From: Sebastian Andrzej Siewior Date: Wed, 30 Mar 2016 13:36:29 +0200 Subject: [PATCH 200/353] net: dev: always take qdisc's busylock in __dev_xmit_skb() Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=07a4adaebe9b2f45b7f9e2a42138e8ca69d2aaeb The root-lock is dropped before dev_hard_start_xmit() is invoked and after setting the __QDISC___STATE_RUNNING bit. If this task is now pushed away by a task with a higher priority then the task with the higher priority won't be able to submit packets to the NIC directly instead they will be enqueued into the Qdisc. The NIC will remain idle until the task(s) with higher priority leave the CPU and the task with lower priority gets back and finishes the job. If we take always the busylock we ensure that the RT task can boost the low-prio task and submit the packet. Signed-off-by: Sebastian Andrzej Siewior --- net/core/dev.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/net/core/dev.c b/net/core/dev.c index 7a316f4ff13c..face03c32651 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -3476,7 +3476,11 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q, * This permits qdisc->running owner to get the lock more * often and dequeue packets faster. */ +#ifdef CONFIG_PREEMPT_RT_FULL + contended = true; +#else contended = qdisc_is_running(q); +#endif if (unlikely(contended)) spin_lock(&q->busylock);