linux-kernel - Re: [RFC][PATCH v6 1/2] printk: Make printk() completely async

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Date:	Wed, 23 Mar 2016 15:41:44 +0100
From:	Jan Kara <jack@...e.cz>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Petr Mladek <pmladek@...e.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...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>
Subject: Re: [RFC][PATCH v6 1/2] printk: Make printk() completely async

On Wed 23-03-16 23:30:20, Sergey Senozhatsky wrote:
> Hello,
> 
> On (03/23/16 14:20), Jan Kara wrote:
> [..]
> > > I though that it actually could be an advantage. console_verbore() is
> > > called also by oops_begin() and it does not need to be fatal. But you
> > > are right that it does not need to be the righ approach.
> > 
> > If we oops, I want printk to be sync regardless whether the machine is able
> > to live afterwards or not. You never know in advance... That's why I've
> > chosen the console_verbose() trigger and I still think it is better than
> > oops_in_progress or special console_panic() trigger.
> 
> console_verbose() is good enough. well, with special console_panic() trigger
> we can have better control and distinguish between "print a lot" and "things
> are bad for sure, print in sync mode". there are 2 archs that call console_verbose()
> in setup_arch():
> 
> - arch/microblaze/kernel/setup.c
> 
> : void __init setup_arch(char **cmdline_p)
> : {
> :         *cmdline_p = boot_command_line;
> : 
> :         console_verbose();			<<<<
> : 
> :         unflatten_device_tree();
> : 
> :         setup_cpuinfo();
> : 
> :         microblaze_cache_init();
> : 
> :         setup_memory();
> 
> - arch/nios2/kernel/setup.c
> 
> : void __init setup_arch(char **cmdline_p)
> : {
> :         int bootmap_size;
> : 
> :         console_verbose();			<<<<
> : 
> : #ifdef CONFIG_EARLY_PRINTK
> :         setup_early_printk();
> : #endif
> 
> 
> so printk will never work in async mode there. hm... should we care?

I don't think we do. After all I believe they do console_verbose() to make
debugging boot issues easier and printk_sync helps that as well. If they
care about printk being async, they could always reset printk_sync to false
after boot is done...
								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR