[PATCH net-next v3 0/5] implement kthread based napi poll

[PATCH net-next v3 0/5] implement kthread based napi poll

[Wei Wang]

The idea of moving the napi poll process out of softirq context to a
kernel thread based context is not new.
Paolo Abeni and Hannes Frederic Sowa have proposed patches to move napi
poll to kthread back in 2016. And Felix Fietkau has also proposed
patches of similar ideas to use workqueue to process napi poll just a
few weeks ago.

The main reason we'd like to push forward with this idea is that the
scheduler has poor visibility into cpu cycles spent in softirq context,
and is not able to make optimal scheduling decisions of the user threads.
For example, we see in one of the application benchmark where network
load is high, the CPUs handling network softirqs has ~80% cpu util. And
user threads are still scheduled on those CPUs, despite other more idle
cpus available in the system. And we see very high tail latencies. In this
case, we have to explicitly pin away user threads from the CPUs handling
network softirqs to ensure good performance.
With napi poll moved to kthread, scheduler is in charge of scheduling both
the kthreads handling network load, and the user threads, and is able to
make better decisions. In the previous benchmark, if we do this and we
pin the kthreads processing napi poll to specific CPUs, scheduler is
able to schedule user threads away from these CPUs automatically.

And the reason we prefer 1 kthread per napi, instead of 1 workqueue
entity per host, is that kthread is more configurable than workqueue,
and we could leverage existing tuning tools for threads, like taskset,
chrt, etc to tune scheduling class and cpu set, etc. Another reason is
if we eventually want to provide busy poll feature using kernel threads
for napi poll, kthread seems to be more suitable than workqueue.
Furthermore, for large platforms with 2 NICs attached to 2 sockets,
kthread is more flexible to be pinned to different sets of CPUs.  

In this patch series, I revived Paolo and Hannes's patch in 2016 and
left them as the first 2 patches. Then there are changes proposed by
Felix, Jakub, Paolo and myself on top of those, with suggestions from
Eric Dumazet.

In terms of performance, I ran tcp_rr tests with 1000 flows with
various request/response sizes, with RFS/RPS disabled, and compared
performance between softirq vs kthread vs workqueue (patchset proposed
by Felix Fietkau).
Host has 56 hyper threads and 100Gbps nic, 8 rx queues and only 1 numa
node. All threads are unpinned.

        req/resp   QPS   50%tile    90%tile    99%tile    99.9%tile
softirq   1B/1B   2.75M   337us       376us      1.04ms     3.69ms
kthread   1B/1B   2.67M   371us       408us      455us      550us
workq     1B/1B   2.56M   384us       435us      673us      822us

softirq 5KB/5KB   1.46M   678us       750us      969us      2.78ms
kthread 5KB/5KB   1.44M   695us       789us      891us      1.06ms
workq   5KB/5KB   1.34M   720us       905us     1.06ms      1.57ms

softirq 1MB/1MB   11.0K   79ms       166ms      306ms       630ms
kthread 1MB/1MB   11.0K   75ms       177ms      303ms       596ms
workq   1MB/1MB   11.0K   79ms       180ms      303ms       587ms

When running workqueue implementation, I found the number of threads
used is usually twice as much as kthread implementation. This probably
introduces higher scheduling cost, which results in higher tail
latencies in most cases.

I also ran an application benchmark, which performs fixed qps remote SSD
read/write operations, with various sizes. Again, both with RFS/RPS
disabled.
The result is as follows:
         op_size  QPS   50%tile 95%tile 99%tile 99.9%tile  
softirq   4K     572.6K   385us   1.5ms  3.16ms   6.41ms
kthread   4K     572.6K   390us   803us  2.21ms   6.83ms
workq     4k     572.6K   384us   763us  3.12ms   6.87ms

softirq   64K    157.9K   736us   1.17ms 3.40ms   13.75ms
kthread   64K    157.9K   745us   1.23ms 2.76ms    9.87ms 
workq     64K    157.9K   746us   1.23ms 2.76ms    9.96ms

softirq   1M     10.98K   2.03ms  3.10ms  3.7ms   11.56ms
kthread   1M     10.98K   2.13ms  3.21ms  4.02ms  13.3ms
workq     1M     10.98K   2.13ms  3.20ms  3.99ms  14.12ms

In this set of tests, the latency is predominant by the SSD operation.
Also, the user threads are much busier compared to tcp_rr tests. We have
to pin the kthreads/workqueue threads to limit to a few CPUs, to not
disturb user threads, and provide some isolation.


Changes since v2:
Corrected typo in patch 1, and updated the cover letter with more
detailed and updated test results.

Changes since v1:
Replaced kthread_create() with kthread_run() in patch 5 as suggested by
Felix Fietkau.

Changes since RFC:
Renamed the kthreads to be napi/<dev>-<napi_id> in patch 5 as suggested
by Hannes Frederic Sowa.

Paolo Abeni (2):
  net: implement threaded-able napi poll loop support
  net: add sysfs attribute to control napi threaded mode
Felix Fietkau (1):
  net: extract napi poll functionality to __napi_poll()
Jakub Kicinski (1):
  net: modify kthread handler to use __napi_poll()
Wei Wang (1):
  net: improve napi threaded config

 include/linux/netdevice.h |   5 ++
 net/core/dev.c            | 143 +++++++++++++++++++++++++++++++++++---
 net/core/net-sysfs.c      | 100 ++++++++++++++++++++++++++
 3 files changed, 239 insertions(+), 9 deletions(-)

-- 
2.29.2.454.gaff20da3a2-goog

[PATCH net-next v3 1/5] net: implement threaded-able napi poll loop support

[Wei Wang]

From: Paolo Abeni <pabeni@redhat.com> 

This patch allows running each napi poll loop inside its
own kernel thread.
The rx mode can be enabled per napi instance via the
newly addded napi_set_threaded() api; the requested kthread
will be created on demand and shut down on device stop.

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.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Wei Wang <weiwan@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
---
 include/linux/netdevice.h |   5 ++
 net/core/dev.c            | 113 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 118 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 03433a4c929e..5ba430f56085 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 {
@@ -357,6 +358,7 @@ enum {
 	NAPI_STATE_LISTED,	/* NAPI added to system lists */
 	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,	/* The poll is performed inside its own thread*/
 };
 
 enum {
@@ -367,6 +369,7 @@ enum {
 	NAPIF_STATE_LISTED	 = BIT(NAPI_STATE_LISTED),
 	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 {
@@ -488,6 +491,8 @@ static inline bool napi_complete(struct napi_struct *n)
 	return napi_complete_done(n, 0);
 }
 
+int napi_set_threaded(struct napi_struct *n, bool threaded);
+
 /**
  *	napi_disable - prevent NAPI from scheduling
  *	@n: NAPI context
diff --git a/net/core/dev.c b/net/core/dev.c
index 4bfdcd6b20e8..a5d2ead8be78 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>
@@ -1488,9 +1489,19 @@ void netdev_notify_peers(struct net_device *dev)
 }
 EXPORT_SYMBOL(netdev_notify_peers);
 
+static int napi_threaded_poll(void *data);
+
+static void napi_thread_start(struct napi_struct *n)
+{
+	if (test_bit(NAPI_STATE_THREADED, &n->state) && !n->thread)
+		n->thread = kthread_create(napi_threaded_poll, n, "%s-%d",
+					   n->dev->name, n->napi_id);
+}
+
 static int __dev_open(struct net_device *dev, struct netlink_ext_ack *extack)
 {
 	const struct net_device_ops *ops = dev->netdev_ops;
+	struct napi_struct *n;
 	int ret;
 
 	ASSERT_RTNL();
@@ -1522,6 +1533,9 @@ static int __dev_open(struct net_device *dev, struct netlink_ext_ack *extack)
 	if (!ret && ops->ndo_open)
 		ret = ops->ndo_open(dev);
 
+	list_for_each_entry(n, &dev->napi_list, dev_list)
+		napi_thread_start(n);
+
 	netpoll_poll_enable(dev);
 
 	if (ret)
@@ -1567,6 +1581,14 @@ int dev_open(struct net_device *dev, struct netlink_ext_ack *extack)
 }
 EXPORT_SYMBOL(dev_open);
 
+static void napi_thread_stop(struct napi_struct *n)
+{
+	if (!n->thread)
+		return;
+	kthread_stop(n->thread);
+	n->thread = NULL;
+}
+
 static void __dev_close_many(struct list_head *head)
 {
 	struct net_device *dev;
@@ -1595,6 +1617,7 @@ static void __dev_close_many(struct list_head *head)
 
 	list_for_each_entry(dev, head, close_list) {
 		const struct net_device_ops *ops = dev->netdev_ops;
+		struct napi_struct *n;
 
 		/*
 		 *	Call the device specific close. This cannot fail.
@@ -1606,6 +1629,9 @@ static void __dev_close_many(struct list_head *head)
 		if (ops->ndo_stop)
 			ops->ndo_stop(dev);
 
+		list_for_each_entry(n, &dev->napi_list, dev_list)
+			napi_thread_stop(n);
+
 		dev->flags &= ~IFF_UP;
 		netpoll_poll_enable(dev);
 	}
@@ -4245,6 +4271,11 @@ int gro_normal_batch __read_mostly = 8;
 static inline void ____napi_schedule(struct softnet_data *sd,
 				     struct napi_struct *napi)
 {
+	if (napi->thread) {
+		wake_up_process(napi->thread);
+		return;
+	}
+
 	list_add_tail(&napi->poll_list, &sd->poll_list);
 	__raise_softirq_irqoff(NET_RX_SOFTIRQ);
 }
@@ -6667,6 +6698,30 @@ static void init_gro_hash(struct napi_struct *napi)
 	napi->gro_bitmask = 0;
 }
 
