linux-kernel - Re: [PATCH printk v3 11/19] printk: nbcon: Rely on kthreads for normal operation

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Message-ID: <Zqta5T-ydHcSaCvR@pathway.suse.cz>
Date: Thu, 1 Aug 2024 11:52:37 +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
Subject: Re: [PATCH printk v3 11/19] printk: nbcon: Rely on kthreads for
 normal operation

On Thu 2024-08-01 11:36:53, Petr Mladek wrote:
> On Wed 2024-07-31 17:31:02, John Ogness wrote:
> > On 2024-07-31, Petr Mladek <pmladek@...e.com> wrote:
> > >> @@ -4102,8 +4139,10 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
> > >>  			 * that they make forward progress, so only increment
> > >>  			 * @diff for usable consoles.
> > >>  			 */
> > >> -			if (!console_is_usable(c, flags, true))
> > >> +			if (!console_is_usable(c, flags, true) &&
> > >> +			    !console_is_usable(c, flags, false)) {
> > >
> > > This looks weird. nbcon console can't make progress when
> > > "write_atomic" is not implemented and the kthreads are not
> > > running.
> > >
> > > I should be:
> > >
> > > 			if (!((console_is_usable(c, flags, true)) ||
> > > 			      (console_is_usable(c, flags, false) && printk_kthreads_running))) {
> > 
> > I would prefer to have the printk_kthreads_running check within
> > console_is_usable() for the !use_atomic case.
> 
> Makes sense.

I have realized right after sending the reply that it actually did
not make sense.

We try to use con->write_thread also in

 + console_flush_all()
   + nbcon_legacy_emit_next_record()
     + nbcon_emit_one()
       + nbcon_emit_next_record()

when (@do_cond_resched == true)   =>   (@use_atomic == false)

=> __pr_flush() actually should be able to flush the messages using
   con->write_thread() even when the kthreads are not running.


Result:

IMHO, we should not check @printk_kthreads_running at all after all.

Sigh, it might make sense to document the design from a top level
view somewhere. We should do it at the end before we forget
the details.

Best Regards,
Petr