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Date:	Mon, 6 Jan 2014 10:46:08 +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 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?

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