+int napi_set_threaded(struct napi_struct *n, bool threaded)
+{
+	ASSERT_RTNL();
+
+	if (n->dev->flags & IFF_UP)
+		return -EBUSY;
+
+	if (threaded == !!test_bit(NAPI_STATE_THREADED, &n->state))
+		return 0;
+	if (threaded)
+		set_bit(NAPI_STATE_THREADED, &n->state);
+	else
+		clear_bit(NAPI_STATE_THREADED, &n->state);
+
+	/* if the device is initializing, nothing todo */
+	if (test_bit(__LINK_STATE_START, &n->dev->state))
+		return 0;
+
+	napi_thread_stop(n);
+	napi_thread_start(n);
+	return 0;
+}
+EXPORT_SYMBOL(napi_set_threaded);
+
 void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
 		    int (*poll)(struct napi_struct *, int), int weight)
 {
@@ -6807,6 +6862,64 @@ 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)) {
+			__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;
+
+	while (!napi_thread_wait(napi)) {
+		struct list_head dummy_repoll;
+		int budget = netdev_budget;
+		unsigned long time_limit;
+		bool again = true;
+
+		INIT_LIST_HEAD(&dummy_repoll);
+		local_bh_disable();
+		time_limit = jiffies + 2;
+		do {
+			/* ensure that the poll list is not empty */
+			if (list_empty(&dummy_repoll))
+				list_add(&napi->poll_list, &dummy_repoll);
+
+			budget -= napi_poll(napi, &dummy_repoll);
+			if (unlikely(budget <= 0 ||
+				     time_after_eq(jiffies, time_limit))) {
+				cond_resched();
+
+				/* refresh the budget */
+				budget = netdev_budget;
+				__kfree_skb_flush();
+				time_limit = jiffies + 2;
+			}
+
+			if (napi_disable_pending(napi))
+				again = false;
+			else if (!test_bit(NAPI_STATE_SCHED, &napi->state))
+				again = false;
+		} while (again);
+
+		__kfree_skb_flush();
+		local_bh_enable();
+	}
+	return 0;
+}
+
 static __latent_entropy void net_rx_action(struct softirq_action *h)
 {
 	struct softnet_data *sd = this_cpu_ptr(&softnet_data);
-- 
2.29.2.454.gaff20da3a2-goog

[PATCH net-next v3 2/5] net: add sysfs attribute to control napi threaded mode

[Wei Wang]

From: Paolo Abeni <pabeni@redhat.com> 

this patch adds a new sysfs attribute to the network
device class. Said attribute is a bitmask that allows controlling
the threaded mode for all the napi instances of the given
network device.

The threaded mode can be switched only if related network device
is down.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Wei Wang <weiwan@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
---
 net/core/net-sysfs.c | 103 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 103 insertions(+)

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 94fff0700bdd..df8dd25e5e4b 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -538,6 +538,108 @@ static ssize_t phys_switch_id_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(phys_switch_id);
 
+static unsigned long *__alloc_thread_bitmap(struct net_device *netdev,
+					    int *bits)
+{
+	struct napi_struct *n;
+
+	*bits = 0;
+	list_for_each_entry(n, &netdev->napi_list, dev_list)
+		(*bits)++;
+
+	return kmalloc_array(BITS_TO_LONGS(*bits), sizeof(unsigned long),
+			     GFP_ATOMIC | __GFP_ZERO);
+}
+
+static ssize_t threaded_show(struct device *dev,
+			     struct device_attribute *attr, char *buf)
+{
+	struct net_device *netdev = to_net_dev(dev);
+	struct napi_struct *n;
+	unsigned long *bmap;
+	size_t count = 0;
+	int i, bits;
+
+	if (!rtnl_trylock())
+		return restart_syscall();
+
+	if (!dev_isalive(netdev))
+		goto unlock;
+
+	bmap = __alloc_thread_bitmap(netdev, &bits);
+	if (!bmap) {
+		count = -ENOMEM;
+		goto unlock;
+	}
+
+	i = 0;
+	list_for_each_entry(n, &netdev->napi_list, dev_list) {
+		if (test_bit(NAPI_STATE_THREADED, &n->state))
+			set_bit(i, bmap);
+		i++;
+	}
+
+	count = bitmap_print_to_pagebuf(true, buf, bmap, bits);
+	kfree(bmap);
+
+unlock:
+	rtnl_unlock();
+
+	return count;
+}
+
+static ssize_t threaded_store(struct device *dev,
+			      struct device_attribute *attr,
+			      const char *buf, size_t len)
+{
+	struct net_device *netdev = to_net_dev(dev);
+	struct napi_struct *n;
+	unsigned long *bmap;
+	int i, bits;
+	size_t ret;
+
+	if (!capable(CAP_NET_ADMIN))
+		return -EPERM;
+
+	if (!rtnl_trylock())
+		return restart_syscall();
+
+	if (!dev_isalive(netdev)) {
+		ret = len;
+		goto unlock;
+	}
+
+	if (netdev->flags & IFF_UP) {
+		ret = -EBUSY;
+		goto unlock;
+	}
+
+	bmap = __alloc_thread_bitmap(netdev, &bits);
+	if (!bmap) {
+		ret = -ENOMEM;
+		goto unlock;
+	}
+
+	ret = bitmap_parselist(buf, bmap, bits);
+	if (ret)
+		goto free_unlock;
+
+	i = 0;
+	list_for_each_entry(n, &netdev->napi_list, dev_list) {
+		napi_set_threaded(n, test_bit(i, bmap));
+		i++;
+	}
+	ret = len;
+
+free_unlock:
+	kfree(bmap);
+
+unlock:
+	rtnl_unlock();
+	return ret;
+}
+static DEVICE_ATTR_RW(threaded);
+
 static struct attribute *net_class_attrs[] __ro_after_init = {
 	&dev_attr_netdev_group.attr,
 	&dev_attr_type.attr,
@@ -570,6 +672,7 @@ static struct attribute *net_class_attrs[] __ro_after_init = {
 	&dev_attr_proto_down.attr,
 	&dev_attr_carrier_up_count.attr,
 	&dev_attr_carrier_down_count.attr,
+	&dev_attr_threaded.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(net_class);
-- 
2.29.2.454.gaff20da3a2-goog

