linux-kernel - Re: [PATCH] printk: Avoid softlockups in console

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Date:	Wed, 30 Jan 2013 16:08:27 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>, jslaby@...e.cz
Subject: Re: [PATCH] printk: Avoid softlockups in console_unlock()

On Tue, 29 Jan 2013 15:54:24 +0100
Jan Kara <jack@...e.cz> wrote:

> >   So I was testing the attached patch which does what we discussed. The bad
> > news is I was able to trigger a situation (twice) when suddently sda
> > disappeared and thus all IO requests failed with EIO. There is no trace of
> > what's happened in the kernel log. I'm guessing that disabled interrupts on
> > the printing CPU caused scsi layer to time out for some request and fail the
> > device. So where do we go from here?
>   Andrew? I guess this fell off your radar via the "hrm, strange, need to
> have a closer look later" path?

urgh.  I was hoping that if we left it long enough, one of both of us
would die :(

I fear we will rue the day when we changed printk() to bounce some of
its work up to a kernel thread.

> Currently I'd be inclined to return to my original solution...

Can we make it smarter?  Say, take a peek at the current
softlockup/nmi-watchdog intervals, work out how for how long we can
afford to keep interrupts disabled and then use that period and
sched_clock() to work out if we're getting into trouble?  IOW, remove
the hard-wired "1000" thing which will always be too high or too low
for all situations.

Implementation-wise, that would probably end up adding a kernel-wide
function along the lines of

/*
 * Return the maximum number of nanosecond for which interrupts may be disabled
 * on the current CPU
 */
u64 max_interrupt_disabled_duration(void)
{
	return min(sortirq duration, nmi watchdog duration);
}

Thinking ahead...

Other kernel sites which know they can disable interrupts for a long
time can perhaps use this.

Later, realtimeish systems (for example machine controllers) might want
to add a kernel tunable so they can set the
max_interrupt_disabled_duration() return value much lower.

To make that more accurate, we could add per-cpu, per-irq variables to
record sched_clock() when each CPU enters the interrupt, so the comment
becomes

/*
 * Return the remaining maximum number of nanosecond for which interrupts may
 * be disabled on the current CPU
 */

This may all be crazy and hopefully we'll never do it, but the design
should permit such things from day one if practical.

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