Disabling interrupts and in the RPS case locking input_pkt_queue is
split into local_irq_disable() and optional spin_lock().
This breaks on PREEMPT_RT because the spinlock_t typed lock can not be
acquired with disabled interrupts.
The sections in which the lock is acquired is usually short in a sense that it
is not causing long und unbounded latiencies. One exception is the
skb_flow_limit() invocation which may invoke a BPF program (and may
require sleeping locks).
By moving local_irq_disable() + spin_lock() into rps_lock(), we can keep
interrupts disabled on !PREEMPT_RT and enabled on PREEMPT_RT kernels.
Without RPS on a PREEMPT_RT kernel, the needed synchronisation happens
as part of local_bh_disable() on the local CPU.
____napi_schedule() is only invoked if sd is from the local CPU. Replace
it with __napi_schedule_irqoff() which already disables interrupts on
PREEMPT_RT as needed. Move this call to rps_ipi_queued() and rename the
function to napi_schedule_rps as suggested by Jakub.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
On 2022-02-02 08:47:35 [-0800], Jakub Kicinski wrote:
>
> I think you can re-jig this a little more - rps_ipi_queued() only return
> 0 if sd is "local" so maybe we can call __napi_schedule_irqoff()
> instead which already has the if () for PREEMPT_RT?
>
> Maybe moving the ____napi_schedule() into rps_ipi_queued() and
> renaming it to napi_schedule_backlog() or napi_schedule_rps()
> would make the code easier to follow in that case?
Something like this then?
net/core/dev.c | 76 ++++++++++++++++++++++++++++----------------------
1 file changed, 42 insertions(+), 34 deletions(-)
diff --git a/net/core/dev.c b/net/core/dev.c
index f43d0580fa11d..18f9941287c2e 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -216,18 +216,38 @@ static inline struct hlist_head *dev_index_hash(struct net *net, int ifindex)
return &net->dev_index_head[ifindex & (NETDEV_HASHENTRIES - 1)];
}
-static inline void rps_lock(struct softnet_data *sd)
+static inline void rps_lock_irqsave(struct softnet_data *sd,
+ unsigned long *flags)
{
-#ifdef CONFIG_RPS
- spin_lock(&sd->input_pkt_queue.lock);
-#endif
+ if (IS_ENABLED(CONFIG_RPS))
+ spin_lock_irqsave(&sd->input_pkt_queue.lock, *flags);
+ else if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_irq_save(*flags);
}
-static inline void rps_unlock(struct softnet_data *sd)
+static inline void rps_lock_irq_disable(struct softnet_data *sd)
{
-#ifdef CONFIG_RPS
- spin_unlock(&sd->input_pkt_queue.lock);
-#endif
+ if (IS_ENABLED(CONFIG_RPS))
+ spin_lock_irq(&sd->input_pkt_queue.lock);
+ else if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_irq_disable();
+}
+
+static inline void rps_unlock_irq_restore(struct softnet_data *sd,
+ unsigned long *flags)
+{
+ if (IS_ENABLED(CONFIG_RPS))
+ spin_unlock_irqrestore(&sd->input_pkt_queue.lock, *flags);
+ else if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_irq_restore(*flags);
+}
+
+static inline void rps_unlock_irq_enable(struct softnet_data *sd)
+{
+ if (IS_ENABLED(CONFIG_RPS))
+ spin_unlock_irq(&sd->input_pkt_queue.lock);
+ else if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_irq_enable();
}
static struct netdev_name_node *netdev_name_node_alloc(struct net_device *dev,
