linux-kernel - Re: [PATCH rcu v2] 4/5] rcu-tasks: Move RCU Tasks self-tests to core

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Message-ID: <2a7774e4-a3fc-48d5-97b0-6f74396e7f93@paulmck-laptop>
Date: Wed, 5 Feb 2025 15:50:14 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: John Ogness <john.ogness@...utronix.de>
Cc: Petr Mladek <pmladek@...e.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, rostedt@...dmis.org,
	Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH rcu v2] 4/5] rcu-tasks: Move RCU Tasks self-tests to
 core_initcall()

On Wed, Feb 05, 2025 at 11:37:25PM +0106, John Ogness wrote:
> On 2025-02-05, John Ogness <john.ogness@...utronix.de> wrote:
> >> OK, so I don't need to add "if (IS_ENABLED(CONFIG_PREEMPT_RT))" to guard
> >> it, then?
> >
> > For !CONFIG_PREEMPT_RT, if there are legacy consoles registered,
> > pr_flush() will additionally perform a blocking lock on the
> > console_lock.
> 
> Sorry, I was just thinking about the flushing component of
> pr_flush(). Later in the function it takes the console_lock even for
> CONFIG_PREEMPT_RT. So please do _not_ have a guard.
> 
> pr_flush() should never hang on the console_lock during shutdown, but if
> does, that is something that will need to be debugged and fixed.
> 
> @pmladek: Looking forward to reading your input on this.

Here is an updated commit, hopefully applying your feedback correctly.

							Thanx, Paul

------------------------------------------------------------------------

commit 35679c18b062368855e183ee6712ca5c16145d8c
Author: Paul E. McKenney <paulmck@...nel.org>
Date:   Wed Feb 5 12:27:23 2025 -0800

    printk: Flush console log from kernel_power_off()
    
    Kernels built with CONFIG_PREEMPT_RT=y can lose significant console output
    and shutdown time, which hides shutdown-time RCU issues from rcutorture.
    Therefore, make pr_flush() public and invoke it after then last print
    in kernel_power_off().
    
    [ paulmck: Apply John Ogness feedback. ]
    
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
    Cc: Petr Mladek <pmladek@...e.com>
    Cc: Steven Rostedt <rostedt@...dmis.org>
    Cc: John Ogness <john.ogness@...utronix.de>
    Cc: Sergey Senozhatsky <senozhatsky@...omium.org>

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 4217a9f412b2..d658d7e81bef 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -207,6 +207,7 @@ void printk_legacy_allow_panic_sync(void);
 extern bool nbcon_device_try_acquire(struct console *con);
 extern void nbcon_device_release(struct console *con);
 void nbcon_atomic_flush_unsafe(void);
+bool pr_flush(int timeout_ms, bool reset_on_progress);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)
@@ -315,6 +316,11 @@ static inline void nbcon_atomic_flush_unsafe(void)
 {
 }
 
+static bool pr_flush(int timeout_ms, bool reset_on_progress)
+{
+	return true;
+}
+
 #endif
 
 bool this_cpu_in_panic(void);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 80910bc3470c..b9699eaacf19 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2461,7 +2461,6 @@ asmlinkage __visible int _printk(const char *fmt, ...)
 }
 EXPORT_SYMBOL(_printk);
 
-static bool pr_flush(int timeout_ms, bool reset_on_progress);
 static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress);
 
 #else /* CONFIG_PRINTK */
@@ -2474,7 +2473,6 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 
 static u64 syslog_seq;
 
-static bool pr_flush(int timeout_ms, bool reset_on_progress) { return true; }
 static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress) { return true; }
 
 #endif /* CONFIG_PRINTK */
@@ -4466,7 +4464,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
  * Context: Process context. May sleep while acquiring console lock.
  * Return: true if all usable printers are caught up.
  */
-static bool pr_flush(int timeout_ms, bool reset_on_progress)
+bool pr_flush(int timeout_ms, bool reset_on_progress)
 {
 	return __pr_flush(NULL, timeout_ms, reset_on_progress);
 }
diff --git a/kernel/reboot.c b/kernel/reboot.c
index a701000bab34..a84828d0757b 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -704,6 +704,7 @@ void kernel_power_off(void)
 	migrate_to_reboot_cpu();
 	syscore_shutdown();
 	pr_emerg("Power down\n");
+	pr_flush(1000, 1);
 	kmsg_dump(KMSG_DUMP_SHUTDOWN);
 	machine_power_off();
 }