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

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Date:	Fri, 1 Apr 2016 10:08:03 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Petr Mladek <pmladek@...e.com>
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>,
	Jan Kara <jack@...e.cz>
Subject: Re: [RFC][PATCH v8 1/2] printk: Make printk() completely async

Hello Petr,

On (03/31/16 13:12), Petr Mladek wrote:
> > +	 * Set printing kthread sleep condition early, under the
> > +	 * logbuf_lock, so it (if RUNNING) will go to console_lock()
> > +	 * and spin on logbuf_lock.
> > +	 */
> > +	if (!in_panic && printk_kthread && !need_flush_console)
> > +		need_flush_console = true;
> 
> I would remove the if-statement and always set it:
> 
>   + It does not matter if we set it in panic. It will not affect
>     anything.

hm... yes, you're right.

>   + The check for printk_kthread is racy. It might be false here
>     and it might be true later when check whether to wakeup
>     the kthread or try to get console directly.

which is fine, isn't it? we will wakeup the printing kthread, it will
console_lock()/console_unlock() (regardless the state of need_flush_console).
printing thread checks need_flush_console only when it decides whether
it shall schedule.

>   + We might set it true even when it was true before.
> 
> I think that this was an attempt to avoid a spurious wake up.
> But we solve it better way now.

we also may have 'printk.synchronous = 1', which will purposelessly
dirty need_flush_console from various CPUs from every printk /* and
upon every return from console_unlock() */; that's why I added both
printk_kthread and !need_flush_console (re-dirty already dirtied)
checks.

> >  	raw_spin_lock(&logbuf_lock);
> >  	retry = console_seq != log_next_seq;
> > +	if (!retry && printk_kthread)
> > +		need_flush_console = false;
> 
> Similar here. I remove the if-statement and always set it. We will
> either retry or it should be false anyway.

well, 'printk.synchronous = 1'. seems that `!retry' check can be
dropped, I agree.

a side nano-note,
apart from 'printk.synchronous = 1', we also can have !printk_kthread
because kthread_run(printk_kthread_func) failed. it's quite unlikely,
but still.

[..]
> > +	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;

oh, wow! that's a major mistake. thanks a lot for catching this!
shame on me.

> 		/*
> 		 * Avoid an infinite loop when console_unlock() cannot
> 		 * access consoles, e.g. because of a suspend. We
> 		 * could get asleep here. Someone else will call
> 		 * consoles if conditions change.
> 		 */

looks ok.

> Also another name might help. If we set it false here, the value
> does describe a global state. The variable describes if this
> kthread needs to flush the console. So, more precise name would be
> 
> 	printk_kthread_need_flush_console

yes, makes sense.

> I think that we are close. I really like the current state of
> the patch and how minimalistic it is.


thanks for your review.

I'll re-spin.

	-ss