linux-kernel - Re: [PATCH 0/8 v4] printk: Cleanups and softlockup avoidance

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Message-ID: <20140408142726.GB9551@quack.suse.cz>
Date:	Tue, 8 Apr 2014 16:27:26 +0200
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, pmladek@...e.cz,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 0/8 v4] printk: Cleanups and softlockup avoidance

On Tue 25-03-14 18:54:53, Jan Kara wrote:
>   Hello,
> 
>   this is another revision of the printk softlockup series.
> 
> Changes since v3:
> Fixed bogus warning in console_try_lock_spin() in non-preemptible kernels.
> Fixed infinite loop in console_flush() when console was suspended.
> 
> Changes since v2:
> I have fixed up some small problems pointed out by Andrew, added possibility to
> configure out the printk offloading logic (for small systems), and offload
> kthreads are now started only once printk.offload_chars is set to value > 0.
> 
> Intro for the newcomers to the series below.
  Ping Andrew?

								Honza
> 
> ---
> 
> 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 since vprintk_emit() calls console_unlock()
> 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 is a revised series using my new approach to the problem which doesn't
> let CPU out of console_unlock() until there's someone else to take over the
> printing. The main difference since the last version is that instead of
> passing printing duty to different CPUs via IPIs we use dedicated kthreads.
> This method is somewhat less reliable (in a sense that there are more
> situations in which handover needn't work at all - e.g. when the currently
> printing CPU holds a spinlock and the CPU where kthread is scheduled to run is
> spinning on this spinlock) but the code is much simpler and in my practical
> testing kthread approach was good enough to avoid any problems (with one
> exception - see patch 8/8).
> 
> 								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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