[PATCH net-next v3 3/5] net: extract napi poll functionality to __napi_poll()

[Wei Wang]

From: Felix Fietkau <nbd@nbd.name> 

This commit introduces a new function __napi_poll() which does the main
logic of the existing napi_poll() function, and will be called by other
functions in later commits.
This idea and implementation is done by Felix Fietkau <nbd@nbd.name> and
is proposed as part of the patch to move napi work to work_queue
context.
This commit by itself is a code restructure.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Wei Wang <weiwan@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
---
 net/core/dev.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index a5d2ead8be78..a739dbbe4d89 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6796,15 +6796,10 @@ void __netif_napi_del(struct napi_struct *napi)
 }
 EXPORT_SYMBOL(__netif_napi_del);
 
-static int napi_poll(struct napi_struct *n, struct list_head *repoll)
+static int __napi_poll(struct napi_struct *n, bool *repoll)
 {
-	void *have;
 	int work, weight;
 
-	list_del_init(&n->poll_list);
-
-	have = netpoll_poll_lock(n);
-
 	weight = n->weight;
 
 	/* This NAPI_STATE_SCHED test is for avoiding a race
@@ -6824,7 +6819,7 @@ static int napi_poll(struct napi_struct *n, struct list_head *repoll)
 			    n->poll, work, weight);
 
 	if (likely(work < weight))
-		goto out_unlock;
+		return work;
 
 	/* Drivers must not modify the NAPI state if they
 	 * consume the entire weight.  In such cases this code
@@ -6833,7 +6828,7 @@ static int napi_poll(struct napi_struct *n, struct list_head *repoll)
 	 */
 	if (unlikely(napi_disable_pending(n))) {
 		napi_complete(n);
-		goto out_unlock;
+		return work;
 	}
 
 	if (n->gro_bitmask) {
@@ -6845,6 +6840,26 @@ static int napi_poll(struct napi_struct *n, struct list_head *repoll)
 
 	gro_normal_list(n);
 
+	*repoll = true;
+
+	return work;
+}
+
+static int napi_poll(struct napi_struct *n, struct list_head *repoll)
+{
+	bool do_repoll = false;
+	void *have;
+	int work;
+
+	list_del_init(&n->poll_list);
+
+	have = netpoll_poll_lock(n);
+
+	work = __napi_poll(n, &do_repoll);
+
+	if (!do_repoll)
+		goto out_unlock;
+
 	/* Some drivers may have called napi_schedule
 	 * prior to exhausting their budget.
 	 */
-- 
2.29.2.454.gaff20da3a2-goog

[PATCH net-next v3 4/5] net: modify kthread handler to use __napi_poll()

[Wei Wang]

From: Jakub Kicinski <kuba@kernel.org> 

The current kthread handler calls napi_poll() and has to pass a dummy
repoll list to the function, which seems redundent. The new proposed
kthread handler calls the newly proposed __napi_poll(), and respects
napi->weight as before. If repoll is needed, cond_resched() is called
first to give other tasks a chance to run before repolling.
This change is proposed by Jakub Kicinski <kuba@kernel.org> on top of
the previous patch.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Wei Wang <weiwan@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
---
 net/core/dev.c | 62 +++++++++++++++++++-------------------------------
 1 file changed, 24 insertions(+), 38 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index a739dbbe4d89..88437cdf29f1 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6840,6 +6840,15 @@ static int __napi_poll(struct napi_struct *n, bool *repoll)
 
 	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;
@@ -6860,15 +6869,6 @@ static int napi_poll(struct napi_struct *n, struct list_head *repoll)
 	if (!do_repoll)
 		goto out_unlock;
 
-	/* 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);
 
 out_unlock:
@@ -6897,40 +6897,26 @@ static int napi_thread_wait(struct napi_struct *napi)
 static int napi_threaded_poll(void *data)
 {
 	struct napi_struct *napi = data;
+	void *have;
 
 	while (!napi_thread_wait(napi)) {
-		struct list_head dummy_repoll;
-		int budget = netdev_budget;
-		unsigned long time_limit;
-		bool again = true;
+		for (;;) {
+			bool repoll = false;
 
-		INIT_LIST_HEAD(&dummy_repoll);
-		local_bh_disable();
-		time_limit = jiffies + 2;
-		do {
-			/* ensure that the poll list is not empty */
-			if (list_empty(&dummy_repoll))
-				list_add(&napi->poll_list, &dummy_repoll);
-
-			budget -= napi_poll(napi, &dummy_repoll);
-			if (unlikely(budget <= 0 ||
-				     time_after_eq(jiffies, time_limit))) {
-				cond_resched();
-
-				/* refresh the budget */
-				budget = netdev_budget;
-				__kfree_skb_flush();
-				time_limit = jiffies + 2;
-			}
+			local_bh_disable();
 
