Commit ffe4ccd3 authored by Eric Dumazet's avatar Eric Dumazet Committed by Paolo Abeni
Browse files

net: add net.core.qdisc_max_burst 



In blamed commit, I added a check against the temporary queue
built in __dev_xmit_skb(). Idea was to drop packets early,
before any spinlock was acquired.

if (unlikely(defer_count > READ_ONCE(q->limit))) {
	kfree_skb_reason(skb, SKB_DROP_REASON_QDISC_DROP);
	return NET_XMIT_DROP;
}

It turned out that HTB Qdisc has a zero q->limit.
HTB limits packets on a per-class basis.
Some of our tests became flaky.

Add a new sysctl : net.core.qdisc_max_burst to control
how many packets can be stored in the temporary lockless queue.

Also add a new QDISC_BURST_DROP drop reason to better diagnose
future issues.

Thanks Neal !

Fixes: 100dfa74 ("net: dev_queue_xmit() llist adoption")
Reported-and-bisected-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Reviewed-by: default avatarNeal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20260107104159.3669285-1-edumazet@google.com


Signed-off-by: default avatarPaolo Abeni <pabeni@redhat.com>
parent dfdf7746

Documentation/admin-guide/sysctl/net.rst

+8 −0
Original line number Diff line number Diff line
@@ -303,6 +303,14 @@ netdev_max_backlog 
Maximum number of packets, queued on the INPUT side, when the interface

receives packets faster than kernel can process them.



qdisc_max_burst

------------------



Maximum number of packets that can be temporarily stored before

reaching qdisc.



Default: 1000



netdev_rss_key

--------------

include/net/dropreason-core.h

+6 −0
Original line number Diff line number Diff line
@@ -67,6 +67,7 @@ 
	FN(TC_EGRESS)			\

	FN(SECURITY_HOOK)		\

	FN(QDISC_DROP)			\

	FN(QDISC_BURST_DROP)		\

	FN(QDISC_OVERLIMIT)		\

	FN(QDISC_CONGESTED)		\

	FN(CAKE_FLOOD)			\
@@ -374,6 +375,11 @@ enum skb_drop_reason { 
	 * failed to enqueue to current qdisc)

	 */

	SKB_DROP_REASON_QDISC_DROP,

	/**

	 * @SKB_DROP_REASON_QDISC_BURST_DROP: dropped when net.core.qdisc_max_burst

	 * limit is hit.

	 */

	SKB_DROP_REASON_QDISC_BURST_DROP,

	/**

	 * @SKB_DROP_REASON_QDISC_OVERLIMIT: dropped by qdisc when a qdisc

	 * instance exceeds its total buffer size limit.

include/net/hotdata.h

+1 −0
Original line number Diff line number Diff line
@@ -42,6 +42,7 @@ struct net_hotdata { 
	int			netdev_budget_usecs;

	int			tstamp_prequeue;

	int			max_backlog;

	int			qdisc_max_burst;

	int			dev_tx_weight;

	int			dev_rx_weight;

	int			sysctl_max_skb_frags;

net/core/dev.c

+3 −3
Original line number Diff line number Diff line
@@ -4203,8 +4203,8 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q, 
	do {

		if (first_n && !defer_count) {

			defer_count = atomic_long_inc_return(&q->defer_count);

			if (unlikely(defer_count > READ_ONCE(q->limit))) {

				kfree_skb_reason(skb, SKB_DROP_REASON_QDISC_DROP);

			if (unlikely(defer_count > READ_ONCE(net_hotdata.qdisc_max_burst))) {

				kfree_skb_reason(skb, SKB_DROP_REASON_QDISC_BURST_DROP);

				return NET_XMIT_DROP;

			}

		}
@@ -4222,7 +4222,7 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q, 
	ll_list = llist_del_all(&q->defer_list);

	/* There is a small race because we clear defer_count not atomically

	 * with the prior llist_del_all(). This means defer_list could grow

	 * over q->limit.

	 * over qdisc_max_burst.

	 */

	atomic_long_set(&q->defer_count, 0);

net/core/hotdata.c

+1 −0
Original line number Diff line number Diff line
@@ -17,6 +17,7 @@ struct net_hotdata net_hotdata __cacheline_aligned = { 


	.tstamp_prequeue = 1,

	.max_backlog = 1000,

	.qdisc_max_burst = 1000,

	.dev_tx_weight = 64,

	.dev_rx_weight = 64,

	.sysctl_max_skb_frags = MAX_SKB_FRAGS,