Re: [PATCH v2] printk: Avoid softlockups in console_unlock()

Jan Kara
2013-02-06T15:19:42Z

On Tue 05-02-13 12:38:38, Andrew Morton wrote:
> On Mon, 4 Feb 2013 23:17:10 +0100
> 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 also managed to trigger a situation when disk
> > disappeared 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().
> >
> > We fix the issue by limiting the time we spend in console_unlock() to
> > watchdog_thresh() / 4 (unless we are in an early boot stage or oops is
> > happening). The rest of the buffer will be printed either by further
> > callers to printk() or by a queued work.
>
> I still hate the patch :(
>
> > ...
> >
> > +void console_unlock(void)
> > +{
> > + if (__console_unlock()) {
> > + /* Let worker do the rest of printing */
> > + schedule_work(&printk_work);
> > + }
> > }
>
> This creates another place from where we cannot call printk(): anywhere
> where worker_pool.lock is held.
>
> And as schedule_work() can do a wakeup it creates a third reason why
> the sched code cannot call printk (along with rq->lock taken by
> wake_up(klogd) and rq->lock taken by up(&console_sem). Hence
> printk_sched(). See the lkml thread "[GIT PULL] printk: Support for
> full dynticks mode".
>
> We already have machinery for doing async tickling in printk: the
> printk_pending stuff. Did you consider adding another
> PRINTK_PENDING_foo in some fashion?
Yes, I noticed that thread just yesterday and also though that using
similar trick might be viable. I'll experiment if we could use the same
method for handling lockup problems I hit. Steven seems to have already
tweaked PRINTK_PENDING stuff to be usable more easily...

Honza

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR