Messages in this thread | | | Date | Fri, 14 Mar 2014 14:30:21 +0100 | From | Jan Kara <> | Subject | Re: [PATCH 5/6] printk: Hand over printing to console if printing too long |
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On Thu 13-03-14 16:06:25, Andrew Morton wrote: > On Thu, 13 Mar 2014 16:58:37 +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 PRINTK_HANDOVER_B bit in > > printk_handover_state variable and wakes up one of dedicated kthreads. > > As soon as the printing CPU notices kthread got scheduled and is > > spinning on console_sem, it drops console_sem and exits > > console_unlock(). kthread then takes over printing instead. This way no > > CPU should spend printing too long even if there is heavy printk > > traffic. > > Sigh ;) > > Creating yet more kernel threads to solve this problem is a bit sad. > Is there no way in which we can borrow one of the existing threads? I realized I didn't react to the above. I've checked which kernel threads are running on my system and I can see: rcu related threads, ksoftirqd, various fs threads, kauditd - nothing really seems as a good fit. Then there are various workqueue related threads - these aren't really suitable either. Finally I can see migration threads (from kernel/stop_machine.c) - I'm not sure I want to mess with these - especially since they can run any function somebody passes to them and when that function calls printk() things might get interesting.
So I would go for starting the threads only if printk.offload_chars ever gets set to value > 0.
Honza -- Jan Kara <jack@suse.cz> SUSE Labs, CR
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