-			if (napi_disable_pending(napi))
-				again = false;
-			else if (!test_bit(NAPI_STATE_SCHED, &napi->state))
-				again = false;
-		} while (again);
+			have = netpoll_poll_lock(napi);
+			__napi_poll(napi, &repoll);
+			netpoll_poll_unlock(have);
 
-		__kfree_skb_flush();
-		local_bh_enable();
+			__kfree_skb_flush();
+			local_bh_enable();
+
+			if (!repoll)
+				break;
+
+			cond_resched();
+		}
 	}
 	return 0;
 }
-- 
2.29.2.454.gaff20da3a2-goog

[PATCH net-next v3 5/5] net: improve napi threaded config

[Wei Wang]

This commit mainly addresses the threaded config to make the switch
between softirq based and kthread based NAPI processing not require
a device down/up.
It also moves the kthread_run() call to the sysfs handler when user
tries to enable "threaded" on napi, and properly handles the
kthread_run() failure. This is because certain drivers do not have
the napi created and linked to the dev when dev_open() is called. So
the previous implementation does not work properly there.

Signed-off-by: Wei Wang <weiwan@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
---
 net/core/dev.c       | 53 ++++++++++++++++++++++++++------------------
 net/core/net-sysfs.c |  9 +++-----
 2 files changed, 35 insertions(+), 27 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 88437cdf29f1..7788899b100f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1491,17 +1491,28 @@ EXPORT_SYMBOL(netdev_notify_peers);
 
 static int napi_threaded_poll(void *data);
 
-static void napi_thread_start(struct napi_struct *n)
+static int napi_kthread_create(struct napi_struct *n)
 {
-	if (test_bit(NAPI_STATE_THREADED, &n->state) && !n->thread)
-		n->thread = kthread_create(napi_threaded_poll, n, "%s-%d",
-					   n->dev->name, n->napi_id);
+	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 int __dev_open(struct net_device *dev, struct netlink_ext_ack *extack)
 {
 	const struct net_device_ops *ops = dev->netdev_ops;
-	struct napi_struct *n;
 	int ret;
 
 	ASSERT_RTNL();
@@ -1533,9 +1544,6 @@ static int __dev_open(struct net_device *dev, struct netlink_ext_ack *extack)
 	if (!ret && ops->ndo_open)
 		ret = ops->ndo_open(dev);
 
-	list_for_each_entry(n, &dev->napi_list, dev_list)
-		napi_thread_start(n);
-
 	netpoll_poll_enable(dev);
 
 	if (ret)
@@ -1586,6 +1594,7 @@ static void napi_thread_stop(struct napi_struct *n)
 	if (!n->thread)
 		return;
 	kthread_stop(n->thread);
+	clear_bit(NAPI_STATE_THREADED, &n->state);
 	n->thread = NULL;
 }
 
@@ -4271,7 +4280,7 @@ int gro_normal_batch __read_mostly = 8;
 static inline void ____napi_schedule(struct softnet_data *sd,
 				     struct napi_struct *napi)
 {
-	if (napi->thread) {
+	if (test_bit(NAPI_STATE_THREADED, &napi->state)) {
 		wake_up_process(napi->thread);
 		return;
 	}
@@ -6700,25 +6709,25 @@ static void init_gro_hash(struct napi_struct *napi)
 
 int napi_set_threaded(struct napi_struct *n, bool threaded)
 {
-	ASSERT_RTNL();
+	int err = 0;
 
-	if (n->dev->flags & IFF_UP)
-		return -EBUSY;
+	ASSERT_RTNL();
 
 	if (threaded == !!test_bit(NAPI_STATE_THREADED, &n->state))
 		return 0;
-	if (threaded)
+	if (threaded) {
+		if (!n->thread) {
+			err = napi_kthread_create(n);
+			if (err)
+				goto out;
+		}
 		set_bit(NAPI_STATE_THREADED, &n->state);
-	else
+	} else {
 		clear_bit(NAPI_STATE_THREADED, &n->state);
+	}
 
-	/* if the device is initializing, nothing todo */
-	if (test_bit(__LINK_STATE_START, &n->dev->state))
-		return 0;
-
-	napi_thread_stop(n);
-	napi_thread_start(n);
-	return 0;
+out:
+	return err;
 }
 EXPORT_SYMBOL(napi_set_threaded);
 
@@ -6763,6 +6772,7 @@ void napi_disable(struct napi_struct *n)
 		msleep(1);
 
 	hrtimer_cancel(&n->timer);
+	napi_thread_stop(n);
 
 	clear_bit(NAPI_STATE_DISABLE, &n->state);
 }
@@ -6883,6 +6893,7 @@ static int napi_thread_wait(struct napi_struct *napi)
 
 	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;
 		}
diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index df8dd25e5e4b..1e24c1e81ad8 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -609,11 +609,6 @@ static ssize_t threaded_store(struct device *dev,
 		goto unlock;
 	}
 
