From 3bcb846ca4cf55415d3719e64bb45a124792c589 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Sat, 4 Jun 2016 20:02:28 -0700 Subject: [PATCH] net: get rid of spin_trylock() in net_tx_action() Note: Tom Herbert posted almost same patch 3 months back, but for different reasons. The reasons we want to get rid of this spin_trylock() are : 1) Under high qdisc pressure, the spin_trylock() has almost no chance to succeed. 2) We loop multiple times in softirq handler, eventually reaching the max retry count (10), and we schedule ksoftirqd. Since we want to adhere more strictly to ksoftirqd being waked up in the future (https://lwn.net/Articles/687617/), better avoid spurious wakeups. 3) calls to __netif_reschedule() dirty the cache line containing q->next_sched, slowing down the owner of qdisc. 4) RT kernels can not use the spin_trylock() here. With help of busylock, we get the qdisc spinlock fast enough, and the trylock trick brings only performance penalty. Depending on qdisc setup, I observed a gain of up to 19 % in qdisc performance (1016600 pps instead of 853400 pps, using prio+tbf+fq_codel) ("mpstat -I SCPU 1" is much happier now) Signed-off-by: Eric Dumazet Cc: Tom Herbert Acked-by: Tom Herbert Signed-off-by: David S. Miller --- net/core/dev.c | 26 +++++++++----------------- 1 file changed, 9 insertions(+), 17 deletions(-) diff --git a/net/core/dev.c b/net/core/dev.c index 904ff431d570..896b686d1966 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -2253,7 +2253,7 @@ int netif_get_num_default_rss_queues(void) } EXPORT_SYMBOL(netif_get_num_default_rss_queues); -static inline void __netif_reschedule(struct Qdisc *q) +static void __netif_reschedule(struct Qdisc *q) { struct softnet_data *sd; unsigned long flags; @@ -3898,22 +3898,14 @@ static void net_tx_action(struct softirq_action *h) head = head->next_sched; root_lock = qdisc_lock(q); - if (spin_trylock(root_lock)) { - smp_mb__before_atomic(); - clear_bit(__QDISC_STATE_SCHED, - &q->state); - qdisc_run(q); - spin_unlock(root_lock); - } else { - if (!test_bit(__QDISC_STATE_DEACTIVATED, - &q->state)) { - __netif_reschedule(q); - } else { - smp_mb__before_atomic(); - clear_bit(__QDISC_STATE_SCHED, - &q->state); - } - } + spin_lock(root_lock); + /* We need to make sure head->next_sched is read + * before clearing __QDISC_STATE_SCHED + */ + smp_mb__before_atomic(); + clear_bit(__QDISC_STATE_SCHED, &q->state); + qdisc_run(q); + spin_unlock(root_lock); } } } -- 2.30.2