linux-kernel - Re: [PATCH 0/4] printk: Softlockup avoidance

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Message-Id: <20150918151411.a3fa65c3e4f33f9f2ddf1fd8@linux-foundation.org>
Date:	Fri, 18 Sep 2015 15:14:11 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, pmladek@...e.com,
	rostedt@...dmis.org, Gavin Hu <gavin.hu.2010@...il.com>,
	KY Srinivasan <kys@...rosoft.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 0/4] printk: Softlockup avoidance

On Wed, 19 Aug 2015 17:38:27 +0200 Jan Kara <jack@...e.com> wrote:

> From: Jan Kara <jack@...e.cz>
> 
> Hello,
> 
> since lately there were several attempts at dealing with softlockups due
> to heavy printk traffic [1] [2] and I've been also privately pinged by
> couple of people about the state of the patch set, I've decided to respin
> the patch set.
> 
> To remind the original problem:
> 
> 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 when printk() gets called under some critical
> spinlock or 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 series addresses the problem in the following way: If CPU has printed
> more that printk_offload (defaults to 1000) characters, it wakes up one
> of dedicated printk kthreads (we don't use workqueue because that has
> deadlock potential if printk was called from workqueue code). Once we find
> out kthread is spinning on a lock, we stop printing, drop console_sem, and
> let kthread continue printing. Since there are two printing kthreads, they
> will pass printing between them and thus no CPU gets hogged by printing.

I still hate your patchset ;)

But nothing better suggests itself.  I have a few review comments -
please let's work through that stuff, get a fresh version out and we'll
see how it goes.

Is this patchset being used in the field?  Perhaps in the suse kernel? 
If so, a mention of that in the changelog would help things along.

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