-	if (netdev->flags & IFF_UP) {
-		ret = -EBUSY;
-		goto unlock;
-	}
-
 	bmap = __alloc_thread_bitmap(netdev, &bits);
 	if (!bmap) {
 		ret = -ENOMEM;
@@ -626,7 +621,9 @@ static ssize_t threaded_store(struct device *dev,
 
 	i = 0;
 	list_for_each_entry(n, &netdev->napi_list, dev_list) {
-		napi_set_threaded(n, test_bit(i, bmap));
+		ret = napi_set_threaded(n, test_bit(i, bmap));
+		if (ret)
+			goto free_unlock;
 		i++;
 	}
 	ret = len;
-- 
2.29.2.454.gaff20da3a2-goog

Re: [PATCH net-next v3 0/5] implement kthread based napi poll

[Wei Wang]

On Wed, Nov 18, 2020 at 12:07 PM Wei Wang <weiwan@google.com> wrote:
>
> The idea of moving the napi poll process out of softirq context to a
> kernel thread based context is not new.
> Paolo Abeni and Hannes Frederic Sowa have proposed patches to move napi
> poll to kthread back in 2016. And Felix Fietkau has also proposed
> patches of similar ideas to use workqueue to process napi poll just a
> few weeks ago.
>
> The main reason we'd like to push forward with this idea is that the
> scheduler has poor visibility into cpu cycles spent in softirq context,
> and is not able to make optimal scheduling decisions of the user threads.
> For example, we see in one of the application benchmark where network
> load is high, the CPUs handling network softirqs has ~80% cpu util. And
> user threads are still scheduled on those CPUs, despite other more idle
> cpus available in the system. And we see very high tail latencies. In this
> case, we have to explicitly pin away user threads from the CPUs handling
> network softirqs to ensure good performance.
> With napi poll moved to kthread, scheduler is in charge of scheduling both
> the kthreads handling network load, and the user threads, and is able to
> make better decisions. In the previous benchmark, if we do this and we
> pin the kthreads processing napi poll to specific CPUs, scheduler is
> able to schedule user threads away from these CPUs automatically.
>
> And the reason we prefer 1 kthread per napi, instead of 1 workqueue
> entity per host, is that kthread is more configurable than workqueue,
> and we could leverage existing tuning tools for threads, like taskset,
> chrt, etc to tune scheduling class and cpu set, etc. Another reason is
> if we eventually want to provide busy poll feature using kernel threads
> for napi poll, kthread seems to be more suitable than workqueue.
> Furthermore, for large platforms with 2 NICs attached to 2 sockets,
> kthread is more flexible to be pinned to different sets of CPUs.
>
> In this patch series, I revived Paolo and Hannes's patch in 2016 and
> left them as the first 2 patches. Then there are changes proposed by
> Felix, Jakub, Paolo and myself on top of those, with suggestions from
> Eric Dumazet.
>
> In terms of performance, I ran tcp_rr tests with 1000 flows with
> various request/response sizes, with RFS/RPS disabled, and compared
> performance between softirq vs kthread vs workqueue (patchset proposed
> by Felix Fietkau).
> Host has 56 hyper threads and 100Gbps nic, 8 rx queues and only 1 numa
> node. All threads are unpinned.
>
>         req/resp   QPS   50%tile    90%tile    99%tile    99.9%tile
> softirq   1B/1B   2.75M   337us       376us      1.04ms     3.69ms
> kthread   1B/1B   2.67M   371us       408us      455us      550us
> workq     1B/1B   2.56M   384us       435us      673us      822us
>
> softirq 5KB/5KB   1.46M   678us       750us      969us      2.78ms
> kthread 5KB/5KB   1.44M   695us       789us      891us      1.06ms
> workq   5KB/5KB   1.34M   720us       905us     1.06ms      1.57ms
>
> softirq 1MB/1MB   11.0K   79ms       166ms      306ms       630ms
> kthread 1MB/1MB   11.0K   75ms       177ms      303ms       596ms
> workq   1MB/1MB   11.0K   79ms       180ms      303ms       587ms
>
> When running workqueue implementation, I found the number of threads
> used is usually twice as much as kthread implementation. This probably
> introduces higher scheduling cost, which results in higher tail
> latencies in most cases.
>
> I also ran an application benchmark, which performs fixed qps remote SSD
> read/write operations, with various sizes. Again, both with RFS/RPS
> disabled.
> The result is as follows:
>          op_size  QPS   50%tile 95%tile 99%tile 99.9%tile
> softirq   4K     572.6K   385us   1.5ms  3.16ms   6.41ms
> kthread   4K     572.6K   390us   803us  2.21ms   6.83ms
> workq     4k     572.6K   384us   763us  3.12ms   6.87ms
>
> softirq   64K    157.9K   736us   1.17ms 3.40ms   13.75ms
> kthread   64K    157.9K   745us   1.23ms 2.76ms    9.87ms
> workq     64K    157.9K   746us   1.23ms 2.76ms    9.96ms
>
> softirq   1M     10.98K   2.03ms  3.10ms  3.7ms   11.56ms
> kthread   1M     10.98K   2.13ms  3.21ms  4.02ms  13.3ms
> workq     1M     10.98K   2.13ms  3.20ms  3.99ms  14.12ms
>
> In this set of tests, the latency is predominant by the SSD operation.
> Also, the user threads are much busier compared to tcp_rr tests. We have
> to pin the kthreads/workqueue threads to limit to a few CPUs, to not
> disturb user threads, and provide some isolation.
>
>
> Changes since v2:
> Corrected typo in patch 1, and updated the cover letter with more
> detailed and updated test results.
>

Hi everyone,

We thought it is a good time to re-push this patch series to get
another round of evaluation after several weeks since last version.
The patch series itself did not have much change. But I updated the
cover letter to include the updated and more detailed test results,
hoping to give more contexts.

