Re: [PATCH 3/4] printk: Defer printing to irq work when we printed too much

[Jan Kara <jack@suse.cz>]

On Wed 14-08-13 21:38:18, Steven Rostedt wrote:
> On Wed, 14 Aug 2013 15:28:27 +0200
> Jan Kara <jack@suse.cz> wrote:
> 
> > A CPU can be caught in console_unlock() for a long time (tens of seconds
> > are reported by our customers) when other CPUs are using printk heavily
> > and serial console makes printing slow. Despite serial console drivers
> > are calling touch_nmi_watchdog() this triggers softlockup warnings
> > because interrupts are disabled for the whole time console_unlock() runs
> > (e.g. vprintk() calls console_unlock() with interrupts disabled). Thus
> > IPIs cannot be processed and other CPUs get stuck spinning in calls like
> > smp_call_function_many(). Also RCU eventually starts reporting lockups.
> > 
> > In my artifical testing I can also easily trigger a situation when disk
> > disappears from the system apparently because commands to / from it
> > could not be delivered for long enough. This is why just silencing
> > watchdogs isn't a reliable solution to the problem and we simply have to
> > avoid spending too long in console_unlock() with interrupts disabled.
> > 
> > The solution this patch works toward is to postpone printing to a later
> > moment / different CPU when we already printed over 1000 characters in
> > current console_unlock() invocation. This is a crude heuristic but
> > measuring time we spent printing doesn't seem to be really viable - we
> > cannot rely on high resolution time being available and with interrupts
> > disabled jiffies are not updated. The value 1000 was chosen so that
> > things are still bearable with 9600 baud serial console and OTOH it
> > shouldn't cause offloading of printing in common cases.
> > 
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  kernel/printk/printk.c | 62 ++++++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 52 insertions(+), 10 deletions(-)
> > 
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 9208e17..28c220a 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -2026,31 +2026,49 @@ out:
> >  	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
> >  }
> >  
> > +/*
> > + * How much characters can we print in one call of printk before offloading
> > + * printing work
> > + */
> > +#define MAX_PRINTK_CHARS 1000
> 
> This really needs to be configurable. 1000 may be good for you, but may
> not be good for others. Heck, we should be able to disable this
> offloading too.
> 
> This should probably be added to the printk /proc/sys/kernel/... files.
> 
> If it is set to zero, then disable any offloading.
  OK, I can easily do this. I'm somewhat skeptical people would want to
tune this but maybe they would.

								Honza

> > +
> > +/* Should we stop printing on this cpu? */
> > +static bool cpu_stop_printing(int printed_chars)
> > +{
> > +	/* Oops? Print everything now to maximize chances user will see it */
> > +	if (oops_in_progress)
> > +		return false;
> > +	return printed_chars > MAX_PRINTK_CHARS;
> > +}
> > +
> >  /**
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR