diff mbox series

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

Message ID 20240503182957.1042122-9-bigeasy@linutronix.de (mailing list archive)
State Changes Requested
Delegated to: Netdev Maintainers
Headers show
Series locking: Introduce nested-BH locking. | expand

Checks

Context Check Description
netdev/series_format success Posting correctly formatted
netdev/tree_selection success Clearly marked for net-next, async
netdev/ynl success Generated files up to date; no warnings/errors; no diff in generated;
netdev/fixes_present success Fixes tag not required for -next series
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 15129 this patch: 15129
netdev/build_tools success Errors and warnings before: 0 this patch: 0
netdev/cc_maintainers success CCed 15 of 15 maintainers
netdev/build_clang success Errors and warnings before: 2066 this patch: 2066
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/deprecated_api success None detected
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 16297 this patch: 16297
netdev/checkpatch success total: 0 errors, 0 warnings, 0 checks, 69 lines checked
netdev/build_clang_rust success No Rust files in patch. Skipping build
netdev/kdoc success Errors and warnings before: 22 this patch: 22
netdev/source_inline success Was 0 now: 0
netdev/contest success net-next-2024-05-05--03-00 (tests: 1003)

Commit Message

Sebastian Andrzej Siewior May 3, 2024, 6:25 p.m. UTC
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@google.com>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/netdevice.h | 11 +++++++++++
 include/linux/sched.h     |  4 +++-
 net/core/dev.h            | 20 ++++++++++++++++++++
 3 files changed, 34 insertions(+), 1 deletion(-)
diff mbox series

Patch

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 41853424b41d7..c551ec235f9af 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3222,7 +3222,9 @@  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;
@@ -3255,10 +3257,19 @@  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 3c2abbc587b49..6779d3b8f2578 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -969,7 +969,9 @@  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