* [PATCH 6.6 1/2] net: Remove conditional threaded-NAPI wakeup based on task state.
2025-12-29 7:53 [PATCH 6.6 0/2] fix do_softirq_post_smp_call_flush warnings caused by NET_RX_SOFTIRQ wen.yang
@ 2025-12-29 7:53 ` wen.yang
2025-12-29 7:53 ` [PATCH 6.6 2/2] net: Allow to use SMP threads for backlog NAPI wen.yang
1 sibling, 0 replies; 3+ messages in thread
From: wen.yang @ 2025-12-29 7:53 UTC (permalink / raw)
To: Greg Kroah-Hartman
Cc: stable, linux-kernel, Sebastian Andrzej Siewior, Jakub Kicinski,
Paolo Abeni, Wen Yang
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
commit 56364c910691f6d10ba88c964c9041b9ab777bd6 upstream.
A NAPI thread is scheduled by first setting NAPI_STATE_SCHED bit. If
successful (the bit was not yet set) then the NAPI_STATE_SCHED_THREADED
is set but only if thread's state is not TASK_INTERRUPTIBLE (is
TASK_RUNNING) followed by task wakeup.
If the task is idle (TASK_INTERRUPTIBLE) then the
NAPI_STATE_SCHED_THREADED bit is not set. The thread is no relying on
the bit but always leaving the wait-loop after returning from schedule()
because there must have been a wakeup.
The smpboot-threads implementation for per-CPU threads requires an
explicit condition and does not support "if we get out of schedule()
then there must be something to do".
Removing this optimisation simplifies the following integration.
Set NAPI_STATE_SCHED_THREADED unconditionally on wakeup and rely on it
in the wait path by removing the `woken' condition.
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Wen Yang <wen.yang@linux.dev>
---
net/core/dev.c | 14 ++------------
1 file changed, 2 insertions(+), 12 deletions(-)
diff --git a/net/core/dev.c b/net/core/dev.c
index 2d3e0e4130c2..d62cab2e4878 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4526,13 +4526,7 @@ static inline void ____napi_schedule(struct softnet_data *sd,
*/
thread = READ_ONCE(napi->thread);
if (thread) {
- /* Avoid doing set_bit() if the thread is in
- * INTERRUPTIBLE state, cause napi_thread_wait()
- * makes sure to proceed with napi polling
- * if the thread is explicitly woken from here.
- */
- if (READ_ONCE(thread->__state) != TASK_INTERRUPTIBLE)
- set_bit(NAPI_STATE_SCHED_THREADED, &napi->state);
+ set_bit(NAPI_STATE_SCHED_THREADED, &napi->state);
wake_up_process(thread);
return;
}
@@ -6688,8 +6682,6 @@ static int napi_poll(struct napi_struct *n, struct list_head *repoll)
static int napi_thread_wait(struct napi_struct *napi)
{
- bool woken = false;
-
set_current_state(TASK_INTERRUPTIBLE);
while (!kthread_should_stop()) {
@@ -6698,15 +6690,13 @@ static int napi_thread_wait(struct napi_struct *napi)
* Testing SCHED bit is not enough because SCHED bit might be
* set by some other busy poll thread or by napi_disable().
*/
- if (test_bit(NAPI_STATE_SCHED_THREADED, &napi->state) || woken) {
+ if (test_bit(NAPI_STATE_SCHED_THREADED, &napi->state)) {
WARN_ON(!list_empty(&napi->poll_list));
__set_current_state(TASK_RUNNING);
return 0;
}
schedule();
- /* woken being true indicates this thread owns this napi. */
- woken = true;
set_current_state(TASK_INTERRUPTIBLE);
}
__set_current_state(TASK_RUNNING);
--
2.25.1
^ permalink raw reply related [flat|nested] 3+ messages in thread* [PATCH 6.6 2/2] net: Allow to use SMP threads for backlog NAPI.
2025-12-29 7:53 [PATCH 6.6 0/2] fix do_softirq_post_smp_call_flush warnings caused by NET_RX_SOFTIRQ wen.yang
2025-12-29 7:53 ` [PATCH 6.6 1/2] net: Remove conditional threaded-NAPI wakeup based on task state wen.yang
@ 2025-12-29 7:53 ` wen.yang
1 sibling, 0 replies; 3+ messages in thread
From: wen.yang @ 2025-12-29 7:53 UTC (permalink / raw)
To: Greg Kroah-Hartman
Cc: stable, linux-kernel, Sebastian Andrzej Siewior, Jakub Kicinski,
Paolo Abeni, Wen Yang
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
commit dad6b97702639fba27a2bd3e986982ad6f0db3a7 upstream.
Backlog NAPI is a per-CPU NAPI struct only (with no device behind it)
used by drivers which don't do NAPI them self, RPS and parts of the
stack which need to avoid recursive deadlocks while processing a packet.
The non-NAPI driver use the CPU local backlog NAPI. If RPS is enabled
then a flow for the skb is computed and based on the flow the skb can be
enqueued on a remote CPU. Scheduling/ raising the softirq (for backlog's
NAPI) on the remote CPU isn't trivial because the softirq is only
scheduled on the local CPU and performed after the hardirq is done.
In order to schedule a softirq on the remote CPU, an IPI is sent to the
remote CPU which schedules the backlog-NAPI on the then local CPU.
On PREEMPT_RT interrupts are force-threaded. The soft interrupts are
raised within the interrupt thread and processed after the interrupt
handler completed still within the context of the interrupt thread. The
softirq is handled in the context where it originated.
With force-threaded interrupts enabled, ksoftirqd is woken up if a
softirq is raised from hardirq context. This is the case if it is raised
from an IPI. Additionally there is a warning on PREEMPT_RT if the
softirq is raised from the idle thread.
This was done for two reasons:
- With threaded interrupts the processing should happen in thread
context (where it originated) and ksoftirqd is the only thread for
this context if raised from hardirq. Using the currently running task
instead would "punish" a random task.
- Once ksoftirqd is active it consumes all further softirqs until it
stops running. This changed recently and is no longer the case.
Instead of keeping the backlog NAPI in ksoftirqd (in force-threaded/
PREEMPT_RT setups) I am proposing NAPI-threads for backlog.
The "proper" setup with threaded-NAPI is not doable because the threads
are not pinned to an individual CPU and can be modified by the user.
Additionally a dummy network device would have to be assigned. Also
CPU-hotplug has to be considered if additional CPUs show up.
All this can be probably done/ solved but the smpboot-threads already
provide this infrastructure.
Sending UDP packets over loopback expects that the packet is processed
within the call. Delaying it by handing it over to the thread hurts
performance. It is not beneficial to the outcome if the context switch
happens immediately after enqueue or after a while to process a few
packets in a batch.
There is no need to always use the thread if the backlog NAPI is
requested on the local CPU. This restores the loopback throuput. The
performance drops mostly to the same value after enabling RPS on the
loopback comparing the IPI and the tread result.
Create NAPI-threads for backlog if request during boot. The thread runs
the inner loop from napi_threaded_poll(), the wait part is different. It
checks for NAPI_STATE_SCHED (the backlog NAPI can not be disabled).
The NAPI threads for backlog are optional, it has to be enabled via the boot
argument "thread_backlog_napi". It is mandatory for PREEMPT_RT to avoid the
wakeup of ksoftirqd from the IPI.