@@ -4456,11 +4476,11 @@ static void rps_trigger_softirq(void *data)
* If yes, queue it to our IPI list and return 1
* If no, return 0
*/
-static int rps_ipi_queued(struct softnet_data *sd)
+static int napi_schedule_rps(struct softnet_data *sd)
{
-#ifdef CONFIG_RPS
struct softnet_data *mysd = this_cpu_ptr(&softnet_data);
+#ifdef CONFIG_RPS
if (sd != mysd) {
sd->rps_ipi_next = mysd->rps_ipi_list;
mysd->rps_ipi_list = sd;
@@ -4469,6 +4489,7 @@ static int rps_ipi_queued(struct softnet_data *sd)
return 1;
}
#endif /* CONFIG_RPS */
+ __napi_schedule_irqoff(&mysd->backlog);
return 0;
}
@@ -4525,9 +4546,7 @@ static int enqueue_to_backlog(struct sk_buff *skb, int cpu,
sd = &per_cpu(softnet_data, cpu);
- local_irq_save(flags);
-
- rps_lock(sd);
+ rps_lock_irqsave(sd, &flags);
if (!netif_running(skb->dev))
goto drop;
qlen = skb_queue_len(&sd->input_pkt_queue);
@@ -4536,26 +4555,21 @@ static int enqueue_to_backlog(struct sk_buff *skb, int cpu,
enqueue:
__skb_queue_tail(&sd->input_pkt_queue, skb);
input_queue_tail_incr_save(sd, qtail);
- rps_unlock(sd);
- local_irq_restore(flags);
+ rps_unlock_irq_restore(sd, &flags);
return NET_RX_SUCCESS;
}
/* Schedule NAPI for backlog device
* We can use non atomic operation since we own the queue lock
*/
- if (!__test_and_set_bit(NAPI_STATE_SCHED, &sd->backlog.state)) {
- if (!rps_ipi_queued(sd))
- ____napi_schedule(sd, &sd->backlog);
- }
+ if (!__test_and_set_bit(NAPI_STATE_SCHED, &sd->backlog.state))
+ napi_schedule_rps(sd);
goto enqueue;
}
drop:
sd->dropped++;
- rps_unlock(sd);
-
- local_irq_restore(flags);
+ rps_unlock_irq_restore(sd, &flags);
atomic_long_inc(&skb->dev->rx_dropped);
kfree_skb(skb);
@@ -5617,8 +5631,7 @@ static void flush_backlog(struct work_struct *work)
local_bh_disable();
sd = this_cpu_ptr(&softnet_data);
- local_irq_disable();
- rps_lock(sd);
+ rps_lock_irq_disable(sd);
skb_queue_walk_safe(&sd->input_pkt_queue, skb, tmp) {
if (skb->dev->reg_state == NETREG_UNREGISTERING) {
__skb_unlink(skb, &sd->input_pkt_queue);
@@ -5626,8 +5639,7 @@ static void flush_backlog(struct work_struct *work)
input_queue_head_incr(sd);
}
}
- rps_unlock(sd);
- local_irq_enable();
+ rps_unlock_irq_enable(sd);
skb_queue_walk_safe(&sd->process_queue, skb, tmp) {
if (skb->dev->reg_state == NETREG_UNREGISTERING) {
@@ -5645,16 +5657,14 @@ static bool flush_required(int cpu)
struct softnet_data *sd = &per_cpu(softnet_data, cpu);
bool do_flush;
- local_irq_disable();
- rps_lock(sd);
+ rps_lock_irq_disable(sd);
/* as insertion into process_queue happens with the rps lock held,
* process_queue access may race only with dequeue
*/
do_flush = !skb_queue_empty(&sd->input_pkt_queue) ||
!skb_queue_empty_lockless(&sd->process_queue);
- rps_unlock(sd);
- local_irq_enable();
+ rps_unlock_irq_enable(sd);
return do_flush;
#endif
@@ -5769,8 +5779,7 @@ static int process_backlog(struct napi_struct *napi, int quota)
}
- local_irq_disable();
- rps_lock(sd);
+ rps_lock_irq_disable(sd);
if (skb_queue_empty(&sd->input_pkt_queue)) {
/*
* Inline a custom version of __napi_complete().
@@ -5786,8 +5795,7 @@ static int process_backlog(struct napi_struct *napi, int quota)
skb_queue_splice_tail_init(&sd->input_pkt_queue,
&sd->process_queue);
}
- rps_unlock(sd);
- local_irq_enable();
+ rps_unlock_irq_enable(sd);
}
return work;
--
2.34.1
