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Date: Mon, 20 Dec 2021 05:57:55 -0800
From: Eric Dumazet <edumazet@...gle.com>
To: Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc: netdev <netdev@...r.kernel.org>,
Jamal Hadi Salim <jhs@...atatu.com>,
Cong Wang <xiyou.wangcong@...il.com>,
Jiri Pirko <jiri@...nulli.us>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Jonathan Lemon <jonathan.lemon@...il.com>,
Alexander Lobakin <alobakin@...me>,
Paolo Abeni <pabeni@...hat.com>,
Talal Ahmad <talalahmad@...gle.com>,
Kevin Hao <haokexin@...il.com>,
Ilias Apalodimas <ilias.apalodimas@...aro.org>,
Kees Cook <keescook@...omium.org>,
Kumar Kartikeya Dwivedi <memxor@...il.com>,
Antoine Tenart <atenart@...nel.org>,
Wei Wang <weiwan@...gle.com>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [net-next v5 1/2] net: sched: use queue_mapping to pick tx queue
On Mon, Dec 20, 2021 at 4:38 AM <xiangxia.m.yue@...il.com> wrote:
>
> From: Tonghao Zhang <xiangxia.m.yue@...il.com>
>
> This patch fixes issue:
> * If we install tc filters with act_skbedit in clsact hook.
> It doesn't work, because netdev_core_pick_tx() overwrites
> queue_mapping.
>
> $ tc filter ... action skbedit queue_mapping 1
>
> And this patch is useful:
> * We can use FQ + EDT to implement efficient policies. Tx queues
> are picked by xps, ndo_select_queue of netdev driver, or skb hash
> in netdev_core_pick_tx(). In fact, the netdev driver, and skb
> hash are _not_ under control. xps uses the CPUs map to select Tx
> queues, but we can't figure out which task_struct of pod/containter
> running on this cpu in most case. We can use clsact filters to classify
> one pod/container traffic to one Tx queue. Why ?
>
> In containter networking environment, there are two kinds of pod/
> containter/net-namespace. One kind (e.g. P1, P2), the high throughput
> is key in these applications. But avoid running out of network resource,
> the outbound traffic of these pods is limited, using or sharing one
> dedicated Tx queues assigned HTB/TBF/FQ Qdisc. Other kind of pods
> (e.g. Pn), the low latency of data access is key. And the traffic is not
> limited. Pods use or share other dedicated Tx queues assigned FIFO Qdisc.
> This choice provides two benefits. First, contention on the HTB/FQ Qdisc
> lock is significantly reduced since fewer CPUs contend for the same queue.
> More importantly, Qdisc contention can be eliminated completely if each
> CPU has its own FIFO Qdisc for the second kind of pods.
>
> There must be a mechanism in place to support classifying traffic based on
> pods/container to different Tx queues. Note that clsact is outside of Qdisc
> while Qdisc can run a classifier to select a sub-queue under the lock.
>
> In general recording the decision in the skb seems a little heavy handed.
> This patch introduces a per-CPU variable, suggested by Eric.
>
> The xmit.skip_txqueue flag is firstly cleared in __dev_queue_xmit().
> - Tx Qdisc may install that skbedit actions, then xmit.skip_txqueue flag
> is set in qdisc->enqueue() though tx queue has been selected in
> netdev_tx_queue_mapping() or netdev_core_pick_tx(). That flag is cleared
> firstly in __dev_queue_xmit(), is useful:
> - Avoid picking Tx queue with netdev_tx_queue_mapping() in next netdev
> in such case: eth0 macvlan - eth0.3 vlan - eth0 ixgbe-phy:
> For example, eth0, macvlan in pod, which root Qdisc install skbedit
> queue_mapping, send packets to eth0.3, vlan in host. In __dev_queue_xmit() of
> eth0.3, clear the flag, does not select tx queue according to skb->queue_mapping
> because there is no filters in clsact or tx Qdisc of this netdev.
> Same action taked in eth0, ixgbe in Host.
> - Avoid picking Tx queue for next packet. If we set xmit.skip_txqueue
> in tx Qdisc (qdisc->enqueue()), the proper way to clear it is clearing it
> in __dev_queue_xmit when processing next packets.