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Wen Yang <wen.yang@linux.dev>
---
net/core/dev.c | 148 +++++++++++++++++++++++++++++++++++++------------
1 file changed, 113 insertions(+), 35 deletions(-)
diff --git a/net/core/dev.c b/net/core/dev.c
index d62cab2e4878..cf5b5a4cbfd7 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -78,6 +78,7 @@
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/sched/mm.h>
+#include <linux/smpboot.h>
#include <linux/mutex.h>
#include <linux/rwsem.h>
#include <linux/string.h>
@@ -217,6 +218,31 @@ static inline struct hlist_head *dev_index_hash(struct net *net, int ifindex)
return &net->dev_index_head[ifindex & (NETDEV_HASHENTRIES - 1)];
}
+#ifndef CONFIG_PREEMPT_RT
+
+static DEFINE_STATIC_KEY_FALSE(use_backlog_threads_key);
+
+static int __init setup_backlog_napi_threads(char *arg)
+{
+ static_branch_enable(&use_backlog_threads_key);
+ return 0;
+}
+early_param("thread_backlog_napi", setup_backlog_napi_threads);
+
+static bool use_backlog_threads(void)
+{
+ return static_branch_unlikely(&use_backlog_threads_key);
+}
+
+#else
+
+static bool use_backlog_threads(void)
+{
+ return true;
+}
+
+#endif
+
static inline void rps_lock_irqsave(struct softnet_data *sd,
unsigned long *flags)
{
@@ -4494,6 +4520,7 @@ EXPORT_SYMBOL(__dev_direct_xmit);
/*************************************************************************
* Receiver routines
*************************************************************************/
+static DEFINE_PER_CPU(struct task_struct *, backlog_napi);
int netdev_max_backlog __read_mostly = 1000;
EXPORT_SYMBOL(netdev_max_backlog);
@@ -4526,12 +4553,16 @@ static inline void ____napi_schedule(struct softnet_data *sd,
*/
thread = READ_ONCE(napi->thread);
if (thread) {
+ if (use_backlog_threads() && thread == raw_cpu_read(backlog_napi))
+ goto use_local_napi;
+
set_bit(NAPI_STATE_SCHED_THREADED, &napi->state);
wake_up_process(thread);
return;
}
}
+use_local_napi:
list_add_tail(&napi->poll_list, &sd->poll_list);
WRITE_ONCE(napi->list_owner, smp_processor_id());
/* If not called from net_rx_action()
@@ -4777,6 +4808,11 @@ static void napi_schedule_rps(struct softnet_data *sd)
#ifdef CONFIG_RPS
if (sd != mysd) {
+ if (use_backlog_threads()) {
+ __napi_schedule_irqoff(&sd->backlog);
+ return;
+ }
+
sd->rps_ipi_next = mysd->rps_ipi_list;
mysd->rps_ipi_list = sd;
@@ -6000,7 +6036,7 @@ static void net_rps_action_and_irq_enable(struct softnet_data *sd)
#ifdef CONFIG_RPS
struct softnet_data *remsd = sd->rps_ipi_list;
- if (remsd) {
+ if (!use_backlog_threads() && remsd) {
sd->rps_ipi_list = NULL;
local_irq_enable();
@@ -6015,7 +6051,7 @@ static void net_rps_action_and_irq_enable(struct softnet_data *sd)
static bool sd_has_rps_ipi_waiting(struct softnet_data *sd)
{
#ifdef CONFIG_RPS
- return sd->rps_ipi_list != NULL;
+ return !use_backlog_threads() && sd->rps_ipi_list;
#else
return false;
#endif
@@ -6059,7 +6095,7 @@ static int process_backlog(struct napi_struct *napi, int quota)
* We can use a plain write instead of clear_bit(),
* and we dont need an smp_mb() memory barrier.
