summaryrefslogtreecommitdiffstats
path: root/debian/patches-rt/0040-printk-nbcon-Add-printer-thread-wakeups.patch
blob: 89e086aa32122bb0baf83438bfc90dfbf98c5a5d (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 26 Sep 2023 13:03:52 +0000
Subject: [PATCH 40/50] printk: nbcon: Add printer thread wakeups
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.7/older/patches-6.7-rt6.tar.xz

Add a function to wakeup the printer threads. Use the new function
when:

  - records are added to the printk ringbuffer
  - consoles are resumed
  - triggered via printk_trigger_flush()

The actual waking is performed via irq_work so that the wakeup can
be triggered from any context.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/console.h  |    3 ++
 kernel/printk/internal.h |    1 
 kernel/printk/nbcon.c    |   56 +++++++++++++++++++++++++++++++++++++++++++++++
 kernel/printk/printk.c   |    7 +++++
 4 files changed, 67 insertions(+)

--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -16,6 +16,7 @@
 
 #include <linux/atomic.h>
 #include <linux/bits.h>
+#include <linux/irq_work.h>
 #include <linux/rculist.h>
 #include <linux/rcuwait.h>
 #include <linux/types.h>
@@ -313,6 +314,7 @@ struct nbcon_write_context {
  * @locked_port:	True, if the port lock is locked by nbcon
  * @kthread:		Printer kthread for this console
  * @rcuwait:		RCU-safe wait object for @kthread waking
+ * @irq_work:		Defer @kthread waking to IRQ work context
  */
 struct console {
 	char			name[16];
@@ -346,6 +348,7 @@ struct console {
 	bool			locked_port;
 	struct task_struct	*kthread;
 	struct rcuwait		rcuwait;
+	struct irq_work		irq_work;
 };
 
 #ifdef CONFIG_LOCKDEP
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -93,6 +93,7 @@ enum nbcon_prio nbcon_get_default_prio(v
 void nbcon_atomic_flush_all(void);
 bool nbcon_atomic_emit_next_record(struct console *con, bool *handover, int cookie);
 void nbcon_kthread_create(struct console *con);
+void nbcon_wake_threads(void);
 
 /*
  * Check if the given console is currently capable and allowed to print
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1052,6 +1052,61 @@ static int nbcon_kthread_func(void *__co
 	goto wait_for_event;
 }
 
+/**
+ * nbcon_irq_work - irq work to wake printk thread
+ * @irq_work:	The irq work to operate on
+ */
+static void nbcon_irq_work(struct irq_work *irq_work)
+{
+	struct console *con = container_of(irq_work, struct console, irq_work);
+
+	nbcon_kthread_wake(con);
+}
+
+static inline bool rcuwait_has_sleeper(struct rcuwait *w)
+{
+	bool has_sleeper;
+
+	rcu_read_lock();
+	/*
+	 * Guarantee any new records can be seen by tasks preparing to wait
+	 * before this context checks if the rcuwait is empty.
+	 *
+	 * This full memory barrier pairs with the full memory barrier within
+	 * set_current_state() of ___rcuwait_wait_event(), which is called
+	 * after prepare_to_rcuwait() adds the waiter but before it has
+	 * checked the wait condition.
+	 *
+	 * This pairs with nbcon_kthread_func:A.
+	 */
+	smp_mb(); /* LMM(rcuwait_has_sleeper:A) */
+	has_sleeper = !!rcu_dereference(w->task);
+	rcu_read_unlock();
+
+	return has_sleeper;
+}
+
+/**
+ * nbcon_wake_threads - Wake up printing threads using irq_work
+ */
+void nbcon_wake_threads(void)
+{
+	struct console *con;
+	int cookie;
+
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(con) {
+		/*
+		 * Only schedule irq_work if the printing thread is
+		 * actively waiting. If not waiting, the thread will
+		 * notice by itself that it has work to do.
+		 */
+		if (con->kthread && rcuwait_has_sleeper(&con->rcuwait))
+			irq_work_queue(&con->irq_work);
+	}
+	console_srcu_read_unlock(cookie);
+}
+
 /* Track the nbcon emergency nesting per CPU. */
 static DEFINE_PER_CPU(unsigned int, nbcon_pcpu_emergency_nesting);
 static unsigned int early_nbcon_pcpu_emergency_nesting __initdata;
@@ -1418,6 +1473,7 @@ void nbcon_init(struct console *con)
 	BUG_ON(!con->pbufs);
 
 	rcuwait_init(&con->rcuwait);
+	init_irq_work(&con->irq_work, nbcon_irq_work);
 	nbcon_seq_force(con, con->seq);
 	nbcon_state_set(con, &state);
 }
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2402,6 +2402,8 @@ asmlinkage int vprintk_emit(int facility
 		}
 	}
 
+	nbcon_wake_threads();
+
 	if (do_trylock_unlock) {
 		/*
 		 * The caller may be holding system-critical or
@@ -2712,6 +2714,10 @@ void resume_console(void)
 	 */
 	synchronize_srcu(&console_srcu);
 
+	/*
+	 * Since this runs in task context, wake the threaded printers
+	 * directly rather than scheduling irq_work to do it.
+	 */
 	cookie = console_srcu_read_lock();
 	for_each_console_srcu(con) {
 		flags = console_srcu_read_flags(con);
@@ -4104,6 +4110,7 @@ void defer_console_output(void)
 
 void printk_trigger_flush(void)
 {
+	nbcon_wake_threads();
 	defer_console_output();
 }