Subject: net: dev: always take qdisc's busylock in __dev_xmit_skb() From: Sebastian Andrzej Siewior Date: Wed Mar 30 13:36:29 2016 +0200 From: Sebastian Andrzej Siewior 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 Signed-off-by: Thomas Gleixner --- net/core/dev.c | 4 ++++ 1 file changed, 4 insertions(+) --- --- a/net/core/dev.c +++ b/net/core/dev.c @@ -3825,7 +3825,11 @@ static inline int __dev_xmit_skb(struct * This permits qdisc->running owner to get the lock more * often and dequeue packets faster. */ +#ifdef CONFIG_PREEMPT_RT + contended = true; +#else contended = qdisc_is_running(q); +#endif if (unlikely(contended)) spin_lock(&q->busylock);