Thanks for reviewing!
Wei

> Changes since v1:
> Replaced kthread_create() with kthread_run() in patch 5 as suggested by
> Felix Fietkau.
>
> Changes since RFC:
> Renamed the kthreads to be napi/<dev>-<napi_id> in patch 5 as suggested
> by Hannes Frederic Sowa.
>
> Paolo Abeni (2):
>   net: implement threaded-able napi poll loop support
>   net: add sysfs attribute to control napi threaded mode
> Felix Fietkau (1):
>   net: extract napi poll functionality to __napi_poll()
> Jakub Kicinski (1):
>   net: modify kthread handler to use __napi_poll()
> Wei Wang (1):
>   net: improve napi threaded config
>
>  include/linux/netdevice.h |   5 ++
>  net/core/dev.c            | 143 +++++++++++++++++++++++++++++++++++---
>  net/core/net-sysfs.c      | 100 ++++++++++++++++++++++++++
>  3 files changed, 239 insertions(+), 9 deletions(-)
>
> --
> 2.29.2.454.gaff20da3a2-goog
>

Re: [PATCH net-next v3 2/5] net: add sysfs attribute to control napi threaded mode

[Randy Dunlap]

On 11/18/20 11:10 AM, Wei Wang wrote:
> From: Paolo Abeni <pabeni@redhat.com> 
> 
> this patch adds a new sysfs attribute to the network
> device class. Said attribute is a bitmask that allows controlling
> the threaded mode for all the napi instances of the given
> network device.
> 
> The threaded mode can be switched only if related network device
> is down.
> 
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
> Signed-off-by: Wei Wang <weiwan@google.com>
> Reviewed-by: Eric Dumazet <edumazet@google.com>

Hi,

Could someone describe the bitmask (is it a bit per instance of the
network device?).
And how to use the sysfs interface, please?

> ---
>  net/core/net-sysfs.c | 103 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 103 insertions(+)
> 
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index 94fff0700bdd..df8dd25e5e4b 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c


thanks.
-- 
~Randy

Re: [PATCH net-next v3 2/5] net: add sysfs attribute to control napi threaded mode

[Wei Wang]

On Wed, Nov 18, 2020 at 12:36 PM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> On 11/18/20 11:10 AM, Wei Wang wrote:
> > From: Paolo Abeni <pabeni@redhat.com>
> >
> > this patch adds a new sysfs attribute to the network
> > device class. Said attribute is a bitmask that allows controlling
> > the threaded mode for all the napi instances of the given
> > network device.
> >
> > The threaded mode can be switched only if related network device
> > is down.
> >
> > Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> > Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
> > Signed-off-by: Wei Wang <weiwan@google.com>
> > Reviewed-by: Eric Dumazet <edumazet@google.com>
>
> Hi,
>
> Could someone describe the bitmask (is it a bit per instance of the
> network device?).
> And how to use the sysfs interface, please?
>

It is 1 bit per napi. Depending on the driver implementation, 1 napi
could correspond to 1 tx queue, or 1 rx queue, or 1 pair of tx/rx
queues.
To set bits in the bit mask, you could do:
echo 0-14 > /sys/class/net/<dev>/threaded
to set consecutive bits.
or:
echo 0,2,4,6,8 >  /sys/class/net/<dev>/threaded
to set individual bits.

To clear the bit mask, you could do:
echo > /sys/class/net/<dev>/threaded

> > ---
> >  net/core/net-sysfs.c | 103 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 103 insertions(+)
> >
> > diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> > index 94fff0700bdd..df8dd25e5e4b 100644
> > --- a/net/core/net-sysfs.c
> > +++ b/net/core/net-sysfs.c
>
>
> thanks.
> --
> ~Randy
>

Re: [PATCH net-next v3 1/5] net: implement threaded-able napi poll loop support

[Jakub Kicinski]

On Wed, 18 Nov 2020 11:10:05 -0800 Wei Wang wrote:
> +int napi_set_threaded(struct napi_struct *n, bool threaded)
> +{
> +	ASSERT_RTNL();
> +
> +	if (n->dev->flags & IFF_UP)
> +		return -EBUSY;
> +
> +	if (threaded == !!test_bit(NAPI_STATE_THREADED, &n->state))
> +		return 0;
> +	if (threaded)
> +		set_bit(NAPI_STATE_THREADED, &n->state);
> +	else
> +		clear_bit(NAPI_STATE_THREADED, &n->state);

Do we really need the per-NAPI control here? Does anyone have use cases
where that makes sense? The user would be guessing which NAPI means
which queue and which bit, currently.

> +	/* if the device is initializing, nothing todo */
> +	if (test_bit(__LINK_STATE_START, &n->dev->state))
> +		return 0;
> +
> +	napi_thread_stop(n);
> +	napi_thread_start(n);
> +	return 0;
> +}
> +EXPORT_SYMBOL(napi_set_threaded);

Why was this exported? Do we still need that?

