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Message-ID: <546c2923-ca6e-00e7-8bcb-3a3eb034a58e@gmail.com>
Date: Sun, 26 Jul 2020 09:49:31 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: Felix Fietkau <nbd@....name>, netdev@...r.kernel.org
Cc: Hillf Danton <hdanton@...a.com>
Subject: Re: [RFC] net: add support for threaded NAPI polling
On 7/26/20 9:31 AM, Felix Fietkau 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).
Note that even with a threaded NAPI, you will not be able to use more than one cpu
to process the traffic.
Also I wonder how this will scale to more than one device using this ?
Say we need 4 NAPI, how the different work queues will mix together ?
We invented years ago RPS and RFS, to be able to spread incoming traffic
to more cpus, for devices having one hardware queue.
>
> Based on a patch by Hillf Danton
>
> Cc: Hillf Danton <hdanton@...a.com>
> Signed-off-by: Felix Fietkau <nbd@....name>
> ---
> include/linux/netdevice.h | 23 ++++++++++++++++++++++
> net/core/dev.c | 40 +++++++++++++++++++++++++++++++++++++++
> 2 files changed, 63 insertions(+)
>
> 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..e140b6a9d5eb 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;
> + }
I do not believe we want to add yet another test in this fast path.
Presumably drivers willing to use thread NAPI can use different interface
and directly call queue_work(), without testing NAPI_STATE_THREADED.
> +
> ____napi_schedule(this_cpu_ptr(&softnet_data), n);
> }
> EXPORT_SYMBOL(__napi_schedule_irqoff);
> @@ -6601,6 +6612,30 @@ static void init_gro_hash(struct napi_struct *napi)
> napi->gro_bitmask = 0;
> }
>
> +static void napi_workfn(struct work_struct *work)
> +{
> + struct napi_struct *n = container_of(work, struct napi_struct, work);
> +
> + for (;;) {
> + if (!test_bit(NAPI_STATE_SCHED, &n->state))
This all looks wrong, some important GRO logic is implemented in napi_poll()
You can not bypass napi_poll()
> + return;
> +
> + if (n->poll(n, n->weight) < n->weight)
> + return;
> +
> + if (!need_resched())
> + continue;
> +
Why not simply using cond_resched() ?
> + /*
> + * have to pay for the latency of task switch even if
> + * napi is scheduled
> + */
> + if (test_bit(NAPI_STATE_SCHED, &n->state))
> + 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 +6656,7 @@ 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);
> }
> @@ -10676,6 +10712,10 @@ 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);
> + BUG_ON(!napi_workq);
> +
> dev_boot_phase = 0;
>
> /* The loopback device is special if any other network devices
>
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