Messages in this thread | | | Date | Mon, 13 Jan 2014 08:28:27 +0100 | From | Jan Kara <> | Subject | Re: [PATCH 9/9] printk: Hand over printing to console if printing too long |
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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@suse.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@suse.cz> SUSE Labs, CR
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