Re: [PATCH v2] net: add support for threaded NAPI polling

[Wei Wang <tracywwnj@gmail.com>]

Nice patch!
One question inline.

On Thu, Aug 6, 2020 at 2:58 AM Felix Fietkau <nbd@nbd.name> wrote:
>
> For some drivers (especially 802.11 drivers), doing a lot of work in the NAPI
> poll function does not perform well. Since NAPI poll is bound to the CPU it
> was scheduled from, we can easily end up with a few very busy CPUs spending
> most of their time in softirq/ksoftirqd and some idle ones.
>
> Introduce threaded NAPI for such drivers based on a workqueue. The API is the
> same except for using netif_threaded_napi_add instead of netif_napi_add.
>
> In my tests with mt76 on MT7621 using threaded NAPI + a thread for tx scheduling
> improves LAN->WLAN bridging throughput by 10-50%. Throughput without threaded
> NAPI is wildly inconsistent, depending on the CPU that runs the tx scheduling
> thread.
>
> With threaded NAPI, throughput seems stable and consistent (and higher than
> the best results I got without it).
>
> Based on a patch by Hillf Danton
>
> Cc: Hillf Danton <hdanton@sina.com>
> Signed-off-by: Felix Fietkau <nbd@nbd.name>
> ---
> Changes since PATCH v1:
> - use WQ_SYSFS to make workqueue configurable from user space
> - cancel work in netif_napi_del
> - add a sysfs file to enable/disable threaded NAPI for a netdev
>
> Changes since RFC v2:
> - fix unused but set variable reported by kbuild test robot
>
> Changes since RFC:
> - disable softirq around threaded poll functions
> - reuse most parts of napi_poll()
> - fix re-schedule condition
>
>  include/linux/netdevice.h |  23 ++++++
>  net/core/dev.c            | 163 ++++++++++++++++++++++++++------------
>  net/core/net-sysfs.c      |  42 ++++++++++
>  3 files changed, 176 insertions(+), 52 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index ac2cd3f49aba..3a39211c7598 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 work_struct      work;
>  };
>
>  enum {
> @@ -357,6 +358,7 @@ enum {
>         NAPI_STATE_HASHED,      /* In NAPI hash (busy polling possible) */
>         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_THREADED,    /* Use threaded NAPI */
>  };
>
>  enum {
> @@ -367,6 +369,7 @@ enum {
>         NAPIF_STATE_HASHED       = BIT(NAPI_STATE_HASHED),
>         NAPIF_STATE_NO_BUSY_POLL = BIT(NAPI_STATE_NO_BUSY_POLL),
>         NAPIF_STATE_IN_BUSY_POLL = BIT(NAPI_STATE_IN_BUSY_POLL),
> +       NAPIF_STATE_THREADED     = BIT(NAPI_STATE_THREADED),
>  };
>
>  enum gro_result {
> @@ -2315,6 +2318,26 @@ static inline void *netdev_priv(const struct net_device *dev)
>  void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
>                     int (*poll)(struct napi_struct *, int), int weight);
>
> +/**
> + *     netif_threaded_napi_add - initialize a NAPI context
> + *     @dev:  network device
> + *     @napi: NAPI context
> + *     @poll: polling function
> + *     @weight: default weight
> + *
> + * This variant of netif_napi_add() should be used from drivers using NAPI
> + * with CPU intensive poll functions.
> + * This will schedule polling from a high priority workqueue that
> + */
> +static inline void netif_threaded_napi_add(struct net_device *dev,
> +                                          struct napi_struct *napi,
> +                                          int (*poll)(struct napi_struct *, int),
> +                                          int weight)
> +{
> +       set_bit(NAPI_STATE_THREADED, &napi->state);
> +       netif_napi_add(dev, napi, poll, weight);
> +}
> +
>  /**
>   *     netif_tx_napi_add - initialize a NAPI context
>   *     @dev:  network device
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 19f1abc26fcd..4b0dbea68a09 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -158,6 +158,7 @@ static DEFINE_SPINLOCK(offload_lock);
>  struct list_head ptype_base[PTYPE_HASH_SIZE] __read_mostly;
>  struct list_head ptype_all __read_mostly;      /* Taps */
>  static struct list_head offload_base __read_mostly;
> +static struct workqueue_struct *napi_workq __read_mostly;
>
>  static int netif_rx_internal(struct sk_buff *skb);
>  static int call_netdevice_notifiers_info(unsigned long val,
> @@ -6286,6 +6287,11 @@ void __napi_schedule(struct napi_struct *n)
>  {
>         unsigned long flags;
>
> +       if (test_bit(NAPI_STATE_THREADED, &n->state)) {
> +               queue_work(napi_workq, &n->work);
> +               return;
> +       }
> +
>         local_irq_save(flags);
>         ____napi_schedule(this_cpu_ptr(&softnet_data), n);
>         local_irq_restore(flags);
> @@ -6333,6 +6339,11 @@ EXPORT_SYMBOL(napi_schedule_prep);
>   */
>  void __napi_schedule_irqoff(struct napi_struct *n)
>  {
> +       if (test_bit(NAPI_STATE_THREADED, &n->state)) {
> +               queue_work(napi_workq, &n->work);
> +               return;
> +       }
> +
>         ____napi_schedule(this_cpu_ptr(&softnet_data), n);
>  }
>  EXPORT_SYMBOL(__napi_schedule_irqoff);
> @@ -6601,6 +6612,95 @@ static void init_gro_hash(struct napi_struct *napi)
>         napi->gro_bitmask = 0;
>  }
>
> +static int __napi_poll(struct napi_struct *n, bool *repoll)
> +{
> +       int work, weight;
> +
> +       weight = n->weight;
> +
> +       /* This NAPI_STATE_SCHED test is for avoiding a race
> +        * with netpoll's poll_napi().  Only the entity which
> +        * obtains the lock and sees NAPI_STATE_SCHED set will
> +        * actually make the ->poll() call.  Therefore we avoid
> +        * accidentally calling ->poll() when NAPI is not scheduled.
> +        */
> +       work = 0;
> +       if (test_bit(NAPI_STATE_SCHED, &n->state)) {
> +               work = n->poll(n, weight);
> +               trace_napi_poll(n, work, weight);
> +       }
> +
> +       if (unlikely(work > weight))
> +               pr_err_once("NAPI poll function %pS returned %d, exceeding its budget of %d.\n",
> +                           n->poll, work, weight);
> +
> +       if (likely(work < weight))
> +               return work;
> +
> +       /* Drivers must not modify the NAPI state if they
> +        * consume the entire weight.  In such cases this code
