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Date: Thu, 18 Mar 2021 08:10:27 +0100
From: Ahmad Fatoum <a.fatoum@...gutronix.de>
To: Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
kuba@...nel.org, olteanv@...il.com
Cc: ast@...nel.org, daniel@...earbox.net, andriin@...com,
edumazet@...gle.com, weiwan@...gle.com, cong.wang@...edance.com,
ap420073@...il.com, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, linuxarm@...neuler.org,
mkl@...gutronix.de, linux-can@...r.kernel.org
Subject: Re: [RFC v2] net: sched: implement TCQ_F_CAN_BYPASS for lockless
qdisc
On 15.03.21 04:10, Yunsheng Lin wrote:
> 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 too, which aviod enqueuing
> and dequeuing operation. qdisc->empty is set to false whenever a
> skb is enqueued, see pfifo_fast_enqueue(), and is set to true when
> skb dequeuing return NULL, see pfifo_fast_dequeue(), a spinlock is
> added to avoid the race between enqueue/dequeue and qdisc->empty
> setting.
>
> If there is requeued skb in q->gso_skb, and qdisc->empty is true,
> do not allow bypassing requeued skb. enqueuing and dequeuing in
> q->gso_skb is always protected by qdisc->seqlock, so is the access
> of q->gso_skb by skb_queue_empty();
>
> Also, qdisc is scheduled at the end of qdisc_run_end() when q->empty
> is false to avoid packet stuck problem.
>
> The performance for ip_forward test increases about 10% with this
> patch.
>
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> ---
> RFC V2: fix requeued skb out of order and data race problem.
cansequence didn't find any frame reordering with 2 FlexCAN's communicating
with each other on a dual core i.MX6. Feel free to add:
Tested-by: Ahmad Fatoum <a.fatoum@...gutronix.de>
> ---
> include/net/pkt_sched.h | 2 ++
> include/net/sch_generic.h | 7 +++++--
> net/core/dev.c | 14 ++++++++++++++
> net/sched/sch_generic.c | 31 ++++++++++++++++++++++++++++++-
> 4 files changed, 51 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
> index f5c1bee..c760f6a 100644
> --- a/include/net/pkt_sched.h
> +++ b/include/net/pkt_sched.h
> @@ -122,6 +122,8 @@ void qdisc_warn_nonwc(const char *txt, struct Qdisc *qdisc);
> bool sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
> struct net_device *dev, struct netdev_queue *txq,
> spinlock_t *root_lock, bool validate);
> +bool sch_may_need_requeuing(struct sk_buff *skb, struct Qdisc *q,
> + struct net_device *dev);
>
> void __qdisc_run(struct Qdisc *q);
>
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index 2d6eb60..6591356 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -161,7 +161,6 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
> if (qdisc->flags & TCQ_F_NOLOCK) {
> if (!spin_trylock(&qdisc->seqlock))
> return false;
> - WRITE_ONCE(qdisc->empty, false);
> } else if (qdisc_is_running(qdisc)) {
> return false;
> }
> @@ -176,8 +175,12 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
> static inline void qdisc_run_end(struct Qdisc *qdisc)
> {
> write_seqcount_end(&qdisc->running);
> - if (qdisc->flags & TCQ_F_NOLOCK)
> + if (qdisc->flags & TCQ_F_NOLOCK) {
> spin_unlock(&qdisc->seqlock);
> +
> + if (unlikely(!READ_ONCE(qdisc->empty)))
> + __netif_schedule(qdisc);
> + }
> }
>
> static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 2bfdd52..8f4afb6 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3791,6 +3791,20 @@ 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 && READ_ONCE(q->empty) &&
> + qdisc_run_begin(q)) {
> + qdisc_bstats_cpu_update(q, skb);
> +
> + if (sch_may_need_requeuing(skb, q, dev))
> + __qdisc_run(q);
> + else if (sch_direct_xmit(skb, q, dev, txq, NULL, true) &&
> + !READ_ONCE(q->empty))
> + __qdisc_run(q);
> +
> + qdisc_run_end(q);
> + return NET_XMIT_SUCCESS;
> + }
> +
> rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
> qdisc_run(q);
>
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 49eae93..0df1462 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -273,6 +273,23 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate,
> return skb;
> }
>
> +bool sch_may_need_requeuing(struct sk_buff *skb, struct Qdisc *q,
> + struct net_device *dev)
> +{
> + bool again = false;
> +
> + if (likely(skb_queue_empty(&q->gso_skb)))
> + return false;
> +
> + /* need validating before requeuing */
> + skb = validate_xmit_skb_list(skb, dev, &again);
> + if (unlikely(!skb))
> + return true;
> +
> + dev_requeue_skb(skb, q);
> + return true;
> +}
> +
> /*
> * Transmit possibly several skbs, and handle the return status as
> * required. Owning running seqcount bit guarantees that
> @@ -606,6 +623,11 @@ static const u8 prio2band[TC_PRIO_MAX + 1] = {
> */
> struct pfifo_fast_priv {
> struct skb_array q[PFIFO_FAST_BANDS];
> +
> + /* protect against data race between enqueue/dequeue and
> + * qdisc->empty setting
> + */
> + spinlock_t lock;
> };
>
> static inline struct skb_array *band2list(struct pfifo_fast_priv *priv,
> @@ -623,7 +645,10 @@ static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc,
> unsigned int pkt_len = qdisc_pkt_len(skb);
> int err;
>
> - err = skb_array_produce(q, skb);
> + spin_lock(&priv->lock);
> + err = __ptr_ring_produce(&q->ring, skb);
> + WRITE_ONCE(qdisc->empty, false);
> + spin_unlock(&priv->lock);
>
> if (unlikely(err)) {
> if (qdisc_is_percpu_stats(qdisc))
> @@ -642,6 +667,7 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
> struct sk_buff *skb = NULL;
> int band;
>
> + spin_lock(&priv->lock);
> for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) {
> struct skb_array *q = band2list(priv, band);
>
> @@ -655,6 +681,7 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
> } else {
> WRITE_ONCE(qdisc->empty, true);
> }
> + spin_unlock(&priv->lock);
>
> return skb;
> }
> @@ -739,6 +766,8 @@ static int pfifo_fast_init(struct Qdisc *qdisc, struct nlattr *opt,
>
> /* Can by-pass the queue discipline */
> qdisc->flags |= TCQ_F_CAN_BYPASS;
> +
> + spin_lock_init(&priv->lock);
> return 0;
> }
>
>
--
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