Re: [PATCH 1/4] printk: Hand over printing to console if printing too long

[Jan Kara]

On Fri 18-09-15 15:14:59, Andrew Morton wrote:
> On Wed, 19 Aug 2015 17:38:28 +0200 Jan Kara <jack@xxxxxxxx> wrote:
> 
> > From: Jan Kara <jack@xxxxxxx>
> > 
> > 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 waking up one of dedicated kthreads.  As
> > soon as the printing CPU notices kthread got scheduled and is spinning
> > on print_lock dedicated for that purpose, it drops console_sem,
> > print_lock, 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.
> > 
> > ...
> >
> > @@ -2230,6 +2292,8 @@ void console_unlock(void)
> >  	unsigned long flags;
> >  	bool wake_klogd = false;
> >  	bool retry;
> > +	bool hand_over = false;
> > +	int printed_chars = 0;
> >  
> >  	if (console_suspended) {
> >  		up_console_sem();
> > @@ -2241,12 +2305,18 @@ void console_unlock(void)
> >  	/* flush buffered message fragment immediately to console */
> >  	console_cont_flush(text, sizeof(text));
> >  again:
> > +	spin_lock(&print_lock);
> 
> I'm surprised this isn't spin_lock_irqsave().  How come this isn't
> deadlockable?

Yes, it should be spin_lock_irqsave(). My original plan was to nest
print_lock inside logbuf_lock which would provide the protection but later
I've ordered them the other way around and forgot to update the irq
protection. Will fix.

> >  	for (;;) {
> >  		struct printk_log *msg;
> >  		size_t ext_len = 0;
> >  		size_t len;
> >  		int level;
> >  
> > +		if (cpu_stop_printing(printed_chars)) {
> > +			hand_over = true;
> > +			break;
> > +		}
> > +
> >  		raw_spin_lock_irqsave(&logbuf_lock, flags);
> >  		if (seen_seq != log_next_seq) {
> >  			wake_klogd = true;
> >
> > ...
> >
> > +/* Kthread which takes over printing from a CPU which asks for help */
> > +static int printing_task(void *arg)
> > +{
> > +	DEFINE_WAIT(wait);
> > +
> > +	while (1) {
> > +		prepare_to_wait_exclusive(&print_queue, &wait,
> > +					  TASK_INTERRUPTIBLE);
> > +		schedule();
> > +		finish_wait(&print_queue, &wait);
> > +		preempt_disable();
> 
> I don't understand the preempt_disable().  Code comment, please?

We don't want to be scheduled away in preemptible kernels when spinning on
print_lock or after we acquired print_lock and before we got console_sem.
I'll add a comment.

Thanks for review!
								Honza

> 
> > +		atomic_inc(&printing_tasks_spinning);
> > +		/*
> > +		 * Store printing_tasks_spinning value before we spin. Matches
> > +		 * the barrier in cpu_stop_printing().
> > +		 */
> > +		smp_mb__after_atomic();
> > +		/*
> > +		 * Wait for currently printing thread to complete. We spin on
> > +		 * print_lock instead of waiting on console_sem since we don't
> > +		 * want to sleep once we got scheduled to make sure we take
> > +		 * over printing without depending on the scheduler.
> > +		 */
> > +		spin_lock(&print_lock);
> > +		atomic_dec(&printing_tasks_spinning);
> > +		spin_unlock(&print_lock);
> > +		if (console_trylock())
> > +			console_unlock();
> > +		preempt_enable();
> > +	}
> > +	return 0;
> > +}
> >
> > ...
> >
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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