netdev - [PATCH v6 net-next 08/15] net: softnet_data: Make xmit.recursion per task.

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Message-ID: <20240612170303.3896084-9-bigeasy@linutronix.de>
Date: Wed, 12 Jun 2024 18:44:34 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
	Daniel Bristot de Oliveira <bristot@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>,
	Will Deacon <will@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Ben Segall <bsegall@...gle.com>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH v6 net-next 08/15] net: softnet_data: Make xmit.recursion per task.

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@...gle.com>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Valentin Schneider <vschneid@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 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