding wakeup
messages to a level as long as the backlog queue has free slots for
the corresponding level, and stop at the moment there are no more such
slots or when there are no more wakeup messages to dequeue.

Fixes: 365ad35 ("tipc: reduce risk of user starvation during link congestion")
Reported-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
---
 net/tipc/link.c | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/net/tipc/link.c b/net/tipc/link.c
index 2c27477..dd3155b 100644
--- a/net/tipc/link.c
+++ b/net/tipc/link.c
@@ -854,18 +854,31 @@ static int link_schedule_user(struct tipc_link *l, struct tipc_msg *hdr)
  */
 static void link_prepare_wakeup(struct tipc_link *l)
 {
+	struct sk_buff_head *wakeupq = &l->wakeupq;
+	struct sk_buff_head *inputq = l->inputq;
 	struct sk_buff *skb, *tmp;
-	int imp, i = 0;
+	struct sk_buff_head tmpq;
+	int avail[5] = {0,};
+	int imp = 0;
+
+	__skb_queue_head_init(&tmpq);
 
-	skb_queue_walk_safe(&l->wakeupq, skb, tmp) {
+	for (; imp <= TIPC_SYSTEM_IMPORTANCE; imp++)
+		avail[imp] = l->backlog[imp].limit - l->backlog[imp].len;
+
+	skb_queue_walk_safe(wakeupq, skb, tmp) {
 		imp = TIPC_SKB_CB(skb)->chain_imp;
-		if (l->backlog[imp].len < l->backlog[imp].limit) {
-			skb_unlink(skb, &l->wakeupq);
-			skb_queue_tail(l->inputq, skb);
-		} else if (i++ > 10) {
-			break;
-		}
+		if (avail[imp] <= 0)
+			continue;
+		avail[imp]--;
+		__skb_unlink(skb, wakeupq);
+		__skb_queue_tail(&tmpq, skb);
 	}
+
+	spin_lock_bh(&inputq->lock);
+	skb_queue_splice_tail(&tmpq, inputq);
+	spin_unlock_bh(&inputq->lock);
+
 }
 
 void tipc_link_reset(struct tipc_link *l)
-- 
2.1.4

Re: [net-next 1/1] tipc: reduce risk of wakeup queue starvation

[David Miller]

From: Jon Maloy <jon.maloy@ericsson.com>
Date: Tue, 30 Jul 2019 16:23:18 +0200

> In commit 365ad353c256 ("tipc: reduce risk of user starvation during
> link congestion") we allowed senders to add exactly one list of extra
> buffers to the link backlog queues during link congestion (aka
> "oversubscription"). However, the criteria for when to stop adding
> wakeup messages to the input queue when the overload abates is
> inaccurate, and may cause starvation problems during very high load.
> 
> Currently, we stop adding wakeup messages after 10 total failed attempts
> where we find that there is no space left in the backlog queue for a
> certain importance level. The counter for this is accumulated across all
> levels, which may lead the algorithm to leave the loop prematurely,
> although there may still be plenty of space available at some levels.
> The result is sometimes that messages near the wakeup queue tail are not
> added to the input queue as they should be.
> 
> We now introduce a more exact algorithm, where we keep adding wakeup
> messages to a level as long as the backlog queue has free slots for
> the corresponding level, and stop at the moment there are no more such
> slots or when there are no more wakeup messages to dequeue.
> 
> Fixes: 365ad35 ("tipc: reduce risk of user starvation during link congestion")
> Reported-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
> Acked-by: Ying Xue <ying.xue@windriver.com>
> Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>

Applied, thank you.