>
> +----+ +----+ +----+
> | P1 | | P2 | | Pn |
> +----+ +----+ +----+
> | | |
> +-----------+-----------+
> |
> | clsact/skbedit
> | MQ
> v
> +-----------+-----------+
> | q0 | q1 | qn
> v v v
> HTB/FQ HTB/FQ ... FIFO
>
> Cc: Jamal Hadi Salim <jhs@...atatu.com>
> Cc: Cong Wang <xiyou.wangcong@...il.com>
> Cc: Jiri Pirko <jiri@...nulli.us>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Jonathan Lemon <jonathan.lemon@...il.com>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Alexander Lobakin <alobakin@...me>
> Cc: Paolo Abeni <pabeni@...hat.com>
> Cc: Talal Ahmad <talalahmad@...gle.com>
> Cc: Kevin Hao <haokexin@...il.com>
> Cc: Ilias Apalodimas <ilias.apalodimas@...aro.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>
> Cc: Antoine Tenart <atenart@...nel.org>
> Cc: Wei Wang <weiwan@...gle.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Suggested-by: Eric Dumazet <edumazet@...gle.com>
I have not suggested this patch, only to not add yet another bit in sk_buff.
> Signed-off-by: Tonghao Zhang <xiangxia.m.yue@...il.com>
> ---
> include/linux/netdevice.h | 19 +++++++++++++++++++
> net/core/dev.c | 7 ++++++-
> net/sched/act_skbedit.c | 4 +++-
> 3 files changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 8b0bdeb4734e..8d02dafb32ba 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3009,6 +3009,7 @@ struct softnet_data {
> /* written and read only by owning cpu: */
> struct {
> u16 recursion;
> + u8 skip_txqueue;
> u8 more;
> } xmit;
> #ifdef CONFIG_RPS
> @@ -4696,6 +4697,24 @@ static inline netdev_tx_t netdev_start_xmit(struct sk_buff *skb, struct net_devi
> return rc;
> }
>
> +static inline void netdev_xmit_skip_txqueue(bool skip)
> +{
> + __this_cpu_write(softnet_data.xmit.skip_txqueue, skip);
> +}
> +
> +static inline bool netdev_xmit_txqueue_skipped(void)
> +{
> + return __this_cpu_read(softnet_data.xmit.skip_txqueue);
> +}
> +
> +static inline struct netdev_queue *
> +netdev_tx_queue_mapping(struct net_device *dev, struct sk_buff *skb)
> +{
> + int qm = skb_get_queue_mapping(skb);
> +
> + return netdev_get_tx_queue(dev, netdev_cap_txqueue(dev, qm));
> +}
> +
> int netdev_class_create_file_ns(const struct class_attribute *class_attr,
> const void *ns);
> void netdev_class_remove_file_ns(const struct class_attribute *class_attr,
> diff --git a/net/core/dev.c b/net/core/dev.c
> index a855e41bbe39..e3f548c54dda 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4048,6 +4048,7 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
> skb_update_prio(skb);
>
> qdisc_pkt_len_init(skb);
> + netdev_xmit_skip_txqueue(false);
> #ifdef CONFIG_NET_CLS_ACT
> skb->tc_at_ingress = 0;
> #endif
> @@ -4073,7 +4074,11 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
> else
> skb_dst_force(skb);
>
> - txq = netdev_core_pick_tx(dev, skb, sb_dev);
> + if (netdev_xmit_txqueue_skipped())
> + txq = netdev_tx_queue_mapping(dev, skb);
> + else
> + txq = netdev_core_pick_tx(dev, skb, sb_dev);
> +
If we really need to add yet another conditional in fast path, I would
suggest using a static key.
Only hosts where SKBEDIT_F_QUEUE_MAPPING is requested would pay the price.
> q = rcu_dereference_bh(txq->qdisc);
>
> trace_net_dev_queue(skb);
> diff --git a/net/sched/act_skbedit.c b/net/sched/act_skbedit.c
> index ceba11b198bb..48504ed3b280 100644
> --- a/net/sched/act_skbedit.c
> +++ b/net/sched/act_skbedit.c
> @@ -58,8 +58,10 @@ static int tcf_skbedit_act(struct sk_buff *skb, const struct tc_action *a,
> }
> }
> if (params->flags & SKBEDIT_F_QUEUE_MAPPING &&
> - skb->dev->real_num_tx_queues > params->queue_mapping)
> + skb->dev->real_num_tx_queues > params->queue_mapping) {
> + netdev_xmit_skip_txqueue(true);
> skb_set_queue_mapping(skb, params->queue_mapping);
> + }
> if (params->flags & SKBEDIT_F_MARK) {
> skb->mark &= ~params->mask;
> skb->mark |= params->mark & params->mask;
> --
> 2.27.0
>
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