> +        * still "owns" the NAPI instance and therefore can
> +        * move the instance around on the list at-will.
> +        */
> +       if (unlikely(napi_disable_pending(n))) {
> +               napi_complete(n);
> +               return work;
> +       }
> +
> +       if (n->gro_bitmask) {
> +               /* flush too old packets
> +                * If HZ < 1000, flush all packets.
> +                */
> +               napi_gro_flush(n, HZ >= 1000);
> +       }
> +
> +       gro_normal_list(n);
> +
> +       /* Some drivers may have called napi_schedule
> +        * prior to exhausting their budget.
> +        */
> +       if (unlikely(!list_empty(&n->poll_list))) {
> +               pr_warn_once("%s: Budget exhausted after napi rescheduled\n",
> +                            n->dev ? n->dev->name : "backlog");
> +               return work;
> +       }
> +
> +       *repoll = true;
> +
> +       return work;
> +}
> +
> +static void napi_workfn(struct work_struct *work)
> +{
> +       struct napi_struct *n = container_of(work, struct napi_struct, work);
> +       void *have;
> +
> +       for (;;) {
> +               bool repoll = false;
> +
> +               local_bh_disable();
> +
> +               have = netpoll_poll_lock(n);
> +               __napi_poll(n, &repoll);
> +               netpoll_poll_unlock(have);
> +
> +               local_bh_enable();
> +
> +               if (!repoll)
> +                       return;
> +
> +               if (!need_resched())
> +                       continue;
> +
> +               /*
> +                * have to pay for the latency of task switch even if
> +                * napi is scheduled
> +                */
> +               queue_work(napi_workq, work);
> +               return;
> +       }
> +}
> +
>  void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
>                     int (*poll)(struct napi_struct *, int), int weight)
>  {
> @@ -6621,6 +6721,7 @@ void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
>  #ifdef CONFIG_NETPOLL
>         napi->poll_owner = -1;
>  #endif
> +       INIT_WORK(&napi->work, napi_workfn);
>         set_bit(NAPI_STATE_SCHED, &napi->state);
>         napi_hash_add(napi);
>  }
> @@ -6659,6 +6760,7 @@ static void flush_gro_hash(struct napi_struct *napi)
>  void netif_napi_del(struct napi_struct *napi)
>  {
>         might_sleep();
> +       cancel_work_sync(&napi->work);
>         if (napi_hash_del(napi))
>                 synchronize_net();
>         list_del_init(&napi->dev_list);
> @@ -6671,65 +6773,18 @@ EXPORT_SYMBOL(netif_napi_del);
>
>  static int napi_poll(struct napi_struct *n, struct list_head *repoll)
>  {
> +       bool do_repoll = false;
>         void *have;
> -       int work, weight;
> +       int work;
>
>         list_del_init(&n->poll_list);
>
>         have = netpoll_poll_lock(n);
>
> -       weight = n->weight;
> -
> -       /* This NAPI_STATE_SCHED test is for avoiding a race
> -        * with netpoll's poll_napi().  Only the entity which
> -        * obtains the lock and sees NAPI_STATE_SCHED set will
> -        * actually make the ->poll() call.  Therefore we avoid
> -        * accidentally calling ->poll() when NAPI is not scheduled.
> -        */
> -       work = 0;
> -       if (test_bit(NAPI_STATE_SCHED, &n->state)) {
> -               work = n->poll(n, weight);
> -               trace_napi_poll(n, work, weight);
> -       }
> -
> -       if (unlikely(work > weight))
> -               pr_err_once("NAPI poll function %pS returned %d, exceeding its budget of %d.\n",
> -                           n->poll, work, weight);
> -
> -       if (likely(work < weight))
> -               goto out_unlock;
> -
> -       /* Drivers must not modify the NAPI state if they
> -        * consume the entire weight.  In such cases this code
> -        * still "owns" the NAPI instance and therefore can
> -        * move the instance around on the list at-will.
> -        */
> -       if (unlikely(napi_disable_pending(n))) {
> -               napi_complete(n);
> -               goto out_unlock;
> -       }
> -
> -       if (n->gro_bitmask) {
> -               /* flush too old packets
> -                * If HZ < 1000, flush all packets.
> -                */
> -               napi_gro_flush(n, HZ >= 1000);
> -       }
> -
> -       gro_normal_list(n);
> -
> -       /* Some drivers may have called napi_schedule
> -        * prior to exhausting their budget.
> -        */
> -       if (unlikely(!list_empty(&n->poll_list))) {
> -               pr_warn_once("%s: Budget exhausted after napi rescheduled\n",
> -                            n->dev ? n->dev->name : "backlog");
> -               goto out_unlock;
> -       }
> -
> -       list_add_tail(&n->poll_list, repoll);
> +       work = __napi_poll(n, &do_repoll);
> +       if (do_repoll)
> +               list_add_tail(&n->poll_list, repoll);
>
> -out_unlock:
>         netpoll_poll_unlock(have);
>
>         return work;
> @@ -10676,6 +10731,10 @@ static int __init net_dev_init(void)
>                 sd->backlog.weight = weight_p;
>         }
>
> +       napi_workq = alloc_workqueue("napi_workq", WQ_UNBOUND | WQ_HIGHPRI,
> +                                    WQ_UNBOUND_MAX_ACTIVE | WQ_SYSFS);
> +       BUG_ON(!napi_workq);
> +
>         dev_boot_phase = 0;
>
>         /* The loopback device is special if any other network devices
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index e353b822bb15..99233e86f4c5 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c
> @@ -471,6 +471,47 @@ static ssize_t proto_down_store(struct device *dev,
>  }
>  NETDEVICE_SHOW_RW(proto_down, fmt_dec);
>
> +static int change_napi_threaded(struct net_device *dev, unsigned long val)
> +{
> +       struct napi_struct *napi;
> +
> +       if (list_empty(&dev->napi_list))
> +               return -EOPNOTSUPP;
> +
> +       list_for_each_entry(napi, &dev->napi_list, dev_list) {
> +               if (val)
> +                       set_bit(NAPI_STATE_THREADED, &napi->state);
> +               else
> +                       clear_bit(NAPI_STATE_THREADED, &napi->state);
> +       }
> +
> +       return 0;
> +}

I am wondering if there will be issues to modify NAPI_STATE_THREADED
when the napi is currently being queued in sd->poll_list. I think it
could happen, right?
In this case, this napi will be processed in softirq context later.
But in the meanwhile, it is possible that this napi also gets queued
to the work_queue. Will there be any issues with it?
Thinking about it more, since __napi_poll() checks for
NAPI_STATE_SCHED bit before calling the poll() function. And the
work_queue handler disables softirq, so it should be OK?

> +
> +static ssize_t napi_threaded_store(struct device *dev,
> +                               struct device_attribute *attr,
> +                               const char *buf, size_t len)
> +{
> +       return netdev_store(dev, attr, buf, len, change_napi_threaded);
> +}
> +
> +static ssize_t napi_threaded_show(struct device *dev,
> +                                 struct device_attribute *attr,
> +                                 char *buf)
> +{
> +       struct net_device *netdev = to_net_dev(dev);
> +       struct napi_struct *napi;
> +       bool enabled = false;
> +
> +       list_for_each_entry(napi, &netdev->napi_list, dev_list) {
> +               if (test_bit(NAPI_STATE_THREADED, &napi->state))
> +                       enabled = true;
> +       }
> +
> +       return sprintf(buf, fmt_dec, enabled);
> +}
> +DEVICE_ATTR_RW(napi_threaded);
> +
>  static ssize_t phys_port_id_show(struct device *dev,
>                                  struct device_attribute *attr, char *buf)
>  {
> @@ -563,6 +604,7 @@ static struct attribute *net_class_attrs[] __ro_after_init = {
>         &dev_attr_tx_queue_len.attr,
>         &dev_attr_gro_flush_timeout.attr,
>         &dev_attr_napi_defer_hard_irqs.attr,
> +       &dev_attr_napi_threaded.attr,
>         &dev_attr_phys_port_id.attr,
>         &dev_attr_phys_port_name.attr,
>         &dev_attr_phys_switch_id.attr,
> --
> 2.28.0
>