On Fri 18-09-15 15:15:21, Andrew Morton wrote:
> On Wed, 19 Aug 2015 17:38:30 +0200 Jan Kara <[email protected]> wrote:
>
> > From: Jan Kara <[email protected]>
> >
> > 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 <[email protected]>
SUSE Labs, CR
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