From ffe4ccd359d006eba559cb1a3c6113144b7fb38c Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 7 Jan 2026 10:41:59 +0000 Subject: 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: 100dfa74cad9 ("net: dev_queue_xmit() llist adoption") Reported-and-bisected-by: Neal Cardwell Signed-off-by: Eric Dumazet Reviewed-by: Neal Cardwell Link: https://patch.msgid.link/20260107104159.3669285-1-edumazet@google.com Signed-off-by: Paolo Abeni --- include/net/dropreason-core.h | 6 ++++++ include/net/hotdata.h | 1 + 2 files changed, 7 insertions(+) (limited to 'include/net') diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h index 58d91ccc56e0..a7b7abd66e21 100644 --- a/include/net/dropreason-core.h +++ b/include/net/dropreason-core.h @@ -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. diff --git a/include/net/hotdata.h b/include/net/hotdata.h index 4acec191c54a..6632b1aa7584 100644 --- a/include/net/hotdata.h +++ b/include/net/hotdata.h @@ -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; -- cgit v1.2.3