Currently pfifo_fast has both TCQ_F_CAN_BYPASS and TCQ_F_NOLOCK flag set, but queue discipline by-pass does not work for lockless qdisc because skb is always enqueued to qdisc even when the qdisc is empty, see __dev_xmit_skb().

This patch calls sch_direct_xmit() to transmit the skb directly to the driver for empty lockless qdisc, which aviod enqueuing and dequeuing operation.

As qdisc->empty is not reliable to indicate a empty qdisc because there is a time window between enqueuing and setting qdisc->empty. So we use the MISSED state added in commit a90c57f2cedd ("net: sched: fix packet stuck problem for lockless qdisc"), which indicate there is lock contention, suggesting that it is better not to do the qdisc bypass in order to avoid packet out of order problem.

In order to make MISSED state reliable to indicate a empty qdisc, we need to ensure that testing and clearing of MISSED state is within the protection of qdisc->seqlock, only setting MISSED state can be done without the protection of qdisc->seqlock. A MISSED state testing is added without the protection of qdisc->seqlock to aviod doing unnecessary spin_trylock() for contention case.

As the enqueuing is not within the protection of qdisc->seqlock, there is still a potential data race as pointed by Jakub [1]:

thread1 thread2 thread3 qdisc_run_begin() # true qdisc_run_begin(q) set(MISSED) pfifo_fast_dequeue clear(MISSED) # recheck the queue qdisc_run_end() enqueue skb1 qdisc empty # true qdisc_run_begin() # true sch_direct_xmit() # skb2 qdisc_run_begin() set(MISSED)

When above happens, skb1 enqueued by thread2 is transmited after skb2 is transmited by thread3 because MISSED state setting and enqueuing is not under the qdisc->seqlock. If qdisc bypass is disabled, skb1 has better chance to be transmited quicker than skb2

This patch does not take care of the above data race, because we view this as similar as below: Even at the same time CPU1 and CPU2 write the skb to two socket which both heading to the same qdisc, there is no guarantee that which skb will hit the qdisc first, because there is a lot of factor like interrupt/softirq/cache miss/scheduling afffecting that.

There are below cases that need special handling: 1. When MISSED state is cleared before another round of dequeuing in pfifo_fast_dequeue(), and __qdisc_run() might not be able to dequeue all skb in one round and call __netif_schedule(), which might result in a non-empty qdisc without MISSED set. In order to avoid this, the MISSED state is set for lockless qdisc and __netif_schedule() will be called at the end of qdisc_run_end.

2. The MISSED state also need to be set for lockless qdisc instead of calling __netif_schedule() directly when requeuing a skb for a similar reason.

3. For netdev queue stopped case, the MISSED case need clearing while the netdev queue is stopped, otherwise there may be unnecessary __netif_schedule() calling. So a new DRAINING state is added to indicate this case, which also indicate a non-empty qdisc.

4. As there is already netif_xmit_frozen_or_stopped() checking in dequeue_skb() and sch_direct_xmit(), which are both within the protection of qdisc->seqlock, but the same checking in __dev_xmit_skb() is without the protection, which might cause empty indication of a lockless qdisc to be not reliable. So remove the checking in __dev_xmit_skb(), and the checking in the protection of qdisc->seqlock seems enough to avoid the cpu consumption problem for netdev queue stopped case.

1. https://lkml.org/lkml/2021/5/29/215 Signed-off-by: Yunsheng Lin linyunsheng@huawei.com --- V2: Adjust the comment and commit log according to discussion in V1. V1: Add nolock_qdisc_is_empty() and do the qdisc empty checking without the protection of qdisc->seqlock to aviod doing unnecessary spin_trylock() for contention case. RFC v4: Use STATE_MISSED and STATE_DRAINING to indicate non-empty qdisc. --- include/net/sch_generic.h | 16 +++++++++++++--- net/core/dev.c | 27 +++++++++++++++++++++++++-- net/sched/sch_generic.c | 20 ++++++++++++++++---- 3 files changed, 54 insertions(+), 9 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h index 3ed6bcc..177f240 100644 --- a/include/net/sch_generic.h +++ b/include/net/sch_generic.h @@ -37,8 +37,15 @@ enum qdisc_state_t { __QDISC_STATE_SCHED, __QDISC_STATE_DEACTIVATED, __QDISC_STATE_MISSED, + __QDISC_STATE_DRAINING, };

