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Date: Fri, 14 Mar 2014 17:21:44 +0100 From: Jan Kara <jack@...e.cz> To: Andrew Morton <akpm@...ux-foundation.org> Cc: LKML <linux-kernel@...r.kernel.org>, pmladek@...e.cz, Steven Rostedt <rostedt@...dmis.org>, Frederic Weisbecker <fweisbec@...il.com>, Jan Kara <jack@...e.cz> Subject: [PATCH 0/8 v3] printk: Cleanups and softlockup avoidance Hello, this is another revision of the printk softlockup series. Since previous version 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. --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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