Subject: [PATCH 36/48] printk: nbcon: Add printer thread wakeups
From: Thomas Gleixner <[email-masked]>
Date: Tue, 26 Sep 2023 13:03:52 +0000

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 <[email-masked]>
Signed-off-by: John Ogness <[email-masked]>
Signed-off-by: Thomas Gleixner (Intel) <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 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(+)

Index: linux-6.8.2-rt10/include/linux/console.h
===================================================================
--- linux-6.8.2-rt10.orig/include/linux/console.h
+++ linux-6.8.2-rt10/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>
@@ -327,6 +328,7 @@ struct nbcon_drvdata {
  * @pbufs:		Pointer to nbcon private buffer
  * @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];
@@ -457,6 +459,7 @@ struct console {
 	struct printk_buffers	*pbufs;
 	struct task_struct	*kthread;
 	struct rcuwait		rcuwait;
+	struct irq_work		irq_work;
 };
 
 #ifdef CONFIG_LOCKDEP
Index: linux-6.8.2-rt10/kernel/printk/internal.h
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/internal.h
+++ linux-6.8.2-rt10/kernel/printk/internal.h
@@ -91,6 +91,7 @@ void nbcon_atomic_flush_pending(void);
 bool nbcon_legacy_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
Index: linux-6.8.2-rt10/kernel/printk/nbcon.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/nbcon.c
+++ linux-6.8.2-rt10/kernel/printk/nbcon.c
@@ -1056,6 +1056,61 @@ wait_for_event:
 	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;
@@ -1451,6 +1506,7 @@ 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, 0);
 	nbcon_state_set(con, &state);
 }
Index: linux-6.8.2-rt10/kernel/printk/printk.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/printk.c
+++ linux-6.8.2-rt10/kernel/printk/printk.c
@@ -2415,6 +2415,8 @@ asmlinkage int vprintk_emit(int facility
 		}
 	}
 
+	nbcon_wake_threads();
+
 	if (do_trylock_unlock) {
 		/*
 		 * The caller may be holding system-critical or
@@ -2721,6 +2723,10 @@ 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);
@@ -4151,6 +4157,7 @@ void defer_console_output(void)
 
 void printk_trigger_flush(void)
 {
+	nbcon_wake_threads();
 	defer_console_output();
 }