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

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Date:	Fri, 1 Apr 2016 13:30:28 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.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>,
	Jan Kara <jack@...e.cz>
Subject: Re: [RFC][PATCH v8 1/2] printk: Make printk() completely async

On Fri 2016-04-01 18:36:02, Sergey Senozhatsky wrote:
> Hello Petr,
> 
> On (04/01/16 10:59), Petr Mladek wrote:
> [..]
> > CPU0					CPU1
> > 
> > printk()
> > 
> >   if (printk_kthread)
> >   # fails and need_flush_console
> >   # stays false
> > 
> > 					init_printk_kthread()
> > 					  # put printk_thread into
> > 					  # run queue
> > 					  printk_kthread = ...;
> > 
> >   if (!in_panic && printk_kthread)
> >     wake_up_process(printk_kthread);
> > 
> > 
> > 					# printk kthread finally gets
> > 					# scheduled
> > 					printk_kthread_func()
> > 
> > 					set_current_state(TASK_INTERRUPTIBLE);
> > 					if (!need_flush_console)
> > 					  schedule();
> > 
> > => printk_kthread is happily sleeping without calling console.
> 
> oohh, that tiny race. well, looks quite harmless, it's unlikely that
> we had printk()-s up until late_initcall(init_printk_kthread) and not
> a single one ever after. but good find!
> 
> so the check
> 	if (printk_kthread)
> 		need_flush_console = 1
> 
> can be replaced with
> 	if (!printk_sync)
> 		need_flush_console = 1
> 
> or... may be dropped.

Yup or yup, see below.

> > I do not see any code that will modify need_flush_console when
> > printk.synchronous is modified at runtime.
> 
> printk.synchronous is RO; no runtime modification.
> 
> > I know that all this is rather theoretical. My main point is to remove
> > unnecessary checks that make the code harder to read and does not bring
> > any big advantage.
> 
> my point is that those checks are just .loads, which help to avoid
> spurious .stores from various CPUs.
> 
> CPU1		CPU2		CPU3		...	CPU1024
> 
> lock logbuf
> need_flush=1
> unlock logbuf
> 		lock logbuf
> 		need_flush=1
> 		unlock logbuf
> 				lock logbuf
> 				need_flush=1
> 				unlock logbuf
> wakeup kthread					
> 						...
> 							lock logbuf
> 							need_flush=1
> 							unlock logbuf
> 
> isn't it a bit useless need_flush=1 traffic?

So, it is a small trade off between code readability and data writes
in a slow path. I do not have strong opinion about it. Feel free to
use what you like, just please use the same approach in both
vprintk_emit() and console_unlock().

Best Regards,
Petr