+#define QDISC_STATE_MISSED BIT(__QDISC_STATE_MISSED) +#define QDISC_STATE_DRAINING BIT(__QDISC_STATE_DRAINING) + +#define QDISC_STATE_NON_EMPTY (QDISC_STATE_MISSED | \ + QDISC_STATE_DRAINING) + struct qdisc_size_table { struct rcu_head rcu; struct list_head list; @@ -145,6 +152,11 @@ static inline bool qdisc_is_running(struct Qdisc *qdisc) return (raw_read_seqcount(&qdisc->running) & 1) ? true : false; }

+static inline bool nolock_qdisc_is_empty(const struct Qdisc *qdisc) +{ + return !(READ_ONCE(qdisc->state) & QDISC_STATE_NON_EMPTY); +} + static inline bool qdisc_is_percpu_stats(const struct Qdisc *q) { return q->flags & TCQ_F_CPUSTATS; @@ -206,10 +218,8 @@ static inline void qdisc_run_end(struct Qdisc *qdisc) spin_unlock(&qdisc->seqlock);

if (unlikely(test_bit(__QDISC_STATE_MISSED, - &qdisc->state))) { - clear_bit(__QDISC_STATE_MISSED, &qdisc->state); + &qdisc->state))) __netif_schedule(qdisc); - } } else { write_seqcount_end(&qdisc->running); } diff --git a/net/core/dev.c b/net/core/dev.c index 50531a2..991d09b 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -3852,10 +3852,33 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q, qdisc_calculate_pkt_len(skb, q);

if (q->flags & TCQ_F_NOLOCK) { + if (q->flags & TCQ_F_CAN_BYPASS && nolock_qdisc_is_empty(q) && + qdisc_run_begin(q)) { + /* Retest nolock_qdisc_is_empty() within the protection + * of q->seqlock to protect from racing with requeuing. + */ + if (unlikely(!nolock_qdisc_is_empty(q))) { + rc = q->enqueue(skb, q, &to_free) & + NET_XMIT_MASK; + __qdisc_run(q); + qdisc_run_end(q); + + goto no_lock_out; + } + + qdisc_bstats_cpu_update(q, skb); + if (sch_direct_xmit(skb, q, dev, txq, NULL, true) && + !nolock_qdisc_is_empty(q)) + __qdisc_run(q); + + qdisc_run_end(q); + return NET_XMIT_SUCCESS; + } + rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK; - if (likely(!netif_xmit_frozen_or_stopped(txq))) - qdisc_run(q); + qdisc_run(q);

+no_lock_out: if (unlikely(to_free)) kfree_skb_list(to_free); return rc; diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c index fc8b56b..83d7f5f 100644 --- a/net/sched/sch_generic.c +++ b/net/sched/sch_generic.c @@ -52,6 +52,8 @@ static void qdisc_maybe_clear_missed(struct Qdisc *q, */ if (!netif_xmit_frozen_or_stopped(txq)) set_bit(__QDISC_STATE_MISSED, &q->state); + else + set_bit(__QDISC_STATE_DRAINING, &q->state); }

/* Main transmission queue. */ @@ -164,9 +166,13 @@ static inline void dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)

skb = next; } - if (lock) + + if (lock) { spin_unlock(lock); - __netif_schedule(q); + set_bit(__QDISC_STATE_MISSED, &q->state); + } else { + __netif_schedule(q); + } }

static void try_bulk_dequeue_skb(struct Qdisc *q, @@ -409,7 +415,11 @@ void __qdisc_run(struct Qdisc *q) while (qdisc_restart(q, &packets)) { quota -= packets; if (quota <= 0) { - __netif_schedule(q); + if (q->flags & TCQ_F_NOLOCK) + set_bit(__QDISC_STATE_MISSED, &q->state); + else + __netif_schedule(q); + break; } } @@ -680,13 +690,14 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc) if (likely(skb)) { qdisc_update_stats_at_dequeue(qdisc, skb); } else if (need_retry && - test_bit(__QDISC_STATE_MISSED, &qdisc->state)) { + READ_ONCE(qdisc->state) & QDISC_STATE_NON_EMPTY) { /* Delay clearing the STATE_MISSED here to reduce * the overhead of the second spin_trylock() in * qdisc_run_begin() and __netif_schedule() calling * in qdisc_run_end(). */ clear_bit(__QDISC_STATE_MISSED, &qdisc->state); + clear_bit(__QDISC_STATE_DRAINING, &qdisc->state);

/* Make sure dequeuing happens after clearing * STATE_MISSED. @@ -1204,6 +1215,7 @@ static void dev_reset_queue(struct net_device *dev, spin_unlock_bh(qdisc_lock(qdisc)); if (nolock) { clear_bit(__QDISC_STATE_MISSED, &qdisc->state); + clear_bit(__QDISC_STATE_DRAINING, &qdisc->state); spin_unlock_bh(&qdisc->seqlock); } }