Date:   Fri, 4 Mar 2022 14:24:30 +0100
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
Subject: Re: [PATCH printk v1 08/13] printk: add pr_flush()

On Wed 2022-03-02 18:29:09, John Ogness wrote:
> On 2022-02-17, Petr Mladek <pmladek@...e.com> wrote:
> >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> >> index 02bde45c1149..1e80fd052bd5 100644
> >> --- a/kernel/printk/printk.c
> >> +++ b/kernel/printk/printk.c
> >> @@ -2802,8 +2804,10 @@ void console_unblank(void)
> >>  	if (oops_in_progress) {
> >>  		if (down_trylock_console_sem() != 0)
> >>  			return;
> >> -	} else
> >> +	} else {
> >> +		pr_flush(1000, true);
> >
> > It would make more sense to flush the consoles after they are
> > unblanked. I mean to move this to the end of the function.
> 
> Agreed.
> 
> > Also it is not obvious why this is not called when oops_in_progress
> > is set. I guess that it is because trylock is needed in this case.
> > It should be handled inside pr_flush().
> >
> > I mean that pr_flush() should internally use trylock when
> > @oops_in_progress is set. It will make it safe even in this
> > mode.
> 
> pr_flush() is a might_sleep() function. We agreed on this at
> LPC2019.

I see.

> Creating a pr_flush() that will directly push out the messages (or
> busy-wait) in non-preemptible contexts is complicated. It might be
> something to attempt for the future, but I would prefer to avoid it at
> this stage.

Sure. It is not needed now, definitely.


> >>  		console_lock();
> >> +	}
> >>  
> >>  	console_locked = 1;
> >>  	console_may_schedule = 0;
> >> @@ -2869,6 +2873,7 @@ struct tty_driver *console_device(int *index)
> >>   */
> >>  void console_stop(struct console *console)
> >>  {
> >> +	pr_flush(1000, true);
> >
> > It would be enough to flush just the given @console.
> 
> For v2 I will create an internal __pr_flush() to allow specifying that
> only a single console is flushed. The high level pr_flush() will then
> call __pr_flush() specifying all consoles.

Feel free to keep it as is when it gets to complicated.
We could always optimize it later.


> > It might be possible to take over the job from the related
> > kthread and flush it in this context. Well, I am not sure if
> > it is a good idea.
> 
> I agree that it might not be a good idea. Let's keep things simple for
> now.

I agree.

> >>  	console_lock();
> >>  	console->flags &= ~CON_ENABLED;
> >>  	console_unlock();
> >> @@ -2880,6 +2885,7 @@ void console_start(struct console *console)
> >>  	console_lock();
> >>  	console->flags |= CON_ENABLED;
> >>  	console_unlock();
> >> +	pr_flush(1000, true);
> >
> > Same here.
> 
> OK.
> 
> >>  }
> >>  EXPORT_SYMBOL(console_start);
> >>  
> >> @@ -3249,6 +3255,71 @@ static int __init printk_late_init(void)
> >>  late_initcall(printk_late_init);
> >>  
> >>  #if defined CONFIG_PRINTK
> >> +/**
> >> + * pr_flush() - Wait for printing threads to catch up.
> >> + *
> >
> > Alternative solution would be to take over the job from the kthreads
> > and flush the consoles in this context. Well, I am not sure
> > if it is a good idea or not.
> 
> Since pr_flush() is might_sleep() this would be relatively simple. Just
> grab the console mutex and go. My concern is that this task may have
> different scheduling parameters that could negatively affect the
> system. For normal operation, I really would prefer that the designated
> kthreads do the work. If "waiting for the kthreads" turns out to be
> problematic, then maybe we could go down this path.

The different scheduling parameters were actually my concern. I though
about a high priority task waiting for normal priority kthreads.

Anyway, let's keep it simple now. Only practice will show if
a more complex solution is needed.

> >> + * @timeout_ms:        The maximum time (in ms) to wait.
> >> + * @reset_on_progress: Reset the timeout if forward progress is seen.
> >> + *
> >> + * A value of 0 for @timeout_ms means no waiting will occur. A value of -1
> >> + * represents infinite waiting.
> >> + *
> >> + * If @reset_on_progress is true, the timeout will be reset whenever any
> >> + * printer has been seen to make some forward progress.
> >> + *
> >> + * Context: Process context. May sleep while acquiring console lock.
> >> + * Return: true if all enabled printers are caught up.
> >> + */
> >> +bool pr_flush(int timeout_ms, bool reset_on_progress)
> >> +{
> >> +	int remaining = timeout_ms;
> >> +	struct console *con;
> >> +	u64 last_diff = 0;
> >> +	u64 printk_seq;
> >> +	u64 diff;
> >> +	u64 seq;
> >> +
> >> +	might_sleep();
> >> +
> >> +	seq = prb_next_seq(prb);
> >> +
> >> +	for (;;) {
> >> +		diff = 0;
> >> +
> >> +		console_lock();
> >> +		for_each_console(con) {
> >> +			if (!console_is_usable(con))
> >> +				continue;
> >> +			printk_seq = con->seq;
> >> +			if (printk_seq < seq)
> >> +				diff += seq - printk_seq;
> >> +		}
> >> +		console_unlock();
> >> +
> >> +		if (diff != last_diff && reset_on_progress)
> >> +			remaining = timeout_ms;
> >> +
> >> +		if (diff == 0 || remaining == 0)
> >> +			break;
> >> +
> >> +		if (remaining < 0) {
> >> +			/* no timeout limit */
> >> +			msleep(100);
> >> +		} else if (remaining < 100) {
> >> +			msleep(remaining);
> >> +			remaining = 0;
> >> +		} else {
> >> +			msleep(100);
> >> +			remaining -= 100;
> >> +		}
> >> +
> >> +		last_diff = diff;
> >> +	}
> >> +
> >> +	return (diff == 0);
> >> +}
> >> +EXPORT_SYMBOL(pr_flush);
> >
> > Summary:
> >
> > The pr_flush() API and the optional timeout look reasonable to me.
> >
> > Please, handle oops_in_progress in pr_flush() and make it safe in this
> > mode. It will allow to move it at the end of console_unblank() where
> > it makes more sense.
> 
> I will add another oops_in_progress check at the end. pr_flush() will
> not be made safe for oops_in_progress. Keep in mind that when
> oops_in_progress is set, direct printing will be active, so there should
> be no need for pr_flush() anyway.

Sounds good.

Best Regards,
Petr

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