linux-kernel - Re: [PATCH 9/9] printk: Hand over printing to console if printing too long

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Message-ID: <20140113072827.GF3837@quack.suse.cz>
Date:	Mon, 13 Jan 2014 08:28:27 +0100
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.cz>, pmladek@...e.cz,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 9/9] printk: Hand over printing to console if printing
 too long

On Mon 06-01-14 10:46:08, Jan Kara wrote:
> On Sat 04-01-14 23:57:43, Andrew Morton wrote:
> > On Mon, 23 Dec 2013 21:39:30 +0100 Jan Kara <jack@...e.cz> wrote:
> > 
> > > Currently, console_unlock() prints messages from kernel printk buffer to
> > > console while the buffer is non-empty. When serial console is attached,
> > > printing is slow and thus other CPUs in the system have plenty of time
> > > to append new messages to the buffer while one CPU is printing. Thus the
> > > CPU can spend unbounded amount of time doing printing in console_unlock().
> > > This is especially serious problem if the printk() calling
> > > console_unlock() was called with interrupts disabled.
> > > 
> > > In practice users have observed a CPU can spend tens of seconds printing
> > > in console_unlock() (usually during boot when hundreds of SCSI devices
> > > are discovered) resulting in RCU stalls (CPU doing printing doesn't
> > > reach quiescent state for a long time), softlockup reports (IPIs for the
> > > printing CPU don't get served and thus other CPUs are spinning waiting
> > > for the printing CPU to process IPIs), and eventually a machine death
> > > (as messages from stalls and lockups append to printk buffer faster than
> > > we are able to print). So these machines are unable to boot with serial
> > > console attached. Also during artificial stress testing SATA disk
> > > disappears from the system because its interrupts aren't served for too
> > > long.
> > > 
> > > This patch implements a mechanism where after printing specified number
> > > of characters (tunable as a kernel parameter printk.offload_chars), CPU
> > > doing printing asks for help by setting a 'hand over' state. The CPU
> > > still keeps printing until another CPU running printk() or a CPU being
> > > pinged by an IPI comes and takes over printing.  This way no CPU should
> > > spend printing too long if there is heavy printk traffic.
> > 
> > It all seems to rely on luck?  If there are 100k characters queued and
> > all the other CPUs stop calling printk(), the CPU which is left in
> > printk is screwed, isn't it?  If so, perhaps it can send an async IPI
> > to ask for help?
>   Let me cite a sentence from the changelog:
> "... until another CPU running printk() or a CPU being pinged by an IPI
> comes and takes over printing."
> 
>   So sending IPI (async one) to another CPU to come and take over printing
> is already implemented :). Do you have a better suggestion how to make that
> more obvious in the changelog?
  Ping Andrew, did you have a look at the patch?

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