linux-kernel - Re: [PATCH printk v2 07/18] printk: nbcon: Add printer thread wakeups

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Date: Thu, 13 Jun 2024 17:08:34 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH printk v2 07/18] printk: nbcon: Add printer thread wakeups

On Tue 2024-06-04 01:30:42, John Ogness wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> Add a function to wakeup the printer threads. The printer threads
> are woken when:
> 
>   - a record is added to the printk ringbuffer
>   - consoles are resumed
>   - triggered via printk_trigger_flush()
>   - consoles should be replayed via sysrq
> 
> The actual waking is performed via irq_work so that the function
> can be called from any context.
> 
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1058,6 +1058,61 @@ static int nbcon_kthread_func(void *__console)
>  	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);

We should use the existing API rcuwait_active().

> +	rcu_read_unlock();
> +
> +	return has_sleeper;
> +}
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2402,6 +2402,8 @@ asmlinkage int vprintk_emit(int facility, int level,
>  		}
>  	}
>  
> +	nbcon_wake_threads();

This need to be called only when there is a nbcon console
and no boot console and it has not been flushed explicitly.

I think that we could move this slightly above:

	if (have_nbcon_console && !have_boot_console) {
[...]
		if (is_panic_context ||
		    !printk_threads_enabled ||
		    (system_state > SYSTEM_RUNNING)) {
			nbcon_atomic_flush_pending();
		} else {
			nbcon_wake_threads();
		}
	}

> +
>  	if (do_trylock_unlock) {
>  		/*
>  		 * The caller may be holding system-critical or
> @@ -2708,6 +2710,10 @@ 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);

The wake up call has already been added in 4th patch of this patchset.

I would slightly prefer to move it from the 4th patch to this one.
Same with start_console(). But it is not super important.


> @@ -4178,6 +4184,7 @@ void defer_console_output(void)
>  
>  void printk_trigger_flush(void)
>  {
> +	nbcon_wake_threads();

IMHO, this is not needed. vprintk_emit() always either flushes nbcon
consoles directly or wakes them.

In each case, it is not needed when printk_trigger_flush() is called
from nbcon_cpu_emergency_exit().

Hmm, I am not sure about the situation in nmi_trigger_cpumask_backtrace().
printk_trigger_flush() is called there to queue the IRQ work yet
another CPU to be on the safe side. But the irq_work used by
nbcon is per-console (not per-CPU). I guess that an attempt to
queue it on 2nd CPU would be a NOP.

>  	defer_console_output();
>  }
>  
> @@ -4513,6 +4520,7 @@ void console_try_replay_all(void)
>  {
>  	if (console_trylock()) {
>  		__console_rewind_all();
> +		nbcon_wake_threads();
>  		/* Consoles are flushed as part of console_unlock(). */
>  		console_unlock();
>  	}

Just an idea. We probably could do better for nbcon consoles. Like try
to flush them directly with emergency prio. But it can be done
in a separate patch later.

Best Regards,
Petr