On (03/21/16 09:56), Byungchul Park wrote:
> > > > + if (!sync_print) {
> > > > + if (in_sched) {
> > > > + /*
> > > > + * @in_sched messages may come too early, when
> > > > we don't
> > > > + * yet have @printk_kthread. We can't print
> > > > deferred
> > > > + * messages directly, because this may
> > > > deadlock, route
> > > > + * them via IRQ context.
> > > > + */
> > > > + __this_cpu_or(printk_pending,
> > > > + PRINTK_PENDING_OUTPUT);
> > > > +
> > > > irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> > > > + } else if (printk_kthread && !in_panic) {
> > > > + /* Offload printing to a schedulable context. */
> > > > + wake_up_process(printk_kthread);
> > >
> > > It will not print the "lockup suspected" message at all, for e.g.
> > > rq->lock,
> > > p->pi_lock and any locks which are used within wake_up_process().
> >
> > this will switch to old SYNC printk() mode should such a lockup ever
> > happen, which is a giant advantage over any other implementation; doing
> > wake_up_process() within the 'we can detect recursive printk() here'
> > gives us better control.
> >
> > why
> >
> > printk()->IRQ->wake_up_process()->spin_dump()->printk()->IRQ->wake_up_process()->spin_dump()->printk()->IRQ...
> > is better?
>
> What is IRQ?
this is how printk() can print the messages in async mode apart from
direct and wake_up_process() in vprintk_emit().
-ss
> > > Furtheremore, any printk() within wake_up_process() cannot work at all, as
> > > well.
> >
> > there is printk_deferred() which has LOGLEVEL_SCHED and which must be used
> > in sched functions.
>
> It would be good for all scheduler code to use the printk_deferred() as you
> said, but it's not true yet.
>
> >
> > -ss
>
