[PATCH v4 net-next 07/14] net: softnet_data: Make xmit.recursion per task.

From: Sebastian Andrzej Siewior
Date: Tue Jun 04 2024 - 11:47:32 EST


Softirq is preemptible on PREEMPT_RT. Without a per-CPU lock in
local_bh_disable() there is no guarantee that only one device is
transmitting at a time.
With preemption and multiple senders it is possible that the per-CPU
recursion counter gets incremented by different threads and exceeds
XMIT_RECURSION_LIMIT leading to a false positive recursion alert.

Instead of adding a lock to protect the per-CPU variable it is simpler
to make the counter per-task. Sending and receiving skbs happens always
in thread context anyway.

Having a lock to protected the per-CPU counter would block/ serialize two
sending threads needlessly. It would also require a recursive lock to
ensure that the owner can increment the counter further.

Make the recursion counter a task_struct member on PREEMPT_RT.

Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
include/linux/netdevice.h | 11 +++++++++++
include/linux/sched.h | 4 +++-
net/core/dev.h | 20 ++++++++++++++++++++
3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index d20c6c99eb887..b5ec072ec2430 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3223,7 +3223,9 @@ struct softnet_data {
#endif
/* written and read only by owning cpu: */
struct {
+#ifndef CONFIG_PREEMPT_RT
u16 recursion;
+#endif
u8 more;
#ifdef CONFIG_NET_EGRESS
u8 skip_txqueue;
@@ -3256,10 +3258,19 @@ struct softnet_data {

DECLARE_PER_CPU_ALIGNED(struct softnet_data, softnet_data);

+#ifdef CONFIG_PREEMPT_RT
+static inline int dev_recursion_level(void)
+{
+ return current->net_xmit_recursion;
+}
+
+#else
+
static inline int dev_recursion_level(void)
{
return this_cpu_read(softnet_data.xmit.recursion);
}
+#endif

void __netif_schedule(struct Qdisc *q);
void netif_schedule_queue(struct netdev_queue *txq);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 61591ac6eab6d..a9b0ca72db55f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -975,7 +975,9 @@ struct task_struct {
/* delay due to memory thrashing */
unsigned in_thrashing:1;
#endif
-
+#ifdef CONFIG_PREEMPT_RT
+ u8 net_xmit_recursion;
+#endif
unsigned long atomic_flags; /* Flags requiring atomic access. */

struct restart_block restart_block;
diff --git a/net/core/dev.h b/net/core/dev.h
index b7b518bc2be55..2f96d63053ad0 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -150,6 +150,25 @@ struct napi_struct *napi_by_id(unsigned int napi_id);
void kick_defer_list_purge(struct softnet_data *sd, unsigned int cpu);

#define XMIT_RECURSION_LIMIT 8
+
+#ifdef CONFIG_PREEMPT_RT
+static inline bool dev_xmit_recursion(void)
+{
+ return unlikely(current->net_xmit_recursion > XMIT_RECURSION_LIMIT);
+}
+
+static inline void dev_xmit_recursion_inc(void)
+{
+ current->net_xmit_recursion++;
+}
+
+static inline void dev_xmit_recursion_dec(void)
+{
+ current->net_xmit_recursion--;
+}
+
+#else
+
static inline bool dev_xmit_recursion(void)
{
return unlikely(__this_cpu_read(softnet_data.xmit.recursion) >
@@ -165,5 +184,6 @@ static inline void dev_xmit_recursion_dec(void)
{
__this_cpu_dec(softnet_data.xmit.recursion);
}
+#endif

#endif
--
2.45.1