From f856ea96a8126ca5444bcdb0a7885f3f2eceaaea Mon Sep 17 00:00:00 2001
From: John Ogness <john.ogness@linutronix.de>
Date: Sat, 10 Sep 2022 23:28:12 +0100
Subject: [PATCH] printk: Add threaded printing support for BKL consoles.
From 7bee14b72628e352498611781c81a7b7d6341144 Mon Sep 17 00:00:00 2001
References: bsc#1214683 (PREEMPT_RT prerequisite backports)
Git-commit: 07173eb6b3b2afa7a263c4d85f6cf5b9c7090f86
Patch-mainline: v6.6.20
Add threaded printing support for BKL consoles on PREEMPT_RT.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
kernel/printk/internal.h | 4 ++
kernel/printk/printk.c | 157 +++++++++++++++++++++++++++++++++++++------
kernel/printk/printk_nobkl.c | 4 +-
3 files changed, 144 insertions(+), 21 deletions(-)
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 53502de4e157..6b21fe0a38f5 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -43,6 +43,8 @@ enum printk_info_flags {
};
extern struct printk_ringbuffer *prb;
+extern bool have_bkl_console;
+extern bool printk_threads_enabled;
extern bool have_boot_console;
@@ -80,6 +82,7 @@ bool cons_alloc_percpu_data(struct console *con);
void cons_kthread_create(struct console *con);
void cons_wake_threads(void);
void cons_force_seq(struct console *con, u64 seq);
+void console_bkl_kthread_create(void);
/*
* Check if the given console is currently capable and allowed to print
@@ -137,6 +140,7 @@ static inline void cons_kthread_wake(struct console *con)
static inline void cons_kthread_wake(struct console *con) { }
static inline void cons_kthread_create(struct console *con) { }
+#define printk_threads_enabled (false)
/*
* In !PRINTK builds we still export console_sem
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 10561c29e3d0..c3aff0ba526d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -448,7 +448,7 @@ static DEFINE_MUTEX(syslog_lock);
* Specifies if a BKL console was ever registered. Used to determine if the
* console lock/unlock dance is needed for console printing.
*/
-static bool have_bkl_console;
+bool have_bkl_console;
/*
* Specifies if a boot console is registered. Used to determine if NOBKL
@@ -2330,7 +2330,7 @@ asmlinkage int vprintk_emit(int facility, int level,
cons_atomic_flush(&wctxt, true);
/* If called from the scheduler, we can not call up(). */
- if (!in_sched && have_bkl_console) {
+ if (!in_sched && have_bkl_console && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
/*
* Try to acquire and then immediately release the console
* semaphore. The release will print out buffers. With the
@@ -2637,6 +2637,9 @@ void resume_console(void)
}
console_srcu_read_unlock(cookie);
+ if (IS_ENABLED(CONFIG_PREEMPT_RT) && have_bkl_console)
+ wake_up_interruptible(&log_wait);
+
pr_flush(1000, true);
}
@@ -3019,19 +3022,7 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
return false;
}
-/**
- * console_unlock - unblock the console subsystem from printing
- *
- * Releases the console_lock which the caller holds to block printing of
- * the console subsystem.
- *
- * While the console_lock was held, console output may have been buffered
- * by printk(). If this is the case, console_unlock(); emits
- * the output prior to releasing the lock.
- *
- * console_unlock(); may be called from any context.
- */
-void console_unlock(void)
+static u64 console_flush_and_unlock(void)
{
bool do_cond_resched;
bool handover;
@@ -3074,6 +3065,34 @@ void console_unlock(void)
* fails, another context is already handling the printing.
*/
} while (prb_read_valid(prb, next_seq, NULL) && console_trylock());
+
+ return next_seq;
+}
+
+/**
+ * console_unlock - unblock the console subsystem from printing
+ *
+ * Releases the console_lock which the caller holds to block printing of
+ * the console subsystem.
+ *
+ * While the console_lock was held, console output may have been buffered
+ * by printk(). If this is the case, console_unlock(); emits
+ * the output prior to releasing the lock.
+ *
+ * console_unlock(); may be called from any context.
+ */
+void console_unlock(void)
+{
+ /*
+ * PREEMPT_RT relies on kthread and atomic consoles for printing.
+ * It never attempts to print from console_unlock().
+ */
+ if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+ __console_unlock();
+ return;
+ }
+
+ console_flush_and_unlock();
}
EXPORT_SYMBOL(console_unlock);
@@ -3295,11 +3314,99 @@ void console_start(struct console *console)
if (flags & CON_NO_BKL)
cons_kthread_wake(console);
+ else if (IS_ENABLED(CONFIG_PREEMPT_RT))
+ wake_up_interruptible(&log_wait);
__pr_flush(console, 1000, true);
}
EXPORT_SYMBOL(console_start);
+static struct task_struct *console_bkl_kthread;
+
+static bool printer_should_wake(u64 seq)
+{
+ bool available = false;
+ struct console *con;
+ int cookie;
+
+ if (kthread_should_stop())
+ return true;
+
+ cookie = console_srcu_read_lock();
+ for_each_console_srcu(con) {
+ short flags = console_srcu_read_flags(con);
+
+ if (flags & CON_NO_BKL)
+ continue;
+ if (!console_is_usable(con, flags))
+ continue;
+ /*
+ * It is safe to read @seq because only this
+ * thread context updates @seq.
+ */
+ if (prb_read_valid(prb, con->seq, NULL)) {
+ available = true;
+ break;
+ }
+ }
+ console_srcu_read_unlock(cookie);
+
+ return available;
+}
+
+static int console_bkl_kthread_func(void *unused)
+{
+ u64 seq = 0;
+ int error;
+
+ for (;;) {
+ error = wait_event_interruptible(log_wait, printer_should_wake(seq));
+
+ if (kthread_should_stop())
+ break;
+
+ if (error)
+ continue;
+
+ console_lock();
+ seq = console_flush_and_unlock();
+ }
+ return 0;
+}
+
+void console_bkl_kthread_create(void)
+{
+ struct task_struct *kt;
+ struct console *c;
+
+ lockdep_assert_held(&console_mutex);
+
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ return;
+
+ if (!printk_threads_enabled || console_bkl_kthread)
+ return;
+
+ for_each_console(c) {
+ if (c->flags & CON_BOOT)
+ return;
+ }
+
+ kt = kthread_run(console_bkl_kthread_func, NULL, "pr/bkl");
+ if (IS_ERR(kt)) {
+ pr_err("unable to start BKL printing thread\n");
+ return;
+ }
+
+ console_bkl_kthread = kt;
+
+ /*
+ * It is important that console printing threads are scheduled
+ * shortly after a printk call and with generous runtime budgets.
+ */
+ sched_set_normal(console_bkl_kthread, -20);
+}
+
static int __read_mostly keep_bootcon;
static int __init keep_bootcon_setup(char *str)
@@ -3538,10 +3645,12 @@ void register_console(struct console *newcon)
newcon->dropped = 0;
console_init_seq(newcon, bootcon_registered);
- if (!(newcon->flags & CON_NO_BKL))
+ if (!(newcon->flags & CON_NO_BKL)) {
have_bkl_console = true;
- else if (!cons_nobkl_init(newcon))
+ console_bkl_kthread_create();
+ } else if (!cons_nobkl_init(newcon)) {
goto unlock;
+ }
if (newcon->flags & CON_BOOT)
have_boot_console = true;
@@ -3914,9 +4023,17 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
int pending = this_cpu_xchg(printk_pending, 0);
if (pending & PRINTK_PENDING_OUTPUT) {
- /* If trylock fails, someone else is doing the printing */
- if (console_trylock())
- console_unlock();
+ if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+ /* The BKL thread waits on @log_wait. */
+ pending |= PRINTK_PENDING_WAKEUP;
+ } else {
+ /*
+ * If trylock fails, some other context
+ * will do the printing.
+ */
+ if (console_trylock())
+ console_unlock();
+ }
}
if (pending & PRINTK_PENDING_WAKEUP)
diff --git a/kernel/printk/printk_nobkl.c b/kernel/printk/printk_nobkl.c
index ff7b0de6ba0d..e0b818a4f8b3 100644
--- a/kernel/printk/printk_nobkl.c
+++ b/kernel/printk/printk_nobkl.c
@@ -907,7 +907,7 @@ static bool __cons_release(struct cons_context *ctxt)
return true;
}
-static bool printk_threads_enabled __ro_after_init;
+bool printk_threads_enabled __ro_after_init;
static bool printk_force_atomic __initdata;
/**
@@ -1750,6 +1750,8 @@ static int __init printk_setup_threads(void)
printk_threads_enabled = true;
for_each_console(con)
cons_kthread_create(con);
+ if (have_bkl_console)
+ console_bkl_kthread_create();
console_list_unlock();
return 0;
}