linux-kernel - Re: [RFC][PATCH v8 1/2] printk: Make printk() completely async

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Date:	Mon, 4 Apr 2016 11:41:08 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>, Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org,
	Byungchul Park <byungchul.park@....com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCH v8 1/2] printk: Make printk() completely async

On Thu 2016-03-31 13:52:50, Jan Kara wrote:
> On Thu 31-03-16 13:12:29, Petr Mladek wrote:
> > >  #if defined CONFIG_PRINTK
> > > +static int printk_kthread_func(void *data)
> > > +{
> > > +	while (1) {
> > > +		set_current_state(TASK_INTERRUPTIBLE);
> > > +		if (!need_flush_console)
> > > +			schedule();
> > > +
> > > +		__set_current_state(TASK_RUNNING);
> > 
> > 
> > We still must do here:
> > 
> > 		need_flush_console = false;
> > 
> > Othrerwise, we might start "busy" cycling. cosole_unlock()
> > sometimes returns earlly, e.g. when console_suspended is set
> > or !can_use_console() returns true.
> > 
> > Sigh, the handling of "need_flush_console" is a bit strange.
> > Part of the logic depends on logbuf_lock and the other part
> > must be lockless.
> 
> Frankly, I think we are overcomplicating this. What we really need to detect
> in printk_kthread_func() is whether someone appended something to the console
> since we woken up. Sure, console_unlock() may have already printed that
> and we would unnecessarily make one more loop over console_lock() and
> console_unlock() but who cares...
> 
> So what about having printk_kthread_func() like:
> 
> 	while (1) {
> 		set_current_state(TASK_INTERRUPTIBLE);
> 		if (!need_flush_console)
> 			schedule();
> 		__set_current_state(TASK_RUNNING);
> 		need_flush_console = false;
> 		console_lock();
> 		console_unlock();
> 	}
> 
> In vprintk_emit() we do:
> 
> 	if (!in_panic && printk_kthread) {
> 		/* Offload printing to a schedulable context. */
> 		need_flush_console = true;
> 		wake_up_process(printk_kthread);
> 	} else {
> 		...
> 
> This guarantees that after message was appended to the buffer in
> vprintk_emit(), the message got either printed by console_unlock() or
> printk_kthread is in TASK_RUNNING state and will call console_unlock() once
> scheduled. It also guarantees that printk_kthread_func() won't loop forever
> when there's nothing to print. And that is all we need...
> 
> I think the simplicity of this is worth the possible extra loops in
> printk_kthread_func().

I do not have strong opinion about this. I agree that the simplicity
of your proposal is nice. You are much more experienced kernel
developer. If you say that the potential extra loop is fine, I am
fine with it as well :-)

Best Regards,
Petr