Re: [PATCH 3/4] kernel: Avoid softlockups in stop_machine() during heavy printing

[Jan Kara]

On Fri 18-09-15 15:15:21, Andrew Morton wrote:
> On Wed, 19 Aug 2015 17:38:30 +0200 Jan Kara <jack@xxxxxxxx> wrote:
> 
> > From: Jan Kara <jack@xxxxxxx>
> > 
> > When there are lots of messages accumulated in printk buffer, printing
> > them (especially over serial console) can take a long time (tens of
> > seconds). stop_machine() will effectively make all cpus spin in
> > multi_cpu_stop() waiting for the CPU doing printing to print all the
> > messages which triggers NMI softlockup watchdog and RCU stall detector
> > which add even more to the messages to print. Since machine doesn't do
> > anything (except serving interrupts) during this time, also network
> > connections are dropped and other disturbances may happen.
> > 
> > Paper over the problem by waiting for printk buffer to be empty before
> > starting to stop CPUs. In theory a burst of new messages can be appended
> > to the printk buffer before CPUs enter multi_cpu_stop() so this isn't a 100%
> > solution but it works OK in practice and I'm not aware of a reasonably
> > simple better solution.
> 
> Confused.  Why don't patches 1 and 2 already fix this problem?

Because stop_machine() will not allow printing threads to run on any CPU
(all but one CPUs are spinning in multi_cpu_stop() without possibility of
preemption) and thus any printk offloading cannot happen.

> > ...
> >
> > @@ -2489,6 +2489,28 @@ struct tty_driver *console_device(int *index)
> >  }
> >  
> >  /*
> > + * Wait until all messages accumulated in the printk buffer are printed to
> > + * console. Note that as soon as this function returns, new messages may be
> > + * added to the printk buffer by other CPUs.
> > + */
> > +void console_flush(void)
> 
> This doesn't seem a very good name.  We already have
> console_cont_flush(), cont_flush(), etc.  Can we think of something
> more specific?  printk_log_buf_drain() perhaps.

Thanks for suggestion. I'll change the name.

> > +{
> > +	bool retry;
> > +	unsigned long flags;
> > +
> > +	while (1) {
> > +		raw_spin_lock_irqsave(&logbuf_lock, flags);
> > +		retry = console_seq != log_next_seq;
> > +		raw_spin_unlock_irqrestore(&logbuf_lock, flags);
> 
> Does this lock/unlock do anything useful?
> 
> > +		if (!retry || console_suspended)
> > +			break;
> > +		/* Cycle console_sem to wait for outstanding printing */
> > +		console_lock();
> > +		console_unlock();
> > +	}
> > +}
> > +
> > +/*
> >   * Prevent further output on the passed console device so that (for example)
> >   * serial drivers can disable console output before suspending a port, and can
> >   * re-enable output afterwards.
> > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> > index fd643d8c4b42..016d34621d2e 100644
> > --- a/kernel/stop_machine.c
> > +++ b/kernel/stop_machine.c
> > @@ -21,6 +21,7 @@
> >  #include <linux/smpboot.h>
> >  #include <linux/atomic.h>
> >  #include <linux/lglock.h>
> > +#include <linux/console.h>
> >  
> >  /*
> >   * Structure to determine completion condition and record errors.  May
> > @@ -543,6 +544,14 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
> >  		return ret;
> >  	}
> >  
> > +	/*
> > +	 * If there are lots of outstanding messages, printing them can take a
> > +	 * long time and all cpus would be spinning waiting for the printing to
> > +	 * finish thus triggering NMI watchdog, RCU lockups etc. Wait for the
> > +	 * printing here to avoid these.
> > +	 */
> > +	console_flush();
> 
> This is pretty pointless if num_possible_cpus==1.  I'd suggest setting
> printk_offload_chars=0 in this case, add some early bale-out into
> console_flush().  Or something along those lines.
> 
> And make console_flush() go away altogether if CONFIG_SMP=n - it's
> pointless bloat.

Sure, I'll do that.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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