*/
- napi->state = 0;
+ napi->state &= NAPIF_STATE_THREADED;
again = false;
} else {
skb_queue_splice_tail_init(&sd->input_pkt_queue,
@@ -6725,43 +6761,48 @@ static void skb_defer_free_flush(struct softnet_data *sd)
}
}
-static int napi_threaded_poll(void *data)
+static void napi_threaded_poll_loop(struct napi_struct *napi)
{
- struct napi_struct *napi = data;
struct softnet_data *sd;
- void *have;
-
- while (!napi_thread_wait(napi)) {
- unsigned long last_qs = jiffies;
+ unsigned long last_qs = jiffies;
- for (;;) {
- bool repoll = false;
+ for (;;) {
+ bool repoll = false;
+ void *have;
- local_bh_disable();
- sd = this_cpu_ptr(&softnet_data);
- sd->in_napi_threaded_poll = true;
+ local_bh_disable();
+ sd = this_cpu_ptr(&softnet_data);
+ sd->in_napi_threaded_poll = true;
- have = netpoll_poll_lock(napi);
- __napi_poll(napi, &repoll);
- netpoll_poll_unlock(have);
+ have = netpoll_poll_lock(napi);
+ __napi_poll(napi, &repoll);
+ netpoll_poll_unlock(have);
- sd->in_napi_threaded_poll = false;
- barrier();
+ sd->in_napi_threaded_poll = false;
+ barrier();
- if (sd_has_rps_ipi_waiting(sd)) {
- local_irq_disable();
- net_rps_action_and_irq_enable(sd);
- }
- skb_defer_free_flush(sd);
- local_bh_enable();
+ if (sd_has_rps_ipi_waiting(sd)) {
+ local_irq_disable();
+ net_rps_action_and_irq_enable(sd);
+ }
+ skb_defer_free_flush(sd);
+ local_bh_enable();
- if (!repoll)
- break;
+ if (!repoll)
+ break;
- rcu_softirq_qs_periodic(last_qs);
- cond_resched();
- }
+ rcu_softirq_qs_periodic(last_qs);
+ cond_resched();
}
+}
+
+static int napi_threaded_poll(void *data)
+{
+ struct napi_struct *napi = data;
+
+ while (!napi_thread_wait(napi))
+ napi_threaded_poll_loop(napi);
+
return 0;
}
@@ -11346,7 +11387,7 @@ static int dev_cpu_dead(unsigned int oldcpu)
list_del_init(&napi->poll_list);
if (napi->poll == process_backlog)
- napi->state = 0;
+ napi->state &= NAPIF_STATE_THREADED;
else
____napi_schedule(sd, napi);
}
@@ -11354,12 +11395,14 @@ static int dev_cpu_dead(unsigned int oldcpu)
raise_softirq_irqoff(NET_TX_SOFTIRQ);
local_irq_enable();
+ if (!use_backlog_threads()) {
#ifdef CONFIG_RPS
- remsd = oldsd->rps_ipi_list;
- oldsd->rps_ipi_list = NULL;
+ remsd = oldsd->rps_ipi_list;
+ oldsd->rps_ipi_list = NULL;
#endif
- /* send out pending IPI's on offline CPU */
- net_rps_send_ipi(remsd);
+ /* send out pending IPI's on offline CPU */
+ net_rps_send_ipi(remsd);
+ }
/* Process offline CPU's input_pkt_queue */
while ((skb = __skb_dequeue(&oldsd->process_queue))) {
@@ -11622,6 +11665,38 @@ static struct pernet_operations __net_initdata default_device_ops = {
*
*/
+static int backlog_napi_should_run(unsigned int cpu)
+{
+ struct softnet_data *sd = per_cpu_ptr(&softnet_data, cpu);
+ struct napi_struct *napi = &sd->backlog;
+
+ return test_bit(NAPI_STATE_SCHED_THREADED, &napi->state);
+}
+
+static void run_backlog_napi(unsigned int cpu)
+{
+ struct softnet_data *sd = per_cpu_ptr(&softnet_data, cpu);
+
+ napi_threaded_poll_loop(&sd->backlog);
+}
+
+static void backlog_napi_setup(unsigned int cpu)
+{
+ struct softnet_data *sd = per_cpu_ptr(&softnet_data, cpu);
+ struct napi_struct *napi = &sd->backlog;
+
+ napi->thread = this_cpu_read(backlog_napi);
+ set_bit(NAPI_STATE_THREADED, &napi->state);
+}
+
+static struct smp_hotplug_thread backlog_threads = {
+ .store = &backlog_napi,
+ .thread_should_run = backlog_napi_should_run,
+ .thread_fn = run_backlog_napi,
+ .thread_comm = "backlog_napi/%u",
+ .setup = backlog_napi_setup,
+};
+
/*
* This is called single threaded during boot, so no need
* to take the rtnl semaphore.
@@ -11672,7 +11747,10 @@ static int __init net_dev_init(void)
init_gro_hash(&sd->backlog);
sd->backlog.poll = process_backlog;
sd->backlog.weight = weight_p;
+ INIT_LIST_HEAD(&sd->backlog.poll_list);
}
+ if (use_backlog_threads())
+ smpboot_register_percpu_thread(&backlog_threads);
dev_boot_phase = 0;
--
2.25.1
^ permalink raw reply related [flat|nested] 3+ messages in thread