Re: [PATCH] kernel/watchdog: flush all printk nmi buffers when hardlockup detected

[Konstantin Khlebnikov]

On 11/02/2020 01.51, Andrew Morton wrote:
> On Mon, 10 Feb 2020 12:48:57 +0300 Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> wrote:
>
> > In NMI context printk() could save messages into per-cpu buffers and
> > schedule flush by irq_work when IRQ are unblocked. This means message
> > about hardlockup appears in kernel log only when/if lockup is gone.
>
> I think I understand what this means.  The hard lockup detector runs at
> NMI time but if it detects a lockup within IRQ context it cannot call
> printk, because it's within NMI context, where synchronous printk
> doesn't work.  Yes?

Yes. Printing from hardlockup watchdog is only special case.
Without it irq-work will flush per-cpu buffer right after enabling irq.

> > Comment in irq_work_queue_on() states that remote IPI aren't NMI safe
> > thus printk() cannot schedule flush work to another cpu.
> >
> > This patch adds simple atomic counter of detected hardlockups and
> > flushes all per-cpu printk buffers in context softlockup watchdog
> > at any other cpu when it sees changes of this counter.
>
> And I think this works because the softlockup detector runs within irq
> context?

Yes. Softlockuo watchdog is a timer. It could use normal printk and
flush per-cpu buffers. Any periodically executed code could do that
but softlockup is most logical place for that.

There is forward signal from softlockup to hardlockup wathdogs in
per-cpu counter hrtimer_interrupts (increment means cpu in't in
hardlockup). This patch adds backward signal from hardlockup to
softlocup detector that some cpus are in hardlockup.

> > ...
> >
> > --- a/kernel/watchdog.c
> > +++ b/kernel/watchdog.c
> > @@ -92,6 +92,26 @@ static int __init hardlockup_all_cpu_backtrace_setup(char *str)
> >   }
> >   __setup("hardlockup_all_cpu_backtrace=", hardlockup_all_cpu_backtrace_setup);
> >   # endif /* CONFIG_SMP */
> > +
> > +atomic_t hardlockup_detected = ATOMIC_INIT(0);
> > +
> > +static inline void flush_hardlockup_messages(void)
>
> I don't think this needs to be inlined?
>
> > +{
> > +	static atomic_t flushed = ATOMIC_INIT(0);
> > +
> > +	/* flush messages from hard lockup detector */
> > +	if (atomic_read(&hardlockup_detected) != atomic_read(&flushed)) {
> > +		atomic_set(&flushed, atomic_read(&hardlockup_detected));
> > +		printk_safe_flush();
> > +	}
> > +}
>
> Could we add some explanatory comments here?  Explain to the reader why
> this code exists, what purpose it serves?  Basically a micro version of
> the above changelog.

Hmm. This seems obvious from names of variables and called function.
Both watchdogs use same patterns: monotonic counter and side variable
with snapshot to detect changes or stalls.

> > ...