linux-kernel - Re: [PATCH 1/7] printk: Hand over printing to console if printing too long

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Date:	Wed, 6 Jan 2016 11:21:46 +0100
From:	Jan Kara <jack@...e.cz>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>,
	KY Sri nivasan <kys@...rosoft.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH 1/7] printk: Hand over printing to console if printing
 too long

On Wed 06-01-16 17:36:53, Sergey Senozhatsky wrote:
> On (01/06/16 12:38), Sergey Senozhatsky wrote:
> > On (01/05/16 15:48), Jan Kara wrote:
> > > > [..]
> > > > > cond_resched() does its job there, of course. well, a user process still can
> > > > > do a lot of call_console_drivers() calls. may be we can check who is calling
> > > > > console_unlock() and if we have "!printk_sync && !oops_in_progress" (or just printk_sync
> > > > > test) AND a user process then return from console_unlock() doing irq_work_queue()
> > > > > and set PRINTK_PENDING_OUTPUT pending bit, the way vprintk_emit() does it.
> > > > 
> > > > attached two patches, I ended up having on top of yours. just in case.
> > > > 
> > > >     printk: factor out can_printk_async()
> > > >     
> > > >     console_unlock() can be called directly or indirectly by a user
> > > >     space process, so it can end up doing call_console_drivers() loop,
> > > >     which will hold it from returning back to user-space from a syscall
> > > >     for unpredictable amount of time.
> > > >     
> > > >     Factor out can_printk_async() function, which queues an irq work and
> > > >     sets a PRINTK_PENDING_OUTPUT pending bit (if we can do async printk).
> > > >     vprintk_emit() already does it, add can_printk_async() call to
> > > >     console_unlock() for !PF_KTHREAD processes.
> > > 
> > > I'd be cautious about changing this userspace visible behavior. Someone may
> > > be relying on it... I agree that sometimes we can block userspace process
> > > in kernel for a long time (e.g. in my testing I often see syslog process
> > > doing the printing) but so far I didn't see / was notified about some real
> > > problem with this. So unless I see some real user issues with user
> > > processes doing printing for too long I would not touch this.
> > 
> > and w/o a lot of effort (no heavy printk message traffic)
> 
> or like this on another setup ([k|u]_ts updated to u64)
> 
> # cat /proc/1/time_in_console_unlock
> kern:[12.755920] user:[38.367332]

So maybe that is worth addressing if it bothers you but please as a
separate patch set. This seems fairly independent and I think even current
version of the patches will be controversial enough...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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