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Date: Wed, 3 Feb 2021 09:20:23 -0800
From: Alexander Duyck <alexander.duyck@...il.com>
To: Wei Wang <weiwan@...gle.com>
Cc: David Miller <davem@...emloft.net>,
Netdev <netdev@...r.kernel.org>,
Jakub Kicinski <kuba@...nel.org>,
Eric Dumazet <edumazet@...gle.com>,
Paolo Abeni <pabeni@...hat.com>,
Hannes Frederic Sowa <hannes@...essinduktion.org>,
Felix Fietkau <nbd@....name>
Subject: Re: [PATCH net-next v9 2/3] net: implement threaded-able napi poll
loop support
On Fri, Jan 29, 2021 at 10:22 AM Wei Wang <weiwan@...gle.com> wrote:
>
> This patch allows running each napi poll loop inside its own
> kernel thread.
> The kthread is created during netif_napi_add() if dev->threaded
> is set. And threaded mode is enabled in napi_enable(). We will
> provide a way to set dev->threaded and enable threaded mode
> without a device up/down in the following patch.
>
> Once that threaded mode is enabled and the kthread is
> started, napi_schedule() will wake-up such thread instead
> of scheduling the softirq.
>
> The threaded poll loop behaves quite likely the net_rx_action,
> but it does not have to manipulate local irqs and uses
> an explicit scheduling point based on netdev_budget.
>
> Co-developed-by: Paolo Abeni <pabeni@...hat.com>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> Co-developed-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Co-developed-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Wei Wang <weiwan@...gle.com>
> ---
> include/linux/netdevice.h | 21 +++----
> net/core/dev.c | 117 ++++++++++++++++++++++++++++++++++++++
> 2 files changed, 124 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 02dcef4d66e2..f1e9fe9017ac 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -347,6 +347,7 @@ struct napi_struct {
> struct list_head dev_list;
> struct hlist_node napi_hash_node;
> unsigned int napi_id;
> + struct task_struct *thread;
> };
>
> enum {
> @@ -358,6 +359,7 @@ enum {
> NAPI_STATE_NO_BUSY_POLL, /* Do not add in napi_hash, no busy polling */
> NAPI_STATE_IN_BUSY_POLL, /* sk_busy_loop() owns this NAPI */
> NAPI_STATE_PREFER_BUSY_POLL, /* prefer busy-polling over softirq processing*/
> + NAPI_STATE_THREADED, /* The poll is performed inside its own thread*/
> };
>
> enum {
> @@ -369,6 +371,7 @@ enum {
> NAPIF_STATE_NO_BUSY_POLL = BIT(NAPI_STATE_NO_BUSY_POLL),
> NAPIF_STATE_IN_BUSY_POLL = BIT(NAPI_STATE_IN_BUSY_POLL),
> NAPIF_STATE_PREFER_BUSY_POLL = BIT(NAPI_STATE_PREFER_BUSY_POLL),
> + NAPIF_STATE_THREADED = BIT(NAPI_STATE_THREADED),
> };
>
> enum gro_result {
> @@ -503,20 +506,7 @@ static inline bool napi_complete(struct napi_struct *n)
> */
> void napi_disable(struct napi_struct *n);
>
> -/**
> - * napi_enable - enable NAPI scheduling
> - * @n: NAPI context
> - *
> - * Resume NAPI from being scheduled on this context.
> - * Must be paired with napi_disable.
> - */
> -static inline void napi_enable(struct napi_struct *n)
> -{
> - BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state));
> - smp_mb__before_atomic();
> - clear_bit(NAPI_STATE_SCHED, &n->state);
> - clear_bit(NAPI_STATE_NPSVC, &n->state);
> -}
> +void napi_enable(struct napi_struct *n);
>
> /**
> * napi_synchronize - wait until NAPI is not running
> @@ -1826,6 +1816,8 @@ enum netdev_priv_flags {
> *
> * @wol_enabled: Wake-on-LAN is enabled
> *
> + * @threaded: napi threaded mode is enabled
> + *
> * @net_notifier_list: List of per-net netdev notifier block
> * that follow this device when it is moved
> * to another network namespace.
> @@ -2143,6 +2135,7 @@ struct net_device {
> struct lock_class_key *qdisc_running_key;
> bool proto_down;
> unsigned wol_enabled:1;
> + unsigned threaded:1;
>
> struct list_head net_notifier_list;
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 7d23bff03864..743dd69fba19 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -91,6 +91,7 @@
> #include <linux/etherdevice.h>
> #include <linux/ethtool.h>
> #include <linux/skbuff.h>
> +#include <linux/kthread.h>
> #include <linux/bpf.h>
> #include <linux/bpf_trace.h>
> #include <net/net_namespace.h>
> @@ -1493,6 +1494,37 @@ void netdev_notify_peers(struct net_device *dev)
> }
> EXPORT_SYMBOL(netdev_notify_peers);
>
> +static int napi_threaded_poll(void *data);
> +
> +static int napi_kthread_create(struct napi_struct *n)
> +{
> + int err = 0;
> +
> + /* Create and wake up the kthread once to put it in
> + * TASK_INTERRUPTIBLE mode to avoid the blocked task
> + * warning and work with loadavg.
> + */
> + n->thread = kthread_run(napi_threaded_poll, n, "napi/%s-%d",
> + n->dev->name, n->napi_id);
> + if (IS_ERR(n->thread)) {
> + err = PTR_ERR(n->thread);
> + pr_err("kthread_run failed with err %d\n", err);
> + n->thread = NULL;
> + }
> +
> + return err;
> +}
> +
> +static void napi_kthread_stop(struct napi_struct *n)
> +{
> + if (!n->thread)
> + return;
> +
> + kthread_stop(n->thread);
> + clear_bit(NAPI_STATE_THREADED, &n->state);
> + n->thread = NULL;
> +}
> +
So I think the napi_kthread_stop should also be split into two parts
and distributed between the napi_disable and netif_napi_del functions.
We should probably be clearing the NAPI_STATE_THREADED bit in
napi_disable, and freeing the thread in netif_napi_del.
> static int __dev_open(struct net_device *dev, struct netlink_ext_ack *extack)
> {
> const struct net_device_ops *ops = dev->netdev_ops;
> @@ -4252,6 +4284,21 @@ int gro_normal_batch __read_mostly = 8;
> static inline void ____napi_schedule(struct softnet_data *sd,
> struct napi_struct *napi)
> {
> + struct task_struct *thread;
> +
> + if (test_bit(NAPI_STATE_THREADED, &napi->state)) {
> + /* Paired with smp_mb__before_atomic() in
> + * napi_enable(). Use READ_ONCE() to guarantee
> + * a complete read on napi->thread. Only call
> + * wake_up_process() when it's not NULL.
> + */
> + thread = READ_ONCE(napi->thread);
> + if (thread) {
> + wake_up_process(thread);
> + return;
> + }
> + }
> +
> list_add_tail(&napi->poll_list, &sd->poll_list);
> __raise_softirq_irqoff(NET_RX_SOFTIRQ);
> }
> @@ -6720,6 +6767,12 @@ void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
> set_bit(NAPI_STATE_NPSVC, &napi->state);
> list_add_rcu(&napi->dev_list, &dev->napi_list);
> napi_hash_add(napi);
> + /* Create kthread for this napi if dev->threaded is set.
> + * Clear dev->threaded if kthread creation failed so that
> + * threaded mode will not be enabled in napi_enable().
> + */
> + if (dev->threaded && napi_kthread_create(napi))
> + dev->threaded = 0;
> }
> EXPORT_SYMBOL(netif_napi_add);
>
> @@ -6734,12 +6787,31 @@ void napi_disable(struct napi_struct *n)
> msleep(1);
>
> hrtimer_cancel(&n->timer);
> + napi_kthread_stop(n);
>
So I think there may be an issue here since we had netif_napi_add
create the thread, but you are freeing it in napi_kthread_stop if I am
not mistaken. That is why I suggested making this only a clear_bit
call like the ones below to just clear the threaded flag from the
state.
> clear_bit(NAPI_STATE_PREFER_BUSY_POLL, &n->state);
> clear_bit(NAPI_STATE_DISABLE, &n->state);
> }
> EXPORT_SYMBOL(napi_disable);
>
> +/**
> + * napi_enable - enable NAPI scheduling
> + * @n: NAPI context
> + *
> + * Resume NAPI from being scheduled on this context.
> + * Must be paired with napi_disable.
> + */
> +void napi_enable(struct napi_struct *n)
> +{
> + BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state));
> + smp_mb__before_atomic();
> + clear_bit(NAPI_STATE_SCHED, &n->state);
> + clear_bit(NAPI_STATE_NPSVC, &n->state);
> + if (n->dev->threaded && n->thread)
> + set_bit(NAPI_STATE_THREADED, &n->state);
> +}
> +EXPORT_SYMBOL(napi_enable);
> +
> static void flush_gro_hash(struct napi_struct *napi)
> {
> int i;
> @@ -6862,6 +6934,51 @@ static int napi_poll(struct napi_struct *n, struct list_head *repoll)
> return work;
> }
>
> +static int napi_thread_wait(struct napi_struct *napi)
> +{
> + set_current_state(TASK_INTERRUPTIBLE);
> +
> + while (!kthread_should_stop() && !napi_disable_pending(napi)) {
> + if (test_bit(NAPI_STATE_SCHED, &napi->state)) {
> + WARN_ON(!list_empty(&napi->poll_list));
> + __set_current_state(TASK_RUNNING);
> + return 0;
> + }
> +
> + schedule();
> + set_current_state(TASK_INTERRUPTIBLE);
> + }
> + __set_current_state(TASK_RUNNING);
> + return -1;
> +}
> +
> +static int napi_threaded_poll(void *data)
> +{
> + struct napi_struct *napi = data;
> + void *have;
> +
> + while (!napi_thread_wait(napi)) {
> + for (;;) {
> + bool repoll = false;
> +
> + local_bh_disable();
> +
> + have = netpoll_poll_lock(napi);
> + __napi_poll(napi, &repoll);
> + netpoll_poll_unlock(have);
> +
> + __kfree_skb_flush();
> + local_bh_enable();
> +
> + if (!repoll)
> + break;
> +
> + cond_resched();
> + }
> + }
> + return 0;
> +}
> +
> static __latent_entropy void net_rx_action(struct softirq_action *h)
> {
> struct softnet_data *sd = this_cpu_ptr(&softnet_data);
> --
> 2.30.0.365.g02bc693789-goog
>
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