Message-ID: <OF5F51C767.3D9CCC17-ON65257608.00598327-65257608.0059D467@in.ibm.com>
Date:	Tue, 4 Aug 2009 21:51:10 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	davem@...emloft.net, herbert@...dor.apana.org.au,
	Jarek Poplawski <jarkao2@...il.com>, kaber@...sh.net,
	netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] Avoid enqueuing skb for default qdiscs

I will send out a final version after any more comments with a fix for a
typo:

-static inline int __qdisc_update_bstats(struct Qdisc *sch, unsigned int
len)
+static inline void __qdisc_update_bstats(struct Qdisc *sch, unsigned int
len)

Thanks,

- KK

Krishna Kumar2/India/IBM@...IN wrote on 08/04/2009 09:21:35 PM:
> Hi Jarek,
>
> (Sorry that you are getting this mail twice since the mail didn't go out
to
> others, but this time I have fixed my mailer)
>
> > Actually I meant:
> > -      qdisc->q.qlen--;
> > +      qdisc->q.qlen = 0;
> > because you can do it after another qdisc->q.qlen = 0.
>
> You are right - I saw some of the sched resets setting it to zero (while
others
> dont touch it). I will reset to zero.
>
> > > > I wonder if we shouldn't update bstats yet. Btw, try checkpatch.
> > >
> > > Rate estimator isn't used for default qdisc, so is anything
> > > required?
> >
> > I meant "Sent" stats from "tc -s qdisc sh" could be misleading.
>
> You are right - for a single netperf run, the stats magically show 0
> packets/bytes sent!
>
> > > Also, checkpatch suggested to change the original code:
> > >         "if (unlikely((skb = dequeue_skb(q)) == NULL))"
> > > since it appears as "new code", hope that change is fine.
> >
> > I mainly meant a whitespace warning.
>
> Yes, I had fixed that also.
>
> A compile and single-netperf-run tested patch is inlined below - the only
> two changes from last patch is setting qlen=0 and updating bstats. Now
> the stats show:
>
> qdisc pfifo_fast 0: root bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
>  Sent 10170288228 bytes 157522 pkt (dropped 0, overlimits 0 requeues 0)
>  rate 0bit 0pps backlog 0b 0p requeues 0
>
> On the original kernel, I got:
> qdisc pfifo_fast 0: root bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
>  Sent 10157266798 bytes 177539 pkt (dropped 0, overlimits 0 requeues 0)
>  rate 0bit 0pps backlog 0b 0p requeues 0
>
> Thanks,
>
> - KK
>
> Signed-off-by: Krishna Kumar <krkumar2@...ibm.com>
> ---
>  include/net/pkt_sched.h   |    3 +
>  include/net/sch_generic.h |   15 +++++
>  net/core/dev.c            |   51 ++++++++++++++-----
>  net/sched/sch_generic.c   |   93 ++++++++++++++++++++++--------------
>  4 files changed, 111 insertions(+), 51 deletions(-)
>
> diff -ruNp org2/include/net/pkt_sched.h new5/include/net/pkt_sched.h
> --- org2/include/net/pkt_sched.h   2009-08-04 11:41:21.000000000 +0530
> +++ new5/include/net/pkt_sched.h   2009-08-04 18:47:16.000000000 +0530
> @@ -87,6 +87,9 @@ extern struct qdisc_rate_table *qdisc_ge
>  extern void qdisc_put_rtab(struct qdisc_rate_table *tab);
>  extern void qdisc_put_stab(struct qdisc_size_table *tab);
>  extern void qdisc_warn_nonwc(char *txt, struct Qdisc *qdisc);
> +extern int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
> +            struct net_device *dev, struct netdev_queue *txq,
> +            spinlock_t *root_lock);
>
>  extern void __qdisc_run(struct Qdisc *q);
>
> diff -ruNp org2/include/net/sch_generic.h new5/include/net/sch_generic.h
> --- org2/include/net/sch_generic.h   2009-03-24 08:54:16.000000000 +0530
> +++ new5/include/net/sch_generic.h   2009-08-04 19:58:26.000000000 +0530
> @@ -45,6 +45,7 @@ struct Qdisc
>  #define TCQ_F_BUILTIN      1
>  #define TCQ_F_THROTTLED      2
>  #define TCQ_F_INGRESS      4
> +#define TCQ_F_CAN_BYPASS   8
>  #define TCQ_F_WARN_NONWC   (1 << 16)
>     int         padded;
>     struct Qdisc_ops   *ops;
> @@ -182,6 +183,11 @@ struct qdisc_skb_cb {
>     char         data[];
>  };
>
> +static inline int qdisc_qlen(struct Qdisc *q)
> +{
> +   return q->q.qlen;
> +}
> +
>  static inline struct qdisc_skb_cb *qdisc_skb_cb(struct sk_buff *skb)
>  {
>     return (struct qdisc_skb_cb *)skb->cb;
> @@ -387,13 +393,18 @@ static inline int qdisc_enqueue_root(str
>     return qdisc_enqueue(skb, sch) & NET_XMIT_MASK;
>  }
>
> +static inline int __qdisc_update_bstats(struct Qdisc *sch, unsigned int
len)
> +{
> +   sch->bstats.bytes += len;
> +   sch->bstats.packets++;
> +}
> +
>  static inline int __qdisc_enqueue_tail(struct sk_buff *skb, struct Qdisc
*sch,
>                     struct sk_buff_head *list)
>  {
>     __skb_queue_tail(list, skb);
>     sch->qstats.backlog += qdisc_pkt_len(skb);
> -   sch->bstats.bytes += qdisc_pkt_len(skb);
> -   sch->bstats.packets++;
> +   __qdisc_update_bstats(sch, qdisc_pkt_len(skb));
>
>     return NET_XMIT_SUCCESS;
>  }
> diff -ruNp org2/net/core/dev.c new5/net/core/dev.c
> --- org2/net/core/dev.c   2009-07-27 09:08:24.000000000 +0530
> +++ new5/net/core/dev.c   2009-08-04 20:26:57.000000000 +0530
> @@ -1786,6 +1786,43 @@ static struct netdev_queue *dev_pick_tx(
>     return netdev_get_tx_queue(dev, queue_index);
>  }
>
> +static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
> +             struct net_device *dev,
> +             struct netdev_queue *txq)
> +{
> +   spinlock_t *root_lock = qdisc_lock(q);
> +   int rc;
> +
> +   spin_lock(root_lock);
> +   if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
> +      kfree_skb(skb);
> +      rc = NET_XMIT_DROP;
> +   } else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&
> +         !test_and_set_bit(__QDISC_STATE_RUNNING, &q->state)) {
> +      unsigned int len = skb->len;
> +
> +      /*
> +       * This is a work-conserving queue; there are no old skbs
> +       * waiting to be sent out; and the qdisc is not running -
> +       * xmit the skb directly.
> +       */
> +      if (sch_direct_xmit(skb, q, dev, txq, root_lock))
> +         __qdisc_run(q);
> +      else
> +         clear_bit(__QDISC_STATE_RUNNING, &q->state);
> +
> +      __qdisc_update_bstats(q, len);
> +
> +      rc = NET_XMIT_SUCCESS;
> +   } else {
> +      rc = qdisc_enqueue_root(skb, q);
> +      qdisc_run(q);
> +   }
> +   spin_unlock(root_lock);
> +
> +   return rc;
> +}
> +
>  /**
>   *   dev_queue_xmit - transmit a buffer
>   *   @skb: buffer to transmit
> @@ -1859,19 +1896,7 @@ gso:
>     skb->tc_verd = SET_TC_AT(skb->tc_verd,AT_EGRESS);
>  #endif
>     if (q->enqueue) {
> -      spinlock_t *root_lock = qdisc_lock(q);
> -
> -      spin_lock(root_lock);
> -
> -      if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
> -         kfree_skb(skb);
> -         rc = NET_XMIT_DROP;
> -      } else {
> -         rc = qdisc_enqueue_root(skb, q);
> -         qdisc_run(q);
> -      }
> -      spin_unlock(root_lock);
> -
> +      rc = __dev_xmit_skb(skb, q, dev, txq);
>        goto out;
>     }
>
> diff -ruNp org2/net/sched/sch_generic.c new5/net/sched/sch_generic.c
> --- org2/net/sched/sch_generic.c   2009-05-25 07:48:07.000000000 +0530
> +++ new5/net/sched/sch_generic.c   2009-08-04 19:38:27.000000000 +0530
> @@ -37,15 +37,11 @@
>   * - updates to tree and tree walking are only done under the rtnl
mutex.
>   */
>
> -static inline int qdisc_qlen(struct Qdisc *q)
> -{
> -   return q->q.qlen;
> -}
> -
>  static inline int dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
>  {
>     q->gso_skb = skb;
>     q->qstats.requeues++;
> +   q->q.qlen++;   /* it's still part of the queue */
>     __netif_schedule(q);
>
>     return 0;
> @@ -61,9 +57,11 @@ static inline struct sk_buff *dequeue_sk
>
>        /* check the reason of requeuing without tx lock first */
>        txq = netdev_get_tx_queue(dev, skb_get_queue_mapping(skb));
> -      if (!netif_tx_queue_stopped(txq) && !netif_tx_queue_frozen(txq))
> +      if (!netif_tx_queue_stopped(txq) &&
> +          !netif_tx_queue_frozen(txq)) {
>           q->gso_skb = NULL;
> -      else
> +         q->q.qlen--;
> +      } else
>           skb = NULL;
>     } else {
>        skb = q->dequeue(q);
> @@ -103,44 +101,23 @@ static inline int handle_dev_cpu_collisi
>  }
>
>  /*
> - * NOTE: Called under qdisc_lock(q) with locally disabled BH.
> - *
> - * __QDISC_STATE_RUNNING guarantees only one CPU can process
> - * this qdisc at a time. qdisc_lock(q) serializes queue accesses for
> - * this queue.
> - *
> - *  netif_tx_lock serializes accesses to device driver.
> - *
> - *  qdisc_lock(q) and netif_tx_lock are mutually exclusive,
> - *  if one is grabbed, another must be free.
> - *
> - * Note, that this procedure can be called by a watchdog timer
> + * Transmit one skb, and handle the return status as required. Holding
the
> + * __QDISC_STATE_RUNNING bit guarantees that only one CPU can execute
this
> + * function.
>   *
>   * Returns to the caller:
>   *            0  - queue is empty or throttled.
>   *            >0 - queue is not empty.
> - *
>   */
> -static inline int qdisc_restart(struct Qdisc *q)
> +int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
> +          struct net_device *dev, struct netdev_queue *txq,
> +          spinlock_t *root_lock)
>  {
> -   struct netdev_queue *txq;
>     int ret = NETDEV_TX_BUSY;
> -   struct net_device *dev;
> -   spinlock_t *root_lock;
> -   struct sk_buff *skb;
> -
> -   /* Dequeue packet */
> -   if (unlikely((skb = dequeue_skb(q)) == NULL))
> -      return 0;
> -
> -   root_lock = qdisc_lock(q);
>
>     /* And release qdisc */
>     spin_unlock(root_lock);
>
> -   dev = qdisc_dev(q);
> -   txq = netdev_get_tx_queue(dev, skb_get_queue_mapping(skb));
> -
>     HARD_TX_LOCK(dev, txq, smp_processor_id());
>     if (!netif_tx_queue_stopped(txq) &&
>         !netif_tx_queue_frozen(txq))
> @@ -177,6 +154,44 @@ static inline int qdisc_restart(struct Q
>     return ret;
>  }
>
> +/*
> + * NOTE: Called under qdisc_lock(q) with locally disabled BH.
> + *
> + * __QDISC_STATE_RUNNING guarantees only one CPU can process
> + * this qdisc at a time. qdisc_lock(q) serializes queue accesses for
> + * this queue.
> + *
> + *  netif_tx_lock serializes accesses to device driver.
> + *
> + *  qdisc_lock(q) and netif_tx_lock are mutually exclusive,
> + *  if one is grabbed, another must be free.
> + *
> + * Note, that this procedure can be called by a watchdog timer
> + *
> + * Returns to the caller:
> + *            0  - queue is empty or throttled.
> + *            >0 - queue is not empty.
> + *
> + */
> +static inline int qdisc_restart(struct Qdisc *q)
> +{
> +   struct netdev_queue *txq;
> +   struct net_device *dev;
> +   spinlock_t *root_lock;
> +   struct sk_buff *skb;
> +
> +   /* Dequeue packet */
> +   skb = dequeue_skb(q);
> +   if (unlikely(!skb))
> +      return 0;
> +
> +   root_lock = qdisc_lock(q);
> +   dev = qdisc_dev(q);
> +   txq = netdev_get_tx_queue(dev, skb_get_queue_mapping(skb));
> +
> +   return sch_direct_xmit(skb, q, dev, txq, root_lock);
> +}
> +
>  void __qdisc_run(struct Qdisc *q)
>  {
>     unsigned long start_time = jiffies;
> @@ -547,8 +562,11 @@ void qdisc_reset(struct Qdisc *qdisc)
>     if (ops->reset)
>        ops->reset(qdisc);
>
> -   kfree_skb(qdisc->gso_skb);
> -   qdisc->gso_skb = NULL;
> +   if (qdisc->gso_skb) {
> +      kfree_skb(qdisc->gso_skb);
> +      qdisc->gso_skb = NULL;
> +      qdisc->q.qlen = 0;
> +   }
>  }
>  EXPORT_SYMBOL(qdisc_reset);
>
> @@ -605,6 +623,9 @@ static void attach_one_default_qdisc(str
>           printk(KERN_INFO "%s: activation failed\n", dev->name);
>           return;
>        }
> +
> +      /* Can by-pass the queue discipline for default qdisc */
> +      qdisc->flags |= TCQ_F_CAN_BYPASS;
>     } else {
>        qdisc =  &noqueue_qdisc;
>     }

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