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Date: Mon, 3 Aug 2009 21:38:47 +0200
From: Jarek Poplawski <jarkao2@...il.com>
To: Krishna Kumar2 <krkumar2@...ibm.com>
Cc: davem@...emloft.net, herbert@...dor.apana.org.au, kaber@...sh.net,
netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] Avoid enqueuing skb for default qdiscs
Krishna Kumar2 wrote, On 08/03/2009 11:17 AM:
> It is not going through for some reason though I sent the patch about
> 1.5 hours back. Is it OK if I attach the patch this one time while I figure
> out what needs to be done to fix my mailer to start working again?
>
> Following attachment contains the description and the patch.
>
> thanks,
>
> - KK
>
>
> (See attached file: patch.dont_queue)
Since this quoted part looks very similarly I'm using it here, so
forget my note if something has changed.
>
>
> Krishna Kumar2/India/IBM@...IN wrote on 08/03/2009 01:40:16 PM:
>
>> Krishna Kumar2/India/IBM@...IN
>> 08/03/2009 01:40 PM
>>
>> To
>>
>> davem@...emloft.net
>>
>> cc
>>
>> Krishna Kumar2/India/IBM@...IN, herbert@...dor.apana.org.au,
> kaber@...sh.net,
>> jarkao2@...il.com, netdev@...r.kernel.org
>>
>> Subject
>>
>> [RFC] [PATCH] Avoid enqueuing skb for default qdiscs
>>
>> From: Krishna Kumar <krkumar2@...ibm.com>
>>
>> dev_queue_xmit enqueue's a skb and calls qdisc_run which
>> dequeue's the skb and xmits it. In most cases (after
>> instrumenting the code), the skb that is enqueue'd is the
>> same one that is dequeue'd (unless the queue gets stopped
>> or multiple cpu's write to the same queue and ends in a
>> race with qdisc_run). For default qdiscs, we can remove
>> this path and simply xmit the skb since this is a work
>> conserving queue.
>>
>> The patch uses a new flag - TCQ_F_CAN_BYPASS to identify
>> the default fast queue. I plan to use this flag for the
>> previous patch also (rename if required). The controversial
>> part of the patch is incrementing qlen when a skb is
>> requeued, this is to avoid checks like the second line below:
>>
>> + } else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&
>>>> THIS LINE: !q->gso_skb &&
>> + !test_and_set_bit(__QDISC_STATE_RUNNING, &q->state)) {
>>
>> Results of a 4 hour testing for multiple netperf sessions
>> (1, 2, 4, 8, 12 sessions on a 4 cpu system-X and 1, 2, 4,
>> 8, 16, 32 sessions on a 16 cpu P6). Aggregate Mb/s across
>> the iterations:
>>
>> -----------------------------------------------------------------
>> | System-X | P6
>> -----------------------------------------------------------------
>> Size | ORG BW NEW BW | ORG BW NEW BW
>> -----|---------------------------|-------------------------------
>> 16K | 154264 156234 | 155350 157569
>> 64K | 154364 154825 | 155790 158845
>> 128K | 154644 154803 | 153418 155572
>> 256K | 153882 152007 | 154784 154596
>> -----------------------------------------------------------------
>>
>> Netperf reported Service demand reduced by 15% on the P6 but
>> no noticeable difference on the system-X box.
>>
>> Please review.
>>
>> Thanks,
>>
>> - KK
>>
>> Signed-off-by: Krishna Kumar <krkumar2@...ibm.com>
>> ---
>>
>> include/net/sch_generic.h | 9 +++
>> net/core/dev.c | 46 ++++++++++++-----
>> net/sched/sch_generic.c | 94 +++++++++++++++++++++---------------
>> 3 files changed, 99 insertions(+), 50 deletions(-)
>>
>> diff -ruNp org/include/net/sch_generic.h new/include/net/sch_generic.h
>> --- org/include/net/sch_generic.h 2009-03-24 08:54:16.000000000 +0530
>> +++ new/include/net/sch_generic.h 2009-07-28 19:37:10.000000000 +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;
>> @@ -306,6 +312,9 @@ extern struct Qdisc *qdisc_create_dflt(s
>> struct Qdisc_ops *ops, u32 parentid);
>> extern void qdisc_calculate_pkt_len(struct sk_buff *skb,
>> struct qdisc_size_table *stab);
>> +extern int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
>> + struct net_device *dev, struct netdev_queue *txq,
>> + spinlock_t *root_lock);
Probably this would look better in pkt_sched.h around __qdisc_run()?
>> extern void tcf_destroy(struct tcf_proto *tp);
>> extern void tcf_destroy_chain(struct tcf_proto **fl);
>>
>> diff -ruNp org2/net/core/dev.c new2/net/core/dev.c
>> --- org2/net/core/dev.c 2009-07-27 09:08:24.000000000 +0530
>> +++ new2/net/core/dev.c 2009-07-28 19:37:10.000000000 +0530
>> @@ -1786,6 +1786,38 @@ static struct netdev_queue *dev_pick_tx(
>> return netdev_get_tx_queue(dev, queue_index);
>> }
>>
>> +static inline int __dev_hw_xmit(struct sk_buff *skb, struct Qdisc *q,
I guess, we don't touch "hw" in this function yet?
>> + 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)) {
>> + /*
>> + * This is the default 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);
>> + rc = 0;
- rc = 0;
+ 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 +1891,7 @@ 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_hw_xmit(skb, q, dev, txq);
>> goto out;
>> }
>>
>> diff -ruNp org2/net/sched/sch_generic.c new2/net/sched/sch_generic.c
>> --- org2/net/sched/sch_generic.c 2009-05-25 07:48:07.000000000 +0530
>> +++ new2/net/sched/sch_generic.c 2009-07-28 19:40:39.000000000 +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);
>> @@ -102,45 +100,24 @@ static inline int handle_dev_cpu_collisi
>> 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
>> +/*
>> + * 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 +154,43 @@ 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 */
>> + if (unlikely((skb = dequeue_skb(q)) == NULL))
>> + return 0;
>> +
>> + root_lock = qdisc_lock(q);
>> + dev = qdisc_dev(q);
>> + txq = netdev_get_tx_queue(dev, skb_get_queue_mapping(skb));
>> +
>> + return sch_xmit_direct(skb, q, dev, txq, root_lock);
>> +}
>> +
>> void __qdisc_run(struct Qdisc *q)
>> {
>> unsigned long start_time = jiffies;
>> @@ -547,8 +561,11 @@ 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--;
You don't need this here: qdiscs should zero it in ops->reset().
>> + }
>> }
>> EXPORT_SYMBOL(qdisc_reset);
>>
>> @@ -605,6 +622,9 @@ 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;
"|=" looks nicer to me.
>> } else {
>> qdisc = &noqueue_qdisc;
>> }
I wonder if we shouldn't update bstats yet. Btw, try checkpatch.
Thanks,
Jarek P.
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