Please rejig the patches into a reviewable form. You can use the
Co-developed-by tag to give people credit on individual patches.

Re: [PATCH net-next v3 1/5] net: implement threaded-able napi poll loop support

[Wei Wang]

On Sat, Nov 21, 2020 at 4:31 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Wed, 18 Nov 2020 11:10:05 -0800 Wei Wang wrote:
> > +int napi_set_threaded(struct napi_struct *n, bool threaded)
> > +{
> > +     ASSERT_RTNL();
> > +
> > +     if (n->dev->flags & IFF_UP)
> > +             return -EBUSY;
> > +
> > +     if (threaded == !!test_bit(NAPI_STATE_THREADED, &n->state))
> > +             return 0;
> > +     if (threaded)
> > +             set_bit(NAPI_STATE_THREADED, &n->state);
> > +     else
> > +             clear_bit(NAPI_STATE_THREADED, &n->state);
>
> Do we really need the per-NAPI control here? Does anyone have use cases
> where that makes sense? The user would be guessing which NAPI means
> which queue and which bit, currently.

Thanks for reviewing this.
I think one use case might be that if the driver uses separate napi
for tx and rx, one might want to only enable threaded mode for rx, and
leave tx completion in interrupt mode.


>
> > +     /* if the device is initializing, nothing todo */
> > +     if (test_bit(__LINK_STATE_START, &n->dev->state))
> > +             return 0;
> > +
> > +     napi_thread_stop(n);
> > +     napi_thread_start(n);
> > +     return 0;
> > +}
> > +EXPORT_SYMBOL(napi_set_threaded);
>
> Why was this exported? Do we still need that?
>

Yea. I guess it is not needed.

> Please rejig the patches into a reviewable form. You can use the
> Co-developed-by tag to give people credit on individual patches.

Will do. Thanks!

Re: [PATCH net-next v3 1/5] net: implement threaded-able napi poll loop support

[Jakub Kicinski]

On Sat, 21 Nov 2020 18:23:33 -0800 Wei Wang wrote:
> On Sat, Nov 21, 2020 at 4:31 PM Jakub Kicinski <kuba@kernel.org> wrote:
> > On Wed, 18 Nov 2020 11:10:05 -0800 Wei Wang wrote:  
> > > +int napi_set_threaded(struct napi_struct *n, bool threaded)
> > > +{
> > > +     ASSERT_RTNL();
> > > +
> > > +     if (n->dev->flags & IFF_UP)
> > > +             return -EBUSY;
> > > +
> > > +     if (threaded == !!test_bit(NAPI_STATE_THREADED, &n->state))
> > > +             return 0;
> > > +     if (threaded)
> > > +             set_bit(NAPI_STATE_THREADED, &n->state);
> > > +     else
> > > +             clear_bit(NAPI_STATE_THREADED, &n->state);  
> >
> > Do we really need the per-NAPI control here? Does anyone have use cases
> > where that makes sense? The user would be guessing which NAPI means
> > which queue and which bit, currently.  
> 
> Thanks for reviewing this.
> I think one use case might be that if the driver uses separate napi
> for tx and rx, one might want to only enable threaded mode for rx, and
> leave tx completion in interrupt mode.

Okay, but with separate IRQs/NAPIs that's really a guessing game in
terms of NAPI -> bit position. I'd rather we held off on the per-NAPI
control. 

If anyone has a strong use for it now, please let us know.

Re: [PATCH net-next v3 1/5] net: implement threaded-able napi poll loop support

[Wei Wang]

On Mon, Nov 23, 2020 at 10:56 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Sat, 21 Nov 2020 18:23:33 -0800 Wei Wang wrote:
> > On Sat, Nov 21, 2020 at 4:31 PM Jakub Kicinski <kuba@kernel.org> wrote:
> > > On Wed, 18 Nov 2020 11:10:05 -0800 Wei Wang wrote:
> > > > +int napi_set_threaded(struct napi_struct *n, bool threaded)
> > > > +{
> > > > +     ASSERT_RTNL();
> > > > +
> > > > +     if (n->dev->flags & IFF_UP)
> > > > +             return -EBUSY;
> > > > +
> > > > +     if (threaded == !!test_bit(NAPI_STATE_THREADED, &n->state))
> > > > +             return 0;
> > > > +     if (threaded)
> > > > +             set_bit(NAPI_STATE_THREADED, &n->state);
> > > > +     else
> > > > +             clear_bit(NAPI_STATE_THREADED, &n->state);
> > >
> > > Do we really need the per-NAPI control here? Does anyone have use cases
> > > where that makes sense? The user would be guessing which NAPI means
> > > which queue and which bit, currently.
> >
> > Thanks for reviewing this.
> > I think one use case might be that if the driver uses separate napi
> > for tx and rx, one might want to only enable threaded mode for rx, and
> > leave tx completion in interrupt mode.
>
> Okay, but with separate IRQs/NAPIs that's really a guessing game in
> terms of NAPI -> bit position. I'd rather we held off on the per-NAPI
> control.
>

Yes. That is true. The bit position is dependent on the driver implementation.

> If anyone has a strong use for it now, please let us know.

OK. Will change it to